Results of Mitigation Effectiveness Survey

Transcription

1 Results of Mitigation Effectiveness Survey &Plans for GEIA-STD Revision Anduin E. Touw Task Team Lead for PERM GEIA-STD Associate Technical Fellow, The Boeing Company 6/24/2010

2 Note: The opinions expressed here are the results of the efforts of the PERM industry group and task teams. They do not necessarily represent the policies or practices of the Boeing Company. 2

3 GEIA-STD Tin Whisker Mitigation GEIA-STD , Standard for Mitigating the Effects of Tin Whiskers in Aerospace In High Performance Electronic Systems, was released in June 2006 Document designed to provide a framework by which suppliers and customers can: Communicate more clearly Establish a common methodology for assigning level of risk Define what actions they are taking as a result Not a step-by-step cookbook for removing tin whisker risk entirely We know more about tin whiskers than we did 4 years ago, but do we know enough to provide more standardization of mitigation practices? 3

4 Some History and Background GEIA-STD was developed by the LEAP Working Group, a joint task team sponsored by AIA and GEIA It was published by GEIA, balloted through G-12 (Solid State Devices Committee) and APMC (Avionic Process Management Counsel) LEAP WG has become PERM, sponsored by AIA GEIA has become TechAmerica and will continue to own / manage the document. 4

5 GEIA-STD : Overview of Control Levels Level 1. No restrictions on Pb-free tin finish use. Level 2. Pb-free tin finish i is allowed under some circumstances. Level 2A. Use of Pb-free tin finish without explicit controls is acceptable under most circumstances but the likelihood of whiskers and methods used to estimate their impact and mitigation strategies will be documented. Pb-free tin finish i may be prohibited in some specific circumstances called out in contractual t documents. Level 2B. Pb-free tin finishes may be used but only with customer approved and specified control measures. These Pb-free tin finish approvals may be blanket approvals for multiple components and applications within the system. Pb-free tin finish may be prohibited in some specific circumstances called out in contractual documents. Level 2C. Restricted use of Pb-free tin finish. Pb-free tin finish is prohibited unless an exception is made. Specific instruction on use of Pb-free tin finish and required control measures to be provided and reviewed on a case-by-case basis. Level 3. Use of Pb-free tin finish is prohibited and measures must be taken to verify compliance. 5

6 Current Activities to Revise Control Levels Team is clarifying guidance statements with regards to control levels Making it clearer that tin whisker mitigation control needs to be consistent with overall program risk strategy For example, if program regularly l uses COTS assembly, they shouldn t select Level 3 or 2C Team is looking at how control levels work at different levels of integration Not designed to be applied at part levels May not be appropriate to have a single control level for an entire program; may need dto control at unit or board dlevel, l based on criticality or other risk metric Team considering providing examples of how different programs have applied 6

7 New Guidelines for Level Selection Level selection should be based on the following criteria: Consequences of Failure of Unit / System Likelihood of Plasma Event Amount of Redundancy (will local failure impact performance) Detectability and Repairability Criteria are incorporated into a decision tree to be included in an informative appendix Use of decision tree and criteria are guidelines only, not requirements 7

8 Other Revision Efforts Thinking about definitions of risk and mitigation Risk of whisker? Risk of electrical failure? Update informative appendix with more recent research references Considering additional appendices Whisker Inspection Guidelines Whisker Removal and Clean-up Guidelines Integrating Tin Whisker Risk into Overall Risk Strategy Guidance on Conformal Coat Application and Inspection for Tin Whisker Mitigation (would rather hand off to IPC) But much of the revision effort focuses on revision to how mitigations are handled d and standardized di d 8

9 Current Mitigation Requirements in GEIA-STD Level 3 No mitigations required because no Pb-free Tin Finishes are allowed (verification of finishes required) Level 2C 2 mitigations from different mitigation categories are required Design to Reduce Impact (spacing, encapsulation) Lower Risk Finishes (nickel underplate, fused tin) Partial Tin Finish Replacement (dipping in SnPb, soldering with SnPb) Coatings (conformal coats, foams) Level 2B 2 mitigations from different categories are recommended (none required) Next revision will include some sort of mitigation requirement for 2B 9

10 Why Were Those Selected as the Requirements? Group wanted to standardize specific mitigations but there was not enough consensus Concerns that readers/users would see specific recommendations or requirements as silver bullets Group looked at including Pinsky Model but some participants had concerns about specific values used in the model and specific functional forms Group agreed with overall goals of the model and with how it performed in most cases but there were concerns about how it behaved in the margins Group considered some sort of scoring for mitigators but decided there was insufficient data to reach consensus Final requirements were a compromise to allow for belt and suspenders but keep the approach very much under program / supplier control and very qualitative 10

11 Concerns about Current Practices No mitigation methods are explicitly included as standard nor explicitly excluded Left for review between supplier and customer Unfortunately, some customers (and suppliers) not well educated Suppliers cannot be confident that their standard practices will be acceptable on multiple programs 2 mitigations from different categories is viewed as arbitrary No ranking of mitigators even though some mitigators are generally more acceptable to our industry than others Currently matte tin is treated equivalently to matte tin with nickel underplate or matte tin with nickel underplate that has been fused All conformal coatings treated equivalently 11

12 What Are We Doing About Mitigations? Conducted (conducting?) a survey of the industry to look at opinions regarding effectiveness of mitigations and the ease at which h they can be implemented Looking for consensus on the relative value of different mitigators Can we create tables of strong versus weak mitigators? t Can we create a more quantitative scoring scheme for determining if the intent of Levels 2B and 2C are met? Plan of Evaluation Conduct Survey Review Survey Results (and demographics) Have technical experts present data to consider faulty perceptions in survey results Determine if we have agreement and technical justification for standardizing mitigators Final standard d recommendations and requirements will NOT be driven by survey results without proper vetting 12

13 General Response Level 32 responses as of first deadline (4/28/10) Additional responses are still being gathered Some mitigations had low response rate due to unfamiliarity All respondents have some level of tin whisker expertise Good mix of applications and control levels Concerns about demographics: Limited government customer response Limited international response No part supplier response Basic format of survey Each mitigator was scored between 0 and 6 for effectiveness 0 means not a mitigator; 6 means not a tin whisker risk 1 to 5 are Very Weak to Very Strong mitigators t 13

14 Average Mitigation Scores Mitigators were not obviously binned Buttheremaybesomeclustersofmitigators there of mitigators Average Mitigation Score 14

15 Average Mitigation Ranking for Each Individual Most respondents centered their responses around 3 (some mitigation) There were 2 respondents that consistently gave low Scores both on programs that are normally Level 3 15

16 Not a Tin Whisker Risk (Out of Scope, >5) >3% Pb (1 resp <=1) 1 respondent said this was a weak mitigator (out of line with rest of responses) SnPb Dip without Standoff (0 resp <=1) These results are consistent with the current revision of the standard Parts made with >3% Pb or (robotically) dipped for full tin replacement are out of scope 16

17 Strong Mitigators (>=4) Physical Barrier (1 resp <=1) Hard potting (2 resp <=1) Partial SnPb dip with Chemical stripping (4 resp <=1) Soldered with SnPb Solder with qualification / verification of coverage (with or without sampling) (4 resp <=1) Some Conformal Coats Parylene C (1 resp <=1) Epoxy >3mil (2 resp <=1) Spacing >500mils (2 resp <=1) Note: Survey result only; not an official PERM position or recommendation. 17

18 Reasonable Mitigators (>3) Many Conformal Coats Uralane >1mil (1 resp <=1) Urethane 1-2mil (4 resp <=1) Epoxy 1-3mil (4 resp <=1) Arcylic >2mil (5 resp <=1) Silicone >3mil (5 resp <=1) ALD (2 resp <=1, low response) Soft potting or encapsulation (6 resp <=1) Soldered with SnPb without verification on bottom termination or small leadless chip parts (6 resp <=1) Spacings mils (2 resp <=1) SnPb with 1% to 3% Pb (errors in 3% process) (7 resp <=1) Component Level Redundancy (2 resp <=1) Note: Survey result only; not an official PERM position or recommendation. 18

19 Weak Mitigators (>2) Spacings mils (4 resp <=1) Nickel Underplate (6 resp <=1) SnPb dip with standoff (7 resp <=1) Soldered with SnPb without verification on large leadless chip and thru-hole (12 resp <=1) SAC finish i or dip (with or without t standoff) (13 resp<=1) SnAg finish (11 resp <=1) Heat treatment above fusing temperature (10 resp <=1) Some Conformal Coats Silicone 2-3mil (7 resp <=1) Acrylic 1-2mil (8 resp <=1) Styrofoam fillers (9 resp <=1; low response rate) Avoiding compressive stress (8 resp <=1) Medium Current (11 resp <=1) Note: Survey result only; not an official PERM position or recommendation. 19

20 Very Weak or Non-Mitigators (<2) Soldered with SnPb without verification on surface-mount (4 resp >=4) Spacing <50 mils (10 resp >=4) Hot dipped tin (7 resp >=4) Silver Underplate (1 resp >=4) SnBi finish i (4 resp >=4) Heat Treatment below fusing (1 resp >=4) Large grain size or claimed matte tin (1 resp >=4) Thick Tin (2 resp >=4) Immersion Tin (1 resp >=4) <1% Pb (1 resp >=4) Note: Survey result only; not an official PERM position or recommendation. 20

21 Some Questions Raised by Survey Results Does current data on ALD put it in the same category as Parylene C or is more study needed? Can we recommend chemical stripping, at least for certain circumstances, and do we know how to prevent secondary damage? Does our history with high temperature solder provide any additional data on SnAg finishes relative to whiskers? Are there guidelines about relative hardness of conformal coats, encapsulants, and foams with regards to their ability to prevent whisker failures? Does the relative ordering of mitigators match with general thinking and research? 21

22 Next Steps Gathering additional survey responses for at least the next few weeks Reviewing the classification of mitigators by the survey and gathering data to re-evaluate evaluate Want to develop new strategy for mitigations (or reconfirm old strategy) this summer Plan on balloting new draft standard this fall 22

23 Conclusions New revision of standard will keep much of structure of last revision Continue with levels (likely same levels) Require notification / documentation and mitigations for some levels New revision WILL provide additional guidance on how to select and use levels New revision WILL NOT be a cookbook that will tell a novice what to do about whiskers But team HOPES new revision will be able to better standardize what mitigations are generally preferred by the military and aerospace industries 23