Environmental and chemical stress is the damage that water, dissolved ions and corrosive gases do to a package, its metallisation and its board, and it can start without any electrical or mechanical load. This page covers the transport of moisture into a plastic package, the humidity acceleration model used in testing, corrosion of aluminium pads, electrochemical migration and dendrites, ionic contamination, creep corrosion by sulfur, and tin whiskers. The cracking and delamination that moisture causes at reflow is on the Mechanical and Package Stress page, and only the moisture and vapour pressure behind it are here. Current-driven transport in metal lines is on the Metal and Interconnect page, field-driven oxide breakdown on the Electrical Stress page, heating on the Thermal Stress page, and the general statistics of lifetime on the Failure Rate and the Bathtub Curve page. Whiskers are included because the sources link them to humidity, corrosion and temperature cycling, although the stress in the tin film that drives them is not quantified in those sources, and the compressive-stress picture is background knowledge. Which technology is affected, for example a particular wide-band-gap device, belongs to the device-specific folder.

1. Why It Happens

Trigger: Water vapour that diffuses into the molding compound, together with ions from the compound or from process residue (Teverovsky; National Semiconductor); a potential difference with enough humidity, a migration-prone alloy and residual contamination for electrochemical migration (Wack); halides, mainly chloride and fluoride, at aluminium pads (Kumar et al.); sulfur compounds in the air with moisture for creep corrosion (Savolainen and Schueller); and tin-rich surface finishes for whiskers (Panashchenko).
Mechanism, moisture: Water enters the compound by Fick diffusion, the equilibrium uptake is linear in the humidity, as Henry’s law requires, and the diffusion coefficient follows an Arrhenius law with an activation energy of 0.43 $eV$ in the best fit (Teverovsky). At reflow the absorbed water becomes steam, the pressure cracks the package, and the crack lets moisture and ionic contamination in (National Semiconductor). The vapour pressure at an interface saturates much faster than the local concentration (Han et al.).
Mechanism, corrosion: Chloride dissolves aluminium to $AlC l_{3}$ , which hydrolyses to $Al (OH)_{3}$ and $H^{+}$ , and where copper touches the aluminium the copper reduces $H^{+}$ to hydrogen gas, which the authors link to deep cracks, pits and dendrites; fluoride forms poorly soluble $Al F_{3}$ that limits the acidification, so the attack goes on with or without copper (Kumar et al.).
Mechanism, migration: Three steps follow each other: metal atoms dissolve as ions at the anode, the ions diffuse to the cathode, and they deposit there, mostly where the field is highest, which forms dendrites (Wack). The bias can be the normal operating voltage (Wack).
Mechanism, creep corrosion: Sulfur compounds dissolve in the surface water to a weak acid that reduces copper oxide and exposes the copper. The product is mainly $C u_{2} S$ with a little $A g_{2} S$ , and no electric field is needed. On immersion silver the copper is anodic to the silver, and a small exposed copper area next to a large silver area corrodes fastest, especially under the edge of the solder mask (Savolainen and Schueller).
Mechanism, whiskers: Conductive crystalline tin grows out of tin-rich surfaces by addition of atoms at the base, not the tip, with long-range diffusion of tin to the growth site (Panashchenko).
What sets the rate: Temperature and humidity, with the exponent and activation energy of the Peck model (Peck; Teverovsky); the package thickness, since the bake-out time scales with its square (Teverovsky); for migration the spacing, voltage and contamination, with halides the most harmful (Bumiller and Hillman; He); the halide and the copper contact for pad corrosion (Kumar et al.); the surface finish and the sulfur level for creep corrosion (Savolainen and Schueller); and time, temperature, humidity and cycling for whiskers (Panashchenko).
Failure criterion: A device that fails its electrical limits after the exposure, used as the time to failure in the Peck model (Peck); for migration a fall in the surface insulation resistance (He); for a coupon the film thickness in 30 days, in the classes G1 to GX (Purafil); and for whiskers a length limit, where one standard lists 50 $μ m$ (Panashchenko).

2. Where to Look

Location: Aluminium bond pads that a copper wire touches, since the Cu/Al couple drives the chloride attack (Kumar et al.); between closely spaced conductors, where the spacings in the examples are 48 to 170 $μ m$ (Bumiller and Hillman); the edge of the solder mask where copper meets silver, where creep corrosion starts (Savolainen and Schueller; iNEMI); the interface between the die-attach film and the chip for vapour pressure (Han et al.); and tin-plated leads and surfaces for whiskers (Panashchenko).
Signature: Corroded pads with pits, cracks and dendrites (Kumar et al.); a failure that cannot be duplicated at first because the corrosion product is fairly resistive, and that returns with humidity, where the resistance of the product fell from over 10 $MΩ$ to below 1 $MΩ$ (Savolainen and Schueller); a falling surface insulation resistance and dendrites that bridge a gap (He; Wack); leakage currents and parametric shifts in linear devices after humidity tests (Teverovsky); and conductive hairs on tin, where a current below the melting current of the whisker gives a permanent short and a larger current an intermittent one (Panashchenko).
Confirm with: SEM with EDX and XPS on the pad, and headspace GC-MS for hydrogen (Kumar et al.); surface insulation resistance on comb structures (He); weight gain against time for moisture (Teverovsky); ionic extraction, which does not find all the contamination by itself (Wack); corrosion coupons for the severity class (Purafil); and SEM for whisker length, with a tilt correction (Panashchenko).
Look-alikes: Creep corrosion looks like electromigration but needs no field, and the usual cleanliness standards were developed for electromigration (Savolainen and Schueller); leakage currents from corrosion and contamination also form bridges, mainly in contaminated atmospheres (Wack); moisture-assisted hot-carrier degradation may have a negative activation energy, so it can be mistaken for something that humidity tests do not accelerate (Teverovsky); and a dendrite and a whisker are different objects, a dendrite being a deposit from migrating ions and a whisker a growth from the tin film itself (background knowledge).

3. Prevention

Design: More spacing, since the time to failure scales with $G / V$ in Hornung’s form (Bumiller and Hillman); a solder mask, which stabilised the insulation resistance by a walling effect (He); for creep corrosion, plug vias that are not test points with solder mask, use non-mask-defined test pads and vias, keep test features more than 2.5 $mm$ apart, round the corners of the pads and print paste to cover the pads fully (Savolainen and Schueller).
Material and process: Lead-free HASL is resistant to creep corrosion and high-temperature OSP survived all but the worst industrial environments, whereas immersion silver is the most prone (Savolainen and Schueller); medium-solids flux gave a longer characteristic life than low-solids flux (He); cleaning before conformal coating (Wack); a Cu-selective azole inhibitor on the copper wire stopped hydrogen evolution under chloride but gave no protection against fluoride (Kumar et al.); and for whiskers lead has been shown to mitigate them, a nickel underlayer was ineffective in one experiment, and conformal coating and annealing affect the growth (Panashchenko).
Operation: Keep parts dry between the bag and the reflow by the floor life of the level, dry pack, a humidity indicator card, and a bake at 125 $^{\circ} C$ or a rebake at 40 $^{\circ} C$ (National Semiconductor); keep the humidity below the 60 to 70 per cent that Wack gives as critical for migration (my reading of a precaution); and avoid a direct air intake onto boards, since high airflow appears to increase creep corrosion, while spray conformal coatings give only marginal protection (Savolainen and Schueller).
Screening and test: Unbiased HAST at 130 $^{\circ} C$ and 85 per cent RH for 96 h, or 110 $^{\circ} C$ for 264 h, biased HAST to JESD22-A110-B, and autoclave at 121 $^{\circ} C$ and 100 per cent RH for 24 to 336 h (Teverovsky); 85 $^{\circ} C$ and 85 per cent RH for 1000 h, which Peck proposed to replace by 20 h at 140 $^{\circ} C$ and 100 per cent RH (Peck); the climatic test of IEC 68-2 at 40 $^{\circ} C$ and 93 per cent RH for 21 days and the CoRe test of at most 10 h (Wack); coupon classes to ISA 71.04 (Purafil); a flowers-of-sulfur chamber with controlled humidity (iNEMI); and the whisker tests of JESD22-A121A and JESD201 (Panashchenko).

Moisture Uptake and Diffusion

Name	Formula	Related Formulae	Variables/Constants
Moisture diffusion in a molding compound and its characteristic times	$\frac{\partial C}{\partial t} = D \frac{\partial ^{2} C}{\partial x ^{2}}, D = D_{0} e^{- U / (k_{B} T)}$	$θ = \frac{4 h ^{2}}{π ^{2} D}$ $t_{bak e} \approx \frac{1.01 h ^{2}}{D}$	$C : m o i s t u r e co n ce n t r a t i o n$ $t : t im e$ $x : d e pt h in t h e p a c k a g e$ $D : d i f f u s i o n coe f f i c i e n t$ $D_{0} : p r e - e x p o n e n t ia l f a c t or$ $U : a c t i v a t i o n e n er g y o f t h e d i f f u s i o n$ $k_{B} : B o l t z mann co n s t an t 8.617 \times 1 0^{- 5} eV/K$ $T : ab so l u t e t e m p er a t u r e$ $θ : c ha r a c t er i s t i c t im e o f a f l a t p a c k a g e o f t hi c k n ess 2 h$ $h : ha l f t hi c k n ess$ $t_{bak e} : bak e - o u t t im e o f a f l a t p a c k a g e, T e v er o v s k y (N E P P r e p or t) : F i c k^{'} s seco n d l a w f or a f l a t p a c k a g e w i t h t h e d i e a t t h e mi d - pl an e, b es t - f i t D_{0} = 7.35 \times 1 0^{- 6} m^{2} /s an d U = 0.43 eV (E q . 11), t h e bak e - o u t t im e i s 0.3 R^{2} / D f or s p h er es an d 0.4 R^{2} / D f or cy l in d er s, a t t = θ t h e s u r f a ce co n ce n t r a t i o n i s ab o u t 55 p er ce n t o f s a t u r a t i o n an d a t 3 θ ab o v e 99 p er ce n t, m e a s u r e d v a l u es f or a QF P - 144 a r e D = 3.0 \times 1 0^{- 9}, 1.2 \times 1 0^{- 7} an d 7.8 \times 1 0^{- 7} c m^{2} /s a t 20, 85 an d 130^{\circ} C w i t h U = 0.50 eV, an d f or a P L C C - 32 3.2 \times 1 0^{- 8} an d 3.2 \times 1 0^{- 7} c m^{2} /s a t 20 an d 85^{\circ} C w i t h U = 0.32 eV, an d D a t r oo m t e m p er a t u r e v a r i es b y m or e t han an or d er o f ma g ni t u d e b e tw ee n co m p o u n d s, H an e t a l . : D o f an e p o x y m o l d in g co m p o u n d a t 85^{\circ} C i s 6.2 \times 1 0^{- 9} m^{2} /h in t h e i r t ab l e (t h e r es u l t s sec t i o n p r in t s o t h er u ni t s), t h e s t an d a r d bak e i s 125^{\circ} C f or 24 h (T e v er o v s k y)$ $(co m p u t e d w i t h t h e f i t : D = 3.0 \times 1 0^{- 9} c m^{2} /s a t 20^{\circ} C, e q u a l t o t h e QF P v a l u e, an d 6.5 \times 1 0^{- 8} a t 85^{\circ} C; f or an a ss u m e d 2 mm p a c k a g e θ = 15.8 d a y s a t 20^{\circ} C, 17.2 h a t 85^{\circ} C an d 4.2 h a t 125^{\circ} C, so 3 θ = 52 h a t 85^{\circ} C; t h e ser i es so l u t i o n o f F i c k^{'} s l a w g i v es 53 an d 94 p er ce n t o f s a t u r a t i o n a t t h e mi d - pl an e (70 an d 96 p er ce n t a v er a g e u pt ak e) a t θ an d 3 θ, so t h e q u o t e d^{'} ab o v e 99 p er ce n t a t 3 θ^{'} i s n o t r e p r o d u ce d an d n ee d s ab o u t 5 θ, a u d i t c h ec k; t_{bak e} = 10.6 h a t 125^{\circ} C, a g ain s t t h e s t an d a r d 24 h)$
Saturated moisture uptake (Henry’s law)	$C_{s a t} = η RH P_{s} (T) = η_{0} P_{0} RH e^{(Δ H - Q) / (k_{B} T)}$	$η = η_{0} e^{Δ H / (k_{B} T)}, P_{s} = P_{0} e^{- Q / (k_{B} T)}$ $\frac{C _{s a t} ( RH _{1} )}{C _{s a t} ( RH _{2} )} = \frac{RH _{1}}{RH _{2}}$	$C_{s a t} : s a t u r a t e d m o i s t u r e co n ce n t r a t i o n$ $η : sor pt i o n coe f f i c i e n t$ $R H : r e l a t i v e h u mi d i t y a s a f r a c t i o n$ $P_{s} : s a t u r a t e d w a t er - v a p o u r p r ess u r e$ $η_{0} an d P_{0} : p r e - e x p o n e n t ia l f a c t or s$ $Δ H : h e a t o f sor pt i o n$ $Q : a c t i v a t i o n e n er g y o f t h e v a p o u r p r ess u r e, 0.42 eV$ $k_{B} : B o l t z mann co n s t an t 8.617 \times 1 0^{- 5} eV/K$ $T : ab so l u t e t e m p er a t u r e$ $R H_{1} an d R H_{2} : tw o h u mi d i t i es a t t h e s am e t e m p er a t u r e, T e v er o v s k y (N E P P r e p or t) : t h e u pt ak e i s l in e a r in h u mi d i t y, a s H e n r y^{'} s l a w r e q u i r es, i t i s 0.2 t o 0.5 p er ce n t o f t h e p a c k a g e ma ss a t 85 p er ce n t R H (0.184 t o 0.329 p er ce n t a f t er 186 h a t 85/85), t h e u pt ak e a t r oo m t e m p er a t u r e an d 50 p er ce n t R H w a s ab o u t 59 p er ce n t o f t ha t a t 85/85, an d w h e n Q i s ab o u t e q u a l t o Δ H t h e s a t u r a t e d u pt ak e ba r e l y c han g es b e tw ee n r oo m t e m p er a t u r e an d 85^{\circ} C, H an e t a l . : s a t u r a t e d co n ce n t r a t i o n s a t 85/85 an d 85/65 a r e 6.1 an d 5.6 kg/ m^{3} f or t h e m o l d in g co m p o u n d, 20.8 an d 14.4 f or t h e d i e - a tt a c h f i l m an d 4.4 an d 3.4 f or t h e b o a r d$ $(co m p u t e d : 50/85 i s 0.588, w hi c h r e p r o d u ces t h e 59 p er ce n t; f or 65 a g ain s t 85 p er ce n t R H a l in e a r l a w g i v es 0.76, b u t H an e t a l . g i v e 0.92 f or t h e m o l d in g co m p o u n d, 0.69 f or t h e d i e - a tt a c h f i l m an d 0.77 f or t h e b o a r d)$
Soak time at a higher temperature and the moisture sensitivity levels	$t_{e q} = t_{r e f} \frac{D ( T _{r e f} )}{D ( T )} = t_{r e f} exp [\frac{U}{k _{B}} (\frac{1}{T} - \frac{1}{T _{r e f}})]$	$\sum_{i} t_{e x p ose d, i} \leq t_{f l oor}$	$t_{e q} : so ak t im e a t t h e t e m p er a t u r e T t ha t g i v es t h e s am e u pt ak e a s t_{r e f} a t T_{r e f}, f or t h e s am e r e l a t i v e h u mi d i t y an d a p a c k a g e t ha t i s n o t y e t s a t u r a t e d$ $D : d i f f u s i o n coe f f i c i e n t o f t h e f i r s t r o w$ $U : a c t i v a t i o n e n er g y o f t h e d i f f u s i o n$ $k_{B} : B o l t z mann co n s t an t 8.617 \times 1 0^{- 5} eV/K$ $t_{e x p ose d, i} : t im e o u t o f t h e d r y ba g in p er i o d i$ $t_{f l oor} : f l oor l i f e o f t h e l e v e l (t h e e q u i v a l e n ce f or m i s m y co n s t r u c t i o n f r o m t h e f i r s t r o w an d t h e s u mm e d f l oor l i f e i s ba c k g r o u n d k n o w l e d g e), N a t i o na l S e mi co n d u c t or : t h e f l oor l i f e a t u p t o 30^{\circ} C an d 60 p er ce n t R H i s 1 y e a r f or l e v e l 2, 4 w ee k s f or 2 a, 168 h f or l e v e l 3, 72 h f or 4, 48 h f or 5, 24 h f or 5 a an d t h e t im e o n t h e l ab e l f or 6, l e v e l 1 i s u n l imi t e d a t u p t o 30^{\circ} C an d 85 p er ce n t R H, l e v e l s 2 t o 6 n ee d d r y p a c k, an d t h e bak e i s a t 125^{\circ} C w i t h a l o w - t e m p er a t u r e r e bak e a t 40^{\circ} C, H an e t a l . : l e v e l 3 n ee d s a so ak o f 192 h a t 30^{\circ} C /60 p er ce n t, e q u i v a l e n t t o 40 or 52 h a t 60^{\circ} C /60 p er ce n t$ $(co m p u t e d w i t h U = 0.43 eV : 192 h a t 30^{\circ} C e q u a l s 43.6 h a t 60^{\circ} C, 34 h w i t h U = 0.50 an d 64 h w i t h 0.32 eV, a g ain s t 40 or 52 h in t h e so u r ce)$
Vapour pressure inside the package at reflow	$p_{v} \approx RH_{in t} p_{s a t} (T_{r e f l o w})$	$RH_{in t} = \frac{C}{η ( T _{r e f l o w} ) P _{s} ( T _{r e f l o w} )}$	$p_{v} : v a p o u r p r ess u r e in a v o i d or a t an in t er f a ce$ $R H_{in t} : e f f ec t i v e r e l a t i v e h u mi d i t y o f t h e m o i s t u r e in t h e in t er f a ce$ $p_{s a t} : s a t u r a t e d v a p o u r p r ess u r e o f w a t er$ $T_{r e f l o w} : p e ak t e m p er a t u r e$ $C : m o i s t u r e co n ce n t r a t i o n a t t h e in t er f a ce$ $η an d P_{s} : a s in t h e p r e v i o u s r o w (t h e seco n d f or m f o l l o w s f r o m H e n r y^{'} s l a w an d i s m y co n s t r u c t i o n), H an e t a l . : t h e v a p o u r p r ess u r e a t t h e in t er f a ce b e tw ee n t h e seco n d d i e - a tt a c h f i l m an d t h e c hi p r e a c h e d ab o u t 2.3 MPa a t r e f l o w a f t er 2 h a t b o t h 85/85 an d 85/65, t h e v a p o u r p r ess u r e s a t u r a t es m u c h f a s t er t han t h e l oc a l co n ce n t r a t i o n, w hi c h s a t u r a t e d a f t er ab o u t 20 h, an d t h e r e f l o w t e m p er a t u r e i s n o t g i v e n in t h e s u mma r y, N a t i o na l S e mi co n d u c t or : ab sor b e d m o i s t u r e t u r n s t o s t e am a t r e f l o w, t h e p r ess u r e cr a c k s t h e p a c k a g e, t h e s t e am t h e n esc a p es t h r o ug h t h e cr a c k an d l e t s m o i s t u r e an d i o ni c co n t amina t i o n in, an d t h e f l e xu r a l s t r e n g t h o f t h e co m p o u n d f a l l s f or t e m p er a t u r es u p t o 240^{\circ} C$ $(co m p u t e d w i t h an a ss u m e d 260^{\circ} C p e ak an d 4.69 MPa f or s a t u r a t e d s t e am (ba c k g r o u n d k n o w l e d g e, s t e am t ab l es) : 2.3 MPa i s 0.49 o f s a t u r a t i o n)$

Humidity Acceleration

Name	Formula	Related Formulae	Variables/Constants
Peck model and acceleration factor	$t_{f} = A RH^{- n} e^{E_{a} / (k_{B} T)}$	$A F = (\frac{RH _{u}}{RH _{t}})^{- n} exp [\frac{E _{a}}{k _{B}} (\frac{1}{T _{u}} - \frac{1}{T _{t}})]$	$t_{f} : t im e t o f ai l u r e$ $A : co n s t an t$ $R H : r e l a t i v e h u mi d i t y in p er ce n t$ $n : h u mi d i t y e x p o n e n t$ $E_{a} : a c t i v a t i o n e n er g y$ $k_{B} : B o l t z mann co n s t an t 8.617 \times 1 0^{- 5} eV/K$ $T : ab so l u t e t e m p er a t u r e$ $A F : a cce l er a t i o n f a c t or, s u b scr i pt s u an d t : u se an d t es t co n d i t i o n s, P ec k (1986) : f or e l ec t r o l y t i c cor r os i o n o f a l u mini u m m e t a l l i s a t i o n in e p o x y p a c k a g es n = 2.66 an d E_{a} = 0.79 eV (r = 0.986), n e a r l y e q u i v a l e n t a r e 2.50 w i t h 0.81 an d 3.00 w i t h 0.77, f r o m 61 d a t a p o in t s o n e p o x y, p h e n o l i c an d s i l i co n e e n c a p s u l a t i o n s s h o u l d b e t r e a t e d se p a r a t e l y, e x t r a p o l a t i o n i s v iab l e d o w n t o ab o u t 30 p er ce n t R H, l i f e ma y b e l o n g er b e l o w t ha t, an d t h e p a p er p r in t s t h e e x p o n e n t w i t h a min u s s i g n in i t s f i t, T e v er o v s k y (s l i d es) : n = 2.7 (r an g e 1 t o 5) an d E_{a} = 0.8 eV (r an g e 0.7 t o 1.1), S c h e nk e l b er g : P ec k 1986 i s l i s t e d a s n = 2.7 an d E_{a} = 0.79 eV (a l so in N e l so n, 1990) an d C o n d r a (2001) a s n = 3 an d E_{a} = 0.9 eV, t h e m o d e l i s f or m o i s t u r e - in d u ce d f ai l u r e o f e p o x y - p a c k a g e d I C s an d i s n o t a g e n er a l m o d e l, co n s t an t s a r e e m p i r i c a l an d t h e d a t a a r e d ec a d es o l d, B u mi l l er an d H i l l man : n = 2.66, l a t er ab o u t 3$ $(co m p u t e d f r o m 85^{\circ} C /85 p er ce n t t o u se a t 30^{\circ} C /20 p er ce n t : A F = 4880 f or 2.66 an d 0.79, a g ain s t ab o u t 4900 in P ec k; a t 40^{\circ} C /20 p er ce n t 1858 a g ain s t 1877; n = 3 w i t h 0.9 eV g i v es 15200, 3.1 t im es m or e)$
Effective humidity at the device and saturated vapour pressure	$RH_{d e v} = RH_{amb} \frac{p _{s a t} ( T _{amb} )}{p _{s a t} ( T _{d e v} )}$	$p_{s a t} (T) = 0.61121 exp [(18.678 - \frac{T}{234.5}) \frac{T}{257.14 + T}] kPa$	$R H_{d e v} : r e l a t i v e h u mi d i t y a t t h e d e v i ce s u r f a ce$ $R H_{amb} : r e l a t i v e h u mi d i t y o f t h e ambi e n t$ $p_{s a t} : s a t u r a t e d v a p o u r p r ess u r e o f w a t er$ $T_{amb} an d T_{d e v} : ambi e n t an d d e v i ce t e m p er a t u r es, T in t h e seco n d f or m i s in^{\circ} C, T e v er o v s k y (s l i d es) : t h e e f f ec t i v e h u mi d i t y i s co m p u t e d f r o m t h e s a t u r a t e d v a p o u r p r ess u r es a t t h e d e v i ce an d ambi e n t t e m p er a t u r es, t h e A r d e n B u c k f or m o f p_{s a t} i s ba c k g r o u n d k n o w l e d g e an d i s v a l i d o n l y f or m o d er a t e t e m p er a t u r es (u p t o ab o u t 50^{\circ} C in i t s or i g ina l r an g e), an d a t 130^{\circ} C in H A S T t h e c hamb er i s p r ess u r i se d so t h e c hamb er h u mi d i t y i s t h e s t a t e d o n e$ $P ec k : t es t in g a t 100 p er ce n t R H ha s o n l y a ma r g ina l in f l u e n ce o n hi s d a t a an d h e s ug g es t s a v o i d in g i t t o o pt imi se t h e e x t r a p o l a t i o n t o l o w s t r ess$ $(co m p u t e d w i t h t h e B u c k f or m, s t r e t c h e d p a s t i t s s t a t e d r an g e : 85 p er ce n t a t 85^{\circ} C f a l l s t o 58 p er ce n t a t a d e v i ce 10 K w a r m er; p_{s a t} = 2.34 k P a a t 20^{\circ} C, 57.9 k P a a t 85^{\circ} C)$

Corrosion, Migration and Contamination

Name	Formula	Related Formulae	Variables/Constants
Corrosion depth from the corrosion current density	$\frac{d h}{d t} = \frac{M j _{cor r}}{z F ρ}$	$m = \frac{M I t}{z F}$ $E_{ce l l} = E_{c a t h o d e} - E_{an o d e}$	$h : t hi c k n ess o f m e t a l l os t$ $t : t im e$ $M : m o l a r ma ss o f t h e m e t a l, 26.98 g/mol f or a l u mini u m$ $j_{cor r} : cor r os i o n c u r r e n t d e n s i t y$ $z : c ha r g e n u mb er, 3 f or A l$ $F : F a r a d a y co n s t an t 96485 C/mol$ $ρ : d e n s i t y, 2.70 g/c m^{3} f or A l$ $m : ma ss l os t$ $I : c u r r e n t$ $E_{ce l l} : ce l l v o l t a g e o f a g a l v ani c co u pl e (M, z, F an d ρ a r e ba c k g r o u n d k n o w l e d g e), K u ma r e t a l . (p H 5, imm er s i o n, 5 pp m o f t h e i o n, A l w i t h 0.5 w t p er ce n t C u) : T a f e l ana l y s i s o n b l ank e t A l g i v es E_{cor r} = - 0.37 V an d j_{cor r} = 1.8 nA/c m^{2} f or c h l or i d e, an d - 0.62 V an d 16.9 nA/c m^{2} f or f l u or i d e, t h e C u / A l s t an d a r d p o t e n t ia l d i f f er e n ce i s c i t e d a s a l m os t 2.0 V, t h e g a l v ani c c u r r e n t i s ab o u t 0.7 μ A f or c h l or i d e (o n se t a f t er 5 min) an d ab o u t 4 μ A a t i t s p e ak f or f l u or i d e, AlC l_{3} d i sso l v es a t 460 g/L a t 30^{\circ} C an d h y d r o l y ses t o Al (OH)_{3} an d H^{+}, w hi l e Al F_{3} d i sso l v es a t o n l y 0.0053 g/L a t 20^{\circ} C$ $(co m p u t e d : 19.6 nm/yr f or c h l or i d e an d 184 nm/yr f or f l u or i d e, a r a t i o o f 9.4 e q u a l t o t h e r a t i o o f t h e c u r r e n t d e n s i t i es; t hi s t r e a t s t h e a tt a c k a s u ni f or m, w hi c h t h e so u r ce d oes n o t, s in ce f l u or i d e g a v e g ian t p i t s, so i t i s a l o w er b o u n d o n t h e l oc a l d e pt h (m y r e a d in g))$
Dendrite growth and time to failure (Hornung and DiGiacomo)	$t_{f} = α \frac{G}{V} e^{Δ H / (k_{B} T)}$	$t_{f} = \frac{Q _{c}}{β J _{t i p}}$ $t_{f} = A_{0} RH^{- n} f (V) e^{Δ H / (k_{B} T)}$	$t_{f} : t im e t o f ai l u r e b y a d e n d r i t e t ha t b r i d g es t h e g a p$ $α : co n s t an t f o u n d b y e x p er im e n t$ $G : e l ec t r o d e s p a c in g$ $V : a ppl i e d v o l t a g e$ $Δ H : a c t i v a t i o n e n er g y$ $k_{B} : B o l t z mann co n s t an t 8.617 \times 1 0^{- 5} eV/K$ $T : ab so l u t e t e m p er a t u r e$ $Q_{c} : cr i t i c a l am o u n t o f m e t a l i o n s f or a b r i d g e$ $β : d e g r ee o f o x i d a t i o n or f r a c t i o n o f a c t i v e s u r f a ce$ $J_{t i p} : c u r r e n t d e n s i t y a t t h e t i p o f t h e d e n d r i t e$ $A_{0} : co n s t an t$ $R H : r e l a t i v e h u mi d i t y in p er ce n t$ $n : h u mi d i t y e x p o n e n t, f (V) : v o l t a g e f u n c t i o n, B u mi l l er an d H i l l man (e q u a t i o n r eco n s t r u c t e d f r o m a g a r b l e d e x t r a c t i o n) : H or n u n g m e a s u r e d Δ H = 1.15 pl u s or min u s 0.15 eV f or s i l v er in b or os i l i c a t e g l a ss, hi s m o d e l co n t ain s n e i t h er h u mi d i t y n or co n t amina t i o n, t h e P ec k f or m ha s n = 2.66 (l a t er ab o u t 3) w i t h f (V) u n d e f in e d an d a p o w er l a w w i t h an e x p o n e n t o f ab o u t 1.5 c i t e d, t h e bia s m u s t e x cee d ab o u t 2 V an d t h e u pp er l imi t i s s p ec u l a t e d t o b e n e a r 100 V, t h e e x am pl e s p a c in g s a r e 120 μ m f or f l i p - c hi p b u m p s, 170 μ m f or T S S O P l e a d s, 100 μ m f or e x t er na l P C B t r a ces an d 48 μ m f or B G A s u b s t r a t e t r a ces, an d t h e a u t h or s s a y t ha t an A r r h e ni u s d e p e n d e n ce o f t h e ini t ia t i o n ha s n o t b ee n co n c l u s i v e l y s h o w n, H e (2014) : co mb s t r u c t u r es a t 65^{\circ} C an d 88 p er ce n t R H f or o v er 1000 h w i t h 25 a g ain s t 12.5 mi l s p a c in g an d 40 a g ain s t 5 V, so l d er a l l oy ha d a l a r g er e f f ec t t han t h e b o a r d f ini s h an d a so l d er ma s k r e d u ce d t h e c han ce o f mi g r a t i o n$ $(co m p u t e d : w i t h 1.15 eV coo l in g f r o m 85 t o 65^{\circ} C l e n g t h e n s t h e t im e b y 9.1, an d b y 6.8 t o 12.1 f or 1.0 t o 1.3 eV; a s p a c in g o f 25 mi l a t 40 V a g ain s t 12.5 mi l a t 5 V g i v es a r a t i o o f G / V o f 0.25, so t h e m o d e l p r e d i c t s a f o u r t im es s h or t er t im e a t t h e w i d e, hi g h - v o l t a g e co n d i t i o n, w hi c h i s a m o d e l s t a t e m e n t an d n o t a r es u l t o f t h e t h es i s)$
Ionic contamination limit and unit conversion	$c_{A} = \frac{m _{i o n}}{A _{e x t r a c t e d}} \leq c_{l im}, 1 μ g/i n^{2} = 0.155 μ g/c m^{2}$	$c_{l im} (μ g/c m^{2}) = \frac{c _{l im} ( μ g/i n ^{2} )}{6.4516}$	$c_{A} : a r e a l d e n s i t y o f t h e e x t r a c t e d i o ni c r es i d u e$ $m_{i o n} : ma ss o f i o n f o u n d b y e x t r a c t i o n$ $A_{e x t r a c t e d} : a r e a t ha t w a s w a s h e d$ $c_{l im} : l imi t, B u mi l l er an d H i l l man (T ab l e 2) : c h l or i d e 6.15 μ g/i n^{2} an d b r o mi d e 7.87 μ g/i n^{2} (I P C), an d c h l or i d e 10 μ g/i n^{2} (A C I), t h e ha l i d es c h l or i d e an d b r o mi d e a r e t h e m os t ha r m f u l$ $W a c k : a n or ma l v a l u e i s b e l o w 0.4 μ g/c m^{2}, co n t amina t i o n i s n o t d e t ec t ab l e b y i o ni c m e a s u r e m e n t s a l o n e, a c l e an s u r f a ce ha s a s u r f a ce t e n s i o n ab o v e 40 m N / m, an d t h e c l ima t i c t es t o f I E C 68 - 2 i s 40^{\circ} C, 93 p er ce n t R H f or 21 d a y s$ $(co m p u t e d : 6.15, 7.87 an d 10 μ g/i n^{2} a r e 0.953, 1.22 an d 1.55 μ g/c m^{2}, an d 0.4 μ g/c m^{2} i s 2.58 μ g/i n^{2}; o n e s q u a r e in c h i s 6.4516 s q u a r e ce n t im e t r es, ba c k g r o u n d k n o w l e d g e)$

Creep Corrosion and Tin Whiskers

Name	Formula	Related Formulae	Variables/Constants
Corrosion severity class and film growth rate (ISA 71.04)	$r (nm/yr) = 0.1 \cdot \frac{365}{30} \cdot d_{30}$	$d_{30} < 300 (Cu), < 200 (Ag) \Rightarrow G1$	$r : f i l m g r o w t h r a t e o n a co u p o n$ $d_{30} : f i l m t hi c k n ess g r o w n in 30 d a y s in an g s t r o m, P u r a f i l b u l l e t in C C C - 3 (A N S I / I S A - 71.04 - 2013) : t h e c l a sses a r e G 1 mi l d (co pp er b e l o w 300, s i l v er b e l o w 200), G 2 m o d er a t e (b e l o w 1000 e a c h), G 3 ha r s h (b e l o w 2000 e a c h) an d GX se v er e (ab o v e 2000 e a c h), a l l in an g s t r o m p er 30 d a y s, t h e t e x t o f t h e b u l l e t in d e f in es G 1 a s 0 t o 299 f or co pp er an d 0 t o 199 f or s i l v er, h y d r o g e n s u l f i d e an d s u l f u r d i o x i d e a r e nam e d b u t n o co n ce n t r a t i o n s a r e g i v e n, S a v o l ain e n an d S c h u e l l er : cr ee p cor r os i o n p r o d u c t i s main l y C u_{2} S w i t h a l i ttl e A g_{2} S, i t n ee d s m o i s t u r e an d a cor r os i v e e l e m e n t b u t n o e l ec t r i c f i e l d, an d s u l f u r i s t h e m os t p r o min e n t e l e m e n t$ $i N E M I : I m A g an d O S P b o a r d s cr e pt o n l y a t 15 t o 50 p er ce n t R H, E N I G a t b o t h l o w an d hi g h R H, in a c hamb er t ha t h o l d s 15 t o 80 p er ce n t R H, t h e co n s t r u c t i o n o f t h e f or m u l a i s min e$ $(co m p u t e d : 300, 1000 an d 2000 an g s t r o m p er 30 d a y s a r e 365, 1217 an d 2433 nm/yr, an d 200 i s 243 nm/yr)$
Tin whisker length from an image and growth rate	$L_{im g} = L cos α, ε = 1 - cos α$	$\overset{v}{ˉ} = \frac{L _{2} - L _{1}}{t _{2} - t _{1}}$	$L_{im g} : l e n g t h m e a s u r e d in o n e ima g e$ $L : t r u e l e n g t h$ $α : an g l e b e tw ee n t h e w hi s k er an d t h e ima g e pl an e$ $ε : r e l a t i v e er r or o f t h e l e n g t h$ $ˉ v : m e an g r o w t h r a t e$ $L_{1} an d L_{2} : l e n g t h s a t t h e t im es t_{1} an d t_{2} (t h e cos in e f or m i s m y r e a d in g o f t h e so u r c e^{'} s er r or f or m u l a), P ana s h c h e nk o : s in g l e - ima g e er r or i s (1 - cos α) t im es 100 p er ce n t, t h e tw o - ima g e t i l t m e t h o d g a v e ab o u t 7 pl u s or min u s 3 p er ce n t a g ain s t 20 pl u s or min u s 11 p er ce n t f or t i l t in g an d a l i g nin g in t h e S E M, w hi s k er s g r o w b y a dd in g a t o m s a t t h e ba se, l e n g t h s r an g e f r o m a f e w mi cr o m e t r es t o mi l l im e t r es, w i t h t hi c k n esses o f 0.5 t o 10 μ m, d e n s i t i es f r o m a f e w t o t h o u s an d s p er co m p o n e n t, an e x am pl e o f 18 μ m a t 5 m o n t h s an d 31 μ m a t 9 t o 12 m o n t h s, an d a f t er 2 y e a r s o f t e m p er a t u r e cy c l in g an a v er a g e l e n g t h o f 125 μ m w i t h a ma x im u m o f 4143 μ m, t h e t es t co n d i t i o n s o f I E C 60068 - 2 - 82, J E S D 22 - A 121 A an d E T - 7410 a r e ab o u t 30^{\circ} C /60 p er ce n t R H or 25^{\circ} C /55 p er ce n t R H f or 4000 h, 55^{\circ} C /85 p er ce n t R H f or 2000 h an d t e m p er a t u r e cy c l in g f or 1000 or 2000 cy c l es$ $(co m p u t e d : ε = 1.5, 6.0, 13.4 an d 29.3 p er ce n t f or 10, 20, 30 an d 45 d e g r ees; t h e m e an g r o w t h r a t e i s 3.6 μ m/month f r o m z er o t o 5 m o n t h s, a ss u min g n o in c u ba t i o n, an d 2.6 μ m/month f r o m 5 t o 10 m o n t h s, a ss u min g t h e 9 t o 12 m o n t h p o in t i s a t 10 m o n t h s)$

Important Questions

Ques: Which constants should the Peck model use?

Peck’s 1986 paper gives $n = 2.66$ and $E_{a} = 0.79$ $eV$ for aluminium corrosion in epoxy packages, and calls 2.50 with 0.81 and 3.00 with 0.77 nearly equivalent. Teverovsky uses 2.7 and 0.8 $eV$ , Schenkelberg lists the Peck values as 2.7 and 0.79 $eV$ , and he also gives 3 and 0.9 $eV$ from Condra. From 85/85 to 30 $^{\circ} C$ and 20 per cent RH the acceleration factor is 4880 for 2.66 and 0.79, 5483 for 2.7 and 0.8, and 15236 for 3 and 0.9 (computed), which is 3.1 times more. The paper’s own pair is more likely the right one for aluminium corrosion, since it gives the paper’s 4900. The Condra pair predicts three times more life from the same test, so it is the less conservative choice (my reading). Reference: D. S. Peck, ‘Comprehensive Model for Humidity Testing Correlation’, IEEE International Reliability Physics Symposium, 1986 (fetch-tool summary of an OCR scan; some values garbled); A. Teverovsky, ‘Moisture Diffusion in Molding Compounds and Quality Assurance of PEMs for Space Applications’, MRQW slides, December 2002 (fetch-tool summary; the model slide is garbled); F. Schenkelberg, ‘Temperature and Humidity Accelerated Life Testing’, Accendo Reliability, 29 May 2017 (modified 2026), including reader comments (fetch-tool summary).

Ques: Is 96 hours of HAST at 130 degrees equal to 1000 hours at 85/85?

Teverovsky says the 130 $^{\circ} C$ and 85 per cent RH condition is equivalent to about 1000 h at 85/85, a ratio of 10.4 for the 96 h test (computed). Peck gives roughly 20 to 1. At equal humidity the Arrhenius factor between 85 and 130 $^{\circ} C$ is 17.4 for 0.79 $eV$ , 12.6 for 0.7 and 53 for 1.1 (computed), which makes 96 h equal to 1207 to 5129 h. The 1000 h figure needs an activation energy of only 0.65 $eV$ , below the 0.7 to 1.1 $eV$ range in the same slides, and 20 to 1 needs 0.83 $eV$ (computed). Peck’s ratio is more likely to be right for aluminium corrosion, and 1000 h is more likely a round, conservative number (my reading). Reference: A. Teverovsky, ‘Moisture Diffusion in Molding Compounds and Quality Assurance of PEMs for Space Applications’, MRQW slides, December 2002 (fetch-tool summary; the model slide is garbled); D. S. Peck, ‘Comprehensive Model for Humidity Testing Correlation’, IEEE International Reliability Physics Symposium, 1986 (fetch-tool summary of an OCR scan; some values garbled).

Ques: What is the diffusion coefficient of a molding compound at 85 degrees?

The sources disagree. The Teverovsky fit $D = D_{0} e^{- U / (k_{B} T)}$ with $D_{0} = 7.35 \times 1 0^{- 6}$ $m^{2} /s$ and $U = 0.43$ $eV$ gives $6.5 \times 1 0^{- 8}$ $c m^{2} /s$ at 85 $^{\circ} C$ (computed). His measured QFP-144 value is $1.2 \times 1 0^{- 7}$ and the PLCC-32 value is $3.2 \times 1 0^{- 7}$ $c m^{2} /s$ . Han et al. print $6.2 \times 1 0^{- 9}$ $m^{2} /h$ in a table for a molding compound, which is $1.7 \times 1 0^{- 8}$ $c m^{2} /s$ (computed), and their results section prints other units. The fit is 3.8 times the table value, the QFP 7.0 times and the PLCC 18.6 times (computed). The compounds differ, and the room-temperature value alone varies by more than an order of magnitude, so the values need not conflict. The Teverovsky values are more likely for a general package, since three of them lie within a factor of five, and the single table entry carries a unit conflict. Reference: A. Teverovsky, ‘Moisture Characteristics of Molding Compounds in PEMs’, QSS Group at NASA Goddard, NEPP report (date not stated, sources to 2002) (fetch-tool summary); Han, Lim, Bae, Hwang, Lee, Oterkus, Madenci, Diyaroglu and Oterkus, ‘Equivalent Acceleration Assessment of JEDEC Moisture Sensitivity Levels using Peridynamics’ (venue and year not stated, ECTC 2015 by the file name) (fetch-tool summary).

Ques: Is the soak time for a moisture sensitivity level set by weight gain or by vapour pressure?

It depends on the criterion. For level 2 in a DDP package (the source summary does not expand the acronym) Han et al. find equivalent times at 85/85 of 5 h by weight gain, 6.5 h by local moisture concentration and 2 h by vapour pressure. The vapour pressure at the interface reached about 2.3 $MPa$ at reflow after 2 h, while the local concentration saturated only after about 20 h. With an assumed 260 $^{\circ} C$ peak and 4.69 $MPa$ for saturated steam, the 2.3 $MPa$ is 0.49 of saturation (computed). Teverovsky takes 168 h at 85 $^{\circ} C$ as enough to saturate packages. Because the pressure is what cracks the package (National Semiconductor), the vapour pressure is more likely the right criterion for cracking, and the weight gain is the easier one to measure (my reading). Reference: Han, Lim, Bae, Hwang, Lee, Oterkus, Madenci, Diyaroglu and Oterkus, ‘Equivalent Acceleration Assessment of JEDEC Moisture Sensitivity Levels using Peridynamics’ (venue and year not stated, ECTC 2015 by the file name) (fetch-tool summary); A. Teverovsky, ‘Moisture Characteristics of Molding Compounds in PEMs’, QSS Group at NASA Goddard, NEPP report (date not stated, sources to 2002) (fetch-tool summary); National Semiconductor, ‘Plastic Package Moisture-Induced Cracking’, literature number ZHCA399, April 2006 (fetch-tool summary).

Ques: Which ionic contamination limit should a board meet?

The sources give different numbers. Bumiller and Hillman list 6.15 $μ g/i n^{2}$ for chloride and 7.87 $μ g/i n^{2}$ for bromide from IPC, and 10 $μ g/i n^{2}$ for chloride from ACI. Wack gives a normal value of below 0.4 $μ g/c m^{2}$ . In common units the first three are 0.95, 1.22 and 1.55 $μ g/c m^{2}$ , and 0.4 $μ g/c m^{2}$ is 2.58 $μ g/i n^{2}$ (computed). Wack’s value is 2.4 times lower than the IPC chloride limit and 3.9 times lower than the ACI one (computed). The summaries do not say whether the quantities are the same, and the 0.4 value may be a total of all ions, so a direct ranking is not safe. The lower value is the safer one to design to (my reading), and Wack warns that ionic measurements alone do not find all contamination. Reference: Bumiller and Hillman, ‘A Review of Models for Time-to-Failure Due to Metallic Migration Mechanisms’, DfR Solutions white paper (year not stated) (fetch-tool summary; equations reconstructed from a garbled extraction); Wack (ZESTRON America), ‘A Closer Look at Why Cleaning Prior to Conformal Coating Becomes Key in Aspects of Climatic Reliability’ (venue and year not stated) (fetch-tool summary).

Ques: How thick must the water film be for electrochemical migration?

Wack says that initiation can occur with only a few monolayers of moisture, and that the critical level is 60 to 70 per cent RH at ambient temperature. Bumiller and Hillman report a critical film of about 20 monolayers, and a path that forms almost instantly above about 70 per cent RH. The two are compatible on RH, 60 to 70 per cent against above 70 per cent. On the film, the few monolayers are more likely for the start of dissolution, and about 20 for a continuous film that carries the ions across a gap (my reading), since the second source also says that higher humidity speeds the initiation. Neither source gives a measurement method in its summary. Both agree that contamination and bias are also needed. Reference: Wack (ZESTRON America), ‘A Closer Look at Why Cleaning Prior to Conformal Coating Becomes Key in Aspects of Climatic Reliability’ (venue and year not stated) (fetch-tool summary); Bumiller and Hillman, ‘A Review of Models for Time-to-Failure Due to Metallic Migration Mechanisms’, DfR Solutions white paper (year not stated) (fetch-tool summary; equations reconstructed from a garbled extraction).

Ques: At what humidity does creep corrosion occur?

The two sources seem to disagree. Savolainen and Schueller say that it takes only about 50 per cent RH to form a thin water film, and that the resistance of the product fell from over 10 $MΩ$ to below 1 $MΩ$ when the humidity rose. The iNEMI work found creep corrosion on OSP and immersion silver boards only between 15 and 50 per cent RH, and on ENIG at both low and high RH. The iNEMI result is more likely for the growth, since it comes from a chamber with controlled humidity and gases. The 50 per cent figure is more likely a statement about the water film. Both fit if the corrosion grows at low humidity and the conduction through the product, which is what shorts the board, needs high humidity (my reading). Reference: Savolainen and Schueller, ‘Creep corrosion of electronic assemblies in harsh environments’, DfR Solutions (venue and year not stated, about 2007 or later) (fetch-tool summary); ‘Relative humidity dependence of creep corrosion on printed circuit boards’, iNEMI subcommittee work, 2017, DOI 10.23919/ICEP.2017.7939321 (abstract only; authors not listed on the page).

Ques: Does the definition of whisker length change the result?

It can. JESD22-A121 of 2005 measured the distance from the surface to the tip if the whisker were straight and perpendicular, while JESD201 of 2006, JESD22-A121A and IEC 60068-2-82 use the straight-line distance from the emergence point to the farthest point. The earlier test also used 60 $^{\circ} C$ and 87 per cent RH, where the later ones use 55 $^{\circ} C$ and 85 per cent RH. A bent whisker is longer under the earlier definition (my reading). A single image also loses length, since the relative error is $1 - cos α$ , which is 1.5, 6.0, 13.4 and 29.3 per cent for 10, 20, 30 and 45 degrees (computed). The two-image tilt method gave about 7 per cent error against 20 per cent for aligning in the SEM. The later definition and the tilt method are more likely to be the ones to use today. Reference: L. Panashchenko, ‘Evaluation of Environmental Tests for Tin Whisker Assessment’, MS thesis, University of Maryland CALCE, December 2009, slides of 20 January 2010 (fetch-tool summary).

Ques: Is chloride or fluoride the worse contaminant for an aluminium pad?

It depends on the copper. On blanket aluminium the fluoride corrosion current density was 16.9 $nA/c m^{2}$ against 1.8 for chloride, a ratio of 9.4, with giant pits at 24 h for fluoride and only minor pitting for chloride. With a copper wire on an aluminium pad, chloride failed the device in about 2 h, while fluoride corroded the pad fully in about 3 h and lifted the wire in about 27 h. The copper makes chloride about as fast as fluoride (my reading), and the authors link the hydrogen from the galvanic couple to cracks. The tests used immersion at pH 5 and 5 ppm, not a molding compound, which the authors say has ion traps, so absolute times are not field times. Fluoride is more likely the worse where there is no copper contact (my reading). Reference: Kumar, Lambert, Caperton, Asokan, Yi and Chyan, ‘Comparative Study of Chloride and Fluoride Induced Aluminum Pad Corrosion in Wire-Bonded Device Packaging Assembly’, Corrosion and Materials Degradation 2, 447-460 (2021) (fetch-tool summary).

Ques: Is HAST at 130 degrees a safe stand-in for 85/85?

Not always. Teverovsky notes that 130 $^{\circ} C$ is above the operating range of many parts, and that the compound can lose its glass transition temperature, creep more and lose adhesion. In a linear-device test at 130 $^{\circ} C$ , 85 per cent RH and 250 h, 11 of 25 part types failed. Moisture-assisted hot-carrier degradation may have a negative activation energy, so HAST may not accelerate it. Peck says that tests should be made on either side of the glass transition and that results around 150 $^{\circ} C$ and above were anomalous. HAST is therefore more likely to be valid for corrosion with an activation energy of about 0.8 $eV$ , and less likely for mechanisms that change at high temperature (my reading). The failure mechanism should be confirmed before the acceleration factor is used. Reference: A. Teverovsky, ‘Moisture Diffusion in Molding Compounds and Quality Assurance of PEMs for Space Applications’, MRQW slides, December 2002 (fetch-tool summary; the model slide is garbled); D. S. Peck, ‘Comprehensive Model for Humidity Testing Correlation’, IEEE International Reliability Physics Symposium, 1986 (fetch-tool summary of an OCR scan; some values garbled).

Ques: Which model describes dendrite growth?

None describes all the factors. Hornung’s form, $t_{f} = α (G / V) e^{Δ H / (k_{B} T)}$ , has spacing, voltage and temperature, and no humidity or contamination. The Peck form has humidity, with n = 2.66 or later about 3, and leaves the voltage function undefined. DiGiacomo’s form, $t_{f} = Q_{c} / (β J_{t i p})$ , has the critical ion amount but needs the tip current density. Bumiller and Hillman say that an Arrhenius dependence of the initiation has not been conclusively shown, so the 1.15 $eV$ of Hornung, measured for silver in glass, should not be taken to a board. For a board, humidity looks more like a threshold, since a film of about 20 monolayers is needed, than a power law (my reading). The models are more likely to be useful for ranking spacing and voltage than for a lifetime. Reference: Bumiller and Hillman, ‘A Review of Models for Time-to-Failure Due to Metallic Migration Mechanisms’, DfR Solutions white paper (year not stated) (fetch-tool summary; equations reconstructed from a garbled extraction); X. He, ‘Evaluation and Modeling of Electrochemical Migration on Printed Circuit Boards’, PhD thesis, University of Maryland, 2014 (abstract only).

Quartz 5

Explorer

Environmental and Chemical