Moisture Characteristics of Molding Compounds and Environmental Effects

Moisture Characteristics of Molding Compounds and Environmental Effects in PEMs Alexander Teverovsky QSS Group/Goddard Operations Alexander. A. Teverovsky. 1@gsfc. nasa. gov

Moisture effects in PEMs Die level: § § Corrosion; Dendrites between metallization lines; Leakage current; Charge instability (lateral, ion drift, hot electron). Package level § Corrosion of leads; § Popcorning; § Dendrite formation (on the surface and inside packages); § Swelling/shrinkage: ü underfills in PBGA and flip-chip technology; ü warpage of large packages; ü parametric shifts in linear devices. 11/21/02 web presentation 2

No humidity in space. Why moisture concerns? An obvious reason: We need to assure that no moisture related failures and no degradation occur during ground phase integration and testing period (2 to 5 years maximum). Not so obvious: We need to be aware of possible effects caused by moisture desorption in space. 11/21/02 web presentation 3

Quality assurance strategy for PEMs Moisture prevention Adequate qualification testing (do we need to use the same environmental testing as for Navy applications? ) Moisture prevention strategy. Military applications: Wafer Applied Seal for PEM Protection (WASPP); Space applications: monitoring of the moisture level and baking of component and assemblies. 11/21/02 web presentation 4

Purpose and Outline To discuss: Moisture diffusion characteristics of MCs: how they can be measured and used for implementation of the moisture prevention strategy. Outline of today’s presentation: • Bake-out conditions for PEMs; • Diffusion characteristics of MCs; • In-situ technique and results of D(T) measurements. 11/21/02 web presentation 5

Future presentations • Do we need HAST? (Is HAST adequate to normal conditions? What are the acceleration factors? What are failure mechanisms and modes? What are possible alternatives to HAST) • Environmentally induced swelling and shrinkage of molding compounds in PEMs. (Technique for swelling and shrinkage measurements; swelling characteristics of different MCs; sorption isotherms; effects of high temperature baking conditions; mechanisms of swelling and shrinkage. ) • Environmental hysteresis in precision voltage reference PEMs. (Effect of external mechanical stresses; effects of moisture induced swelling and bake induced shrinkage; hysteresis due to low and high temperature exposure; relaxation of parametric shifts. ) • Effect of moisture on characteristics, qualification testing, and reliability of chip solid tantalum capacitors. 11/21/02 web presentation 6

Master curve for moisture diffusion At t = t C/Co=0. 1, M/Mo=0. 06 The first step in any MC or PEM degradation process is moisture diffusion => the characteristic times of diffusion are important for implementing the moisture prevention strategy. Bake-out time: t(T) = h 2/D(T) 11/21/02 t Kinetics of water molecules concentration at the die surface and mass losses during baking of a flat package saturated with moisture. web presentation 7

Diffusion characteristics of epoxy encapsulating materials Data reported in literature D 85 varies ~ 10 times Averaged characteristics of MC: Do = 7. 35 10 -6 m 2/sec U = 0. 43 e. V 11/21/02 web presentation 8

Calculated moisture bake-out times for packages of different shapes and sizes 11/21/02 web presentation 9

Comparison of the calculated bake-out conditions with JEDEC recommendations Package Type DIP-24 Thickness mm Bake Temperature 40 C 90 C 125 C 3. 8 1996 (1608 -1608) 206 (168 -240) 59 (48 -48) DIP-8 3. 2 1416 (1608 -1608) 146 (168 -240) 41 (48 -48) PQFP-44 2 553 (528 -1608) 57 (48 -144) 16 (16 -40) PLCC-32 3 1244 (1608 -1608) 128 (168 -240) 36 (48 -48) TSOP-32 1 138 (120 -240) 14 (11 -24) 4 (3 -10) *Data in brackets are J-STD-033 ‘ 02 recommendations 11/21/02 web presentation 10

Calculated bake times at 125 o. C and JEDEC recommendations Three body thickness groups per IPC/JEDEC J-STD-033 A, July 2002: <1. 4 mm; <2 mm; < 4. 5 mm Note: • 2 a and 5 a are levels of moisture sensitivity. • part saturated at 30 o. C/85% RH 11/21/02 web presentation • Ignoring real size of the parts might cause significant errors. • JEDEC bake recommendations are focused on SMT solder reflow process and might be not adequate for moisture control purposes. 11

D(T) technique. Conventional (isothermal) measurements: Time domain T = Ti d. Msat is necessary for calculations duration of test 11/21/02 web presentation + Next i 12

Non-isothermal technique: Temperature domain Preconditioning: 85 o. C/85%RH/168 hrs Equipment: TGA or T - chamber and balance. Linear temperature increase 11/21/02 web presentation 13

Example: QFP-144 package Weighting is performed after fast cooling to RT. 11/21/02 web presentation 14

Diffusion coefficients calculated using different techniques Sorption/desorption kinetics at 85 o. C D 85 (cm 2/s) at 85 o. C Pack. Isotherm. Isot. Mater. sorption desorp. Different techniques agree within 30% error 11/21/02 Non. Variatio isot. n, % desorp. QFP 144 7. 1 E-08 6. 3 E-08 7. 7 E-08 10 MC 4. 4 E-08 4. 6 E-08 6. 8 E-08 25. 3 DIP 28 5. 8 E-08 6. 2 E-08 8. 8 E-08 23. 5 web presentation 15

D(T) measurement results Mfr. : Actel V 3 Semi AMD XILINX Package: QFP 144 QFP 160 PLCC 32 QFP 240 2. 09 0. 253 0. 011 0. 081 D 0, cm 2/s 0. 5 0. 45 0. 32 0. 41 U, e. V 2. 4 E-9 3. 2 E-8 4. 5 E-9 D 20, cm 2/s 3 E-9 3. 2 E-7 9. 4 E-8 D 85, cm 2/s 1. 2 E-7 6. 9 E-8 1 E-6 4. 3 E-7 D 130, cm 2/s 7. 8 E-7 3. 7 E-7 Literature data: 0. 32 < U < 0. 52; 8 E-9 < D 85 < 2 E-7 11/21/02 web presentation 16

Effect of Lead Frame Delamination CSAM resolution: ~ 100 nm Water molecule ~ 0. 3 nm - delamination coefficient; k = SLF/SP, LF size factor. k ~ 0. 5; 0< < 1 at = 1 Dpac = 2. 25 DMC An increase in Deff indicates severe delaminations. 11/21/02 web presentation 17

Sorption isotherm d. M = F(RH, T) Henry’s law: - sorption coefficient; P -water vapor pressure; Ps -saturated water pressure; f - relative humidity Test conditions: 168 hrs at 85 o. C at each RH 11/21/02 web presentation Moisture uptake is a linear function of relative humidity 18

d. M = F(RH, T) Sorption at 85 % RH and T from 70 o. C to 130 o. C Sorption coefficient : = o exp( H/k. T) H - the heat of moisture solution. Ps=Po exp(-Q/k. T) Q - the heat of water vaporization (~0. 42 e. V). Test conditions: 168 hr to 72 hr at 85% RH at each temperature 11/21/02 web presentation Moisture uptake does not depend significantly on temperature 19

Do we really need the moisture prevention strategy? Burn-in testing at 85 o. C. 1 month storage at 25 o. C and 70% RH Will storage of the parts at normal laboratory conditions affect BI testing results? Note: C/Co = 1 corresponds to the equilibrium moisture saturation at 100% RH. C(t) calculations for an initially dry part (2 mm) stored one month at laboratory conditions 11/21/02 web presentation 20

Summary A moisture prevention strategy, which includes monitoring of moisture content and adequate baking of plastic parts and assemblies, is suggested. This strategy can be implemented by calculations of the characteristic times of moisture diffusion and bake-out conditions. Software for simulation of the evolution of moisture content and recommendations for baking can be developed. JEDEC recommendations for baking are in agreement with calculations based on the averaged D(T) characteristics. However, the calculation provides much more flexibility and accuracy. A technique for in-situ measurements of temperature dependence of moisture diffusion characteristics has been demonstrated. 11/21/02 web presentation 21