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Forward/Backward Compatibility

Definitions:

  • Backward compatibility:   A lead-free component is said to be backward compatible if it can be reliably attached to a PCB using tin/lead solder paste with a tin/lead solder profile (typical peak reflow temperatures ranging from 220°C to 235°C).

  • Forward Compatibility:  A SnPb component is said to be forward compatible if it can be reliably attached to a PCB using lead-free solder paste with a lead-free profile (typical peak reflow temperatures ranging from 240°C to 260°C).

  • For leadframe-based surface mount packages1,2
    Lead finish Reflow Process and Peak Temperature Range
    Tin/Lead Process
    (220°C - 235°C)    		 Lead-Free Process
    (240°C - 260°C)
    Tin/Lead (SnPb) OK OK if device ordered with suffix
    260°C (forward compatible)
    Lead-free (matte Sn) OK (backward compatible) OK

    1. Based on board level evaluations, it is recommended that a reflow temperature of 235°C be used for LLP® backward compatibility (SnPb solder paste/Pb-free components).
    2. Through-hole packages have not been evaluated for forward or backward compatibility.

     

  • For micro SMD and BGA packages
    Solder bump/ball composition Reflow Process and Peak Temperature Range
    Tin/Lead Process (220°C - 235°C) Lead-Free Process (240°C - 260°C)
    Tin/Lead (SnPb) OK NOT RECOMMENDED
    (not forward compatible)
    Lead-free (SnAg [BGA] or SnAgCu [micro SMD]) NOT RECOMMENDED
    (not backward compatible)    		 OK

Reflow Profile

  • Generally, the reflow profile will depend on the melting point of the solder paste and the geometry and layout of the PCB.
  • With sufficient engineering characterization, the throughput for the Pb-free profile can be made the same as that of the SnPb profile.
  • For lead-free qualification, National used the Pb-free profile given in J-STD-020C.  This profile is also acceptable for production, provided that adequate characterization is done by the board level assembler.

Tin Whiskers

  • Tin whiskers are recognized as a reliability concern throughout the industry.
  • To mitigate whisker growth National has done the following:
    • Established the minimum plating thickness of 8 microns, with a nominal thickness of 12 microns.
    • Established a post-plate annealing of 1 hour at 150°C within 24 hours of plating.
  • Additional information is available on the Tin Whiskers page.

 

Packaging | Lead-Free Status | Impact to Customers | Tin Whiskers
Green Compliance | Industry Consortia | Questions & Answers | Literature