Commercial components vs lead-free terminations; how to manage it?

LEAD-FREE Transition schedule

Finishes of components VS mounting process

In case of incompatibility, the material must be “respected”

Points of attention during commercial parts selection

• TSSOP/SOIC package are preferred options compared to QFN.

• Lead-frame material : copper alloy is preferred as closer to PCB dilatation
  • • Packages with Iron nickel Lead-frame could affect the mounting reliability

• Molding compound type varies with package and manufacturer (different Tg et CTE) 
  • • Qualification of package and/or manufacturer by similarity is less obvious with commercial parts  than HIREL parts
    • PCN affecting molding (new molding, new assembly site) or bonding (gold wire -> copper wire) : requalification of package.

• Manufacturer lead finish transition : HIREL SnPb part switched in lead-free variante.
  • • • • Parts to be selected with precaution because HIREL manufacturers processes are not (yet) mature.

Lead finish synthesis

• • • The lead finish is obviously the main change  for lead-free process.

• • • For others finishes, compatibility of soldering process has to be checked “case by case”

PURE TIN MANAGEMENT

• Current space standard do not recommend the use of Pure Tin (Tin Whiskers).

• ADS strategy and material restriction
  • • Parts with bright pure tin plating (>97% tin) on terminations shall not be used.
    • Parts with mate pure tin finish (>97% tin) are allowed, provided they pass the JESD-201 class 2 requirements or meet the GEIA-STD-0005-2/Class 2B requirements.
    • Parts with pure tin finish not to be used in power functions (V > 15V and I > 2A) or screwed on board.
    • When JESD-201 class 2 is not demonstrated, the use of conformal coating is considered as an acceptable risk mitigation.

• • Most of manufacturers mitigate at component level, (1hours baking @ 150°C after tin deposit, thickness of the under-layer (Ni), or Low percentage other metals to reduce whiskers growth (and others).

Pure Tin finish is manageable

Airbus Group status on lead free assembly reliability

• Many studies conducted within Airbus Group on lead free assembly reliability
• For Space activities, CNES study conducted in 2016-2017. 
  • • Several types of SMT packages with different finishes, assembled on lead free assembly line.

• • • Tested through vibration & thermal cycles [-55°C ; 100°C], until 2000 cycles.

– Results presented during COMET event, May 16 2019 – 

• Main outcomes : As soon as the assembly process is well defined. 
  • • Reliable assemblies with tin lead process remain reliable with lead free process
    • Critical assemblies (ex : TSOP with iron nickel leads) remain critical with-lead free process 
    • No difference was identified compare to different component finishes (similar solder joint reliability)
    • In general, failure mode remains the same with lead-free process 
    • BGA not evaluated in the frame of this study

These statements were confirmed by other French industrials during COMET workshop

SYNTHESIS

• Most of the commercial parts  have pure tin finish
  • • Whiskers need mitigation, but most of the commercial manufacturers know how to manage it.

• Commercial parts  are designed and validated for lead-free mounting process. 
  • • Attention have to be brought to the package selection (lead-frame material, package type…). 
    • Validation step is necessary (take care of the package similarity).

• The  compatibility with the SnPb mounting process is generally good (similar to Pb free solder process), except BGA with SAC balls, not recommended to be SnPb soldered. 
• The main encountered difficulties are not with commercial parts , but with specific components or adapted from HIREL.

Most of the lead-free commercial parts problematics are manageable