Reliability Issues and Mitigation Strategies for RoHS Compliant Assemblies

Dan Amiralian

RoHS -Potential Reliability Issues

RoHS – The EU Mandate to eliminate lead and enhance the recycling of electronics has generated seemingly equal amounts of concern and confusion. This presentation is intended to be a BRIEF overview of major reliability concerns and real-world mitigation strategies.

Don’t Let This Happen to YOU

Knowledge Overcomes Fear

Widespread Technical Concerns

Component Plating Connector Plating Board Plating Black Pad Kirkendall Voiding Board Flexing Fillet Cracking Ceramic Capacitors Tin Whiskers Fretting Corrosion Long-Term Storage Solder Wetting Electromigration Mechanical Shock Pad Liftoff PCB Material Creeping Corrosion Stress Relaxation Champagne Voiding Tin Pesting Solder Contamination Insufficient Hole Fill Popcorning (MSD) Conductive Anodic Filaments

What do the experts say?

"There are three kinds of lies: lies, damn lies, and statistics."

– Mark Twain

Range of Opinion

 Lead-free (SAC) is better than lead  SAC is no different than lead  SAC has issues, but they can be overcome  SAC has issues that present real-world problems that require mitigation  We need to go back to lead

Which Watch Swiss Swatch Switch

 Is applying for exemption to go back to SN63/37 due to reliability issues  30% Failure Rate  Crystals Shorting  Sub 32mil (.8mm) parts shorting  5% field failure rate to date  This is not a unique experience

Keep it Simple – Two Major Concerns

 Process  Alloy used to solder  Process Parameters  Specific Concerns  SMT  BGA  PTH  Components  Potential Contamination  Inability to withstand process  Finishes  Unacceptable number of variables

A Reasoned Approach

 RoHS isn’t going away   There

IS

a reason High-Reliability electronics are exempt There ARE ways to mitigate reliability issues

Focusing on the Real World

 Not enough attention paid to COMPONENT issues  Most mitigation plans do not fit the real world  Too much academic information and propaganda – not enough “in the trenches” information.

Component Issues

     The Most Common failures are with the MOST COMMON Components Highest Incidence of Failures: Capacitors, PEMs and PCBs Inventory Control issues can lead to Pb contamination BGAs Package Sizes and Finishes are a concern

Chip Capacitors Major Concern

 UMD study of field failures from 70 companies revealed 30% of ALL failures were Capacitors – mainly MLCCs

Multi-Layer Chip Capacitor Failure Characteristics

 Faults not recognized by normal screening techniques  Most likely in high capacitance devices of low voltage rating  Not visible on the outside of the device  Capacitance does not change but the leakage current may change dramatically

Why Do MLCC Fail at a Greater Rate with Pb Free Alloys?

 Pb-Free alloys are stiffer  More stress on component  Pb-Free alloys require higher temperature and longer dwell times  Cooling rate needs to be tightly controlled  MLCCs may have inherent problems  On cutting edge of material use

How Do I Mitigate MLCC Cracking in the Real World?

  Existing Designs:  Review Design and Access Potential   Limit Requirement for Reworking MLCCs Develop Reserve New Designs:  Place on top side only – be mindful of stress  Select more Robust Components   Limit Requirement for Reworking MLCCs Develop Reserve

# 2 PEMs

Plastic Encapsulated Microelectronics        Tin Whiskers (Matte Finish) Popcorning Wire Bonds Flux Process Compatibility Wetting (Ag Pd) Process Cycles Processes and Practices Previously Permissible with SN63 need to be revisited

Mitigating PEM Failures

  Review Allowed Processes  Wavesolder  Heat Cycles Review Component Specifications  MSD   Peak Temperature and Dwell Lead Frame Scoring vis à-vis Wavesolder

Develop Valid Failure Reporting and Reserve

RoHS Printed Circuit Board (WEEE Compliant)

 A variety of RoHS Compatible Finishes OSP ENIG Silver Tin SAC HASL  Material RoHS FR4 ISOLA Polyclad  Special Issues Shelf Life Environment Appearance

Printed Circuit Board

 Finish Survey – EU  31% ENIG  30% Don't Know  12% Pure Sn  9% Silver  9% Other  6% Sn Ag Cu  3% Pd/Au

Pad Finish, Tg, Td and Z-Axis Expansion are Important Factors Traditional FR4 after RoHS Processing

PCB Recommendations

 ENIG  Long History  Best Overall Performance  ISOLA 410 / 620 or Equivalent  Designed for RoHS  Best overall characteristics

Through Hole Considerations

 RoHS Alloys do not act like SN63   MUCH longer wetting time Higher Temperature – STIFFER Material

Poor up-flow, Cracking and Pad Lift are real concerns

Mitigation of PTH Concerns

 PROCESS MUST BE MORE FINELY DESIGNED AND CONTROLLED  Trade-Offs need to be accessed  Aspect Ratio to Specification Performance has to be reconciled  Board FLEX / SHOCK

Ball Grid Array Concerns

RoHS Concerns -Ball Grid Array

 Voiding  Solder Fragility  Thermal Issues  Mechanical Shock Issues  Plastic Devices  High Pin Count Devices  Process Cycles

Mitigation of BGA Concerns

 PROCESS VALIDATION  LIMIT VARIABLES  Understand Risk

Over Time it will become obvious that High Pin Count BGAs and Multi Cycled BGAs processed RoHS WILL fail much more often, especially under certain conditions

The BEST Mitigation Strategy for RoHS Compliant Assemblies is Choosing the RIGHT PARTNER

Dan Amiralian