# Presentation: 670501

## Moisture Diffusion in Molding Compounds and Quality Assurance of PEMs for Space Applications

- Alexander Teverovsky
- QSS Group, Inc./Goddard Operations
- Alexander.A.Teverovsky.1@gsfc.nasa.gov

## No humidity in space. Why moisture concerns?

**We need to assure that no moisture related failures occur during the ground phase integration and testing period (2 to 5 years).**

**Quality assurance strategy for PEMs:**
**Moisture prevention**
**Adequate qualification testing**

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## Quality assurance strategy for PEMs for military and space applications

_**Moisture prevention strategy:**_
-  **Military applications:** Wafer Applied Seal for PEM Protection (WASPP)**;**
**Space applications**: Virtual monitoring/simulation of the moisture level variations and baking of components and assemblies.

_**Adequate testing:**_
-  **Military applications**: To assure reliability for 15 to 20 years of storage and operation in harsh humid environments.
**Space applications**: To assure reliability for 5 years maximum in controlled laboratory conditions.

- MRQW  Dec '02

## Purpose and Outline

_**The  purpose is**__ __**to discuss:**_
- Moisture characteristics of MCs and how they can be used for the moisture prevention strategy and for development of adequate testing.
- The relevance of HAST for PEMs intended for space applications.
_**Outline:**_
**Part I:**
- Bake-out conditions for PEMs;
- Diffusion characteristics of molding compounds .
**Part II:**
- Acceleration factors of HAST.
- What might be an adequate testing?

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## Part I.  Moisture Diffusion Characteristics and Their Measurement

**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.**

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## Characteristic times of moisture diffusion in plastic encapsulated parts 

- Moisture concentration at the die surface and mass losses of a flat package with rated baking time.

- At t = C/Co=0.1, M/Mo=0.06

- 

**Bake-out time:**** ********(T) = h****2****/D(T)**

- 2h – package thickness

- The master curve for moisture diffusion

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## Diffusion characteristics of epoxy encapsulating materials 

- Averaged characteristics of MC:
- Do = 7.3510-6 m2/sec
- U = 0.43 eV

**Data reported in literature.**

- D85 varies ~ 10 times

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## Calculated bake times at 125 oC and JEDEC recommendations

**Three body thickness groups per IPC/JEDEC J-STD-033A, July 2002:**** <1.4 mm; <2 mm;< 4.5 mm**

**Ignoring real size****  of the parts****  causes significant****  errors.**
**JEDEC is focused****  on SMT reflow****  soldering and****  might be not****  adequate for****  moisture control****  purposes.**

_**Note:**_** **
**2a and 5a are levels****   of moisture sensitivity.**
**part saturated at****   30 ****o****C/85% RH**

- IPC-SM-786A, ’95: < >2 mm

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## In-situ non-isothermal technique for D(T) measurements

_**Areas of application**_
- Moisture sorption simulation (virtual monitoring of moisture level).
- Calculation of bake-out conditions.
- Lot characterization of molding compound.(ROBOCOTS: need rapid assessment methods)
- Evaluation of moisture leaks along the leads of a plastic package.
- Development of adequate moisture stress testing (HAST alternative).

- A. Teverovsky, “A Rapid Technique for Moisture Diffusion Characterization of Molding Compounds in PEMs“, http://nepp.nasa.gov

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## Part II.  Highly Accelerated Stress Test (HAST)

**Do we need to use same environmental stress testing as for PEMs intended for harsh humid conditions?**

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## Accelerated moisture resistance tests

**JESD22-A102-C (unbiased autoclave):**    121 oC, 100% RH, 24 to 336 hrs (96 hr typical).
**JESD22-A118 (unbiased HAST):**    Cond. A: 130 oC, 85% RH, 96 hrs.   Cond. B: 110 oC, 85% RH, 264 hrs.
**JESD22-A110-B (biased HAST): **   Cond. A: 130 oC, 85% RH, 96 hrs.   Cond. B: 110 oC, 85% RH, 264 hrs.

_**Typical HAST conditions: bias at 130 **__**o**__**C, 85% RH**_
- 96 hrs per JESD22-A110-B.
- 150 hrs per NAVSEA SD18 (Part Requirement & Application Guide).
- 500 hrs per PRF38535 spec. (for technology characterization).
**NASA projects:  250 hrs?**

**Equivalent to 1000 hours at ****85 ****o****C and 85% RH**

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## HAST failure mechanisms

**HAST failure mechanisms**

_**Package level**_
**Corrosion of the leads.**
**Dendrite formation (on the****  surface and inside****  packages).**
**Swelling/shrinkage:**
- Delaminations;
- Solder ball failures in PBGA and flip-chip technology;
- warpage of large packages;
- parametric shifts in linear devices.

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## HAST failure mechanisms

_**Die level:**_
**Corrosion of Al metallization.**
**Dendrites between metallization****  lines.**
**Leakage currents.**
**Charge instability (lateral, ion****  drift, hot electron).**

- Curtsy of C.Greenwell

**HEXFET failures**

- Mechanisms of moisture induced parametric degradation require additional analysis

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## What is the HAST acceleration?

- Best-fit model:S. Peck [86];Hallberg and Peck [91];S. Tam [95]; SSB-1 [2000];J. Sweet [02]:

**Life time increases with device temperature**

- Ea = 0.8 eV;  n = 2.7

- 0.7 < Ea < 1.1 eV;
- 1 < n < 5

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## Test results on linear devices

| Part | DC | Pack.type | QTYtested | QTYfailed | Failuremode |
| --- | --- | --- | --- | --- | --- |
| Vref | 0112 | SOIC8 | 31 | 3 | 1- increased Vout
2 - catastrophic |
| Opamp | 0101 | SOIC16 | 30 | 2 | - |
| Instr. amp | 0033 | SOIC8 | 30 | 30 | Excessive input currents |
| FET1 | 0041 | SOT223 | 29 | 28 | IDS, VGTH |
| FET2 | 0040 | D2Pak | 30 | 29 | ~ 60% parametric shift |

**HAST: 130 ****o****C, 85% RH, 250 hours**

**11 part types out of 25 failed HAST**

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## Is HAST adequate for normal conditions?

**Testing temperature (130 ****o****C) is above the operational range for many parts.**
**Degradation of molding compound:**
- Decrease of Tg (up to 100 oC in resins and up to 30 oC in MCs).
- Enhanced creep.
- Decrease of the tensile stress and adhesion.
**The model accelerates mostly corrosion failures, but corrosion is**** no longer a prime concern.**
**Moisture assisted hot-carrier degradation might****have U < 0.**

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## Irreversible changes in MCs: An example

_**Test:**_ moisture uptake and swelling after HAST were measured on two epoxy molding compounds.
_**Conditions:**_ saturation with moisture at 85% RH and different temperatures.

**Additional mechanical stresses due to swelling:**
-   AE[(MC - LF)T + m],
**A****t **MC  LF  **mechanical stresses are due only to moisture sorption.**

**An increase in volume might cause  delaminations and mechanical failures.**

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## Do we need moisture testing?

**Ionic dissociation in polymer:** Charge carriers are impurity ions generated by dissociation of a salt MA

**The equilibrium constant:** *n**o* is the concentration of salt molecules;  *f*  is the fractional degree of dissociation

**In a medium with low ********:** Uo is the energy of ions separation in vacuum

**2. Another reason:**
**Moisture activates ionic impurities similar to temperature. Moisture testing might accelerate degradation mechanisms related to charge instability.**

**1.  Moisture concentration at the die surface at RT is approximately the same as at high T/RH conditions.**

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## A possible alternative to HAST: MS + HTB

- Soaking in humidity chamber for 48 hrs at 110 oC and 85% RH

- Note: C/Co = 1 corresponds to the equilibrium moisture saturation at 100% RH.

- Testing at 85 oC.

**Moisture evolution in a 2 mm thick package**

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## Calculated test time and equivalent time of operation

_**Soaking conditions:**_
- RH = 95%,
- T = 110 oC,
- t = 30 hr
_**Test conditions:**_
- RHeff  90%
- 70 oC < T < 105 oC
_**Operating conditions**_**(environment):**
- RH =50%
- T = 20 oC

**Package thickness 2 mm**

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## HAST alternative: MS + HTB testing 

**HAST alternative: MS + HTB testing **

_**Input:**_ Package size; D(T) data; ground phase conditions; maximum storage and operation temperatures.

- Set RH and T for MS conditions

- MS + HTB Test conditions

- C(t) simulation

- Estimate MS time

- Set T for HTB conditions

- Calculate A and 

- C(t) simulation

- Estimate HTB time

- Is A & suitable?

**Algorithm for calculation of MS+HTB testing conditions**

**+**

**-**

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## Conclusion

**Suggested quality assurance strategy: moisture content control (virtual monitoring and baking) + adequate testing.**
**The moisture prevention strategy can be implemented by assessment of moisture content and calculations of the bake-out conditions based on D(T) data. **
**The existing HAST conditions are too harsh for PEMs intended for space applications.  A possible alternative to HAST might be MS + HTB testing. **
**Additional analysis of moisture induced parametric degradation and acceleration factors is necessary.**

- MRQW  Dec '02