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NASA DOD Phase 2: Copper Dissolution Testing



NASA DOD Phase 2: Copper Dissolution Testing
This paper describes copper dissolution measurements of two lead-free solder alloys to better define allowable rework process windows.
Analysis Lab

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Authored By:


Jeff Kennedy
Celestica
Minneapolis MN USA

Dave Hillman and Ross Wilcoxon
Rockwell Collins
Cedar Rapids IA USA

Summary


Copper dissolution is a concern for products making the conversion to lead-free solder alloys. In these alloys, the more rapid reaction of tin/copper in lead-free solder alloys compared to tin-lead solder alloys, can accelerate the degradation of the plated copper connections. No copper dissolution testing was conducted during Phase 1 of the JCAA/JGPP program testing, which focused on the reliability of solder joints.

The Phase 2 effort included testing to validate copper dissolution measurements reported by the commercial electronics industry. Copper dissolution is of particular concern when components are reworked, which is much more commonly used on highreliability electronics than in consumer electronics. Reworking product that has lead-free solder joints may impact the repair depot operations as the copper dissolution may remove over half of the Plated Through Hole (PTH) copper in a single rework cycle.

This paper describes copper dissolution measurements of two lead-free solder alloys, for both plated through holes and surface mount pads, to better define allowable rework process windows. These results showed that due to the higher copper dissolution rates, lead-free assembles may require design changes such as PTH copper thickness to ensure that they can be reworked.

Conclusions


A number of issues related to copper dissolution should be addressed for products making the transition to lead-free assembly. These include:
  • The amount of initial copper plated in the PTH hole may need to be increased to establish a greater margin of safety. The current requirement for 1 mil copper plating minimum may need to be increased to as high as 2.0 mils to provide this margin.
  • A resultant minimum copper thickness after rework process may need to be specified and validation methods to ensure compliance would need to be established.
  • Alloy selection for rework may be different than for primary attach depending on the expected number of rework cycle requirements for the given product lifetime. Some initial studies have indicated that mixing various Pb-free alloys will not degrade solder joint quality or solder joint reliability.
  • Copper dissolution rates vary somewhat with the PTH diameter. This study included only two hole sizes: 0.036" and 0.015". The smaller hole may inhibit material flow up and down the PTH barrel, which affects the copper dissolution rate. Product design consideration may require some additional testing to validate product parameters and associated process requirements.
  • Rework locations need to be identified by reference designator.
  • Control and recording of rework exposure time may also be required to ensure the connection will meet lifetime requirements of the product.
  • Tighter controls on solder pot contaminant levels and maintenance of pot composition may be required to reduce variance of the copper dissolution effect during rework operations.
  • Consideration for larger component sizes with regard to nozzle design and alloy flow during the rework procedure may be necessary.


Initially Published in the SMTA Proceedings

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