Newsroom

---

Highlights: NEMI Lead-Free Assembly Project

APEX Forum, January 21, 2002

Press Contacts at bottom of page

Process Group
Component Group
Reliability Group
Alloy Group

Process Group

1. Evaluated five Sn3.9Ag06.Cu and five Sn37Pb solder pastes and selected one (of each) for use on NEMI reliability test board builds.
   Results: Printability of the first generation experimental lead-free pastes was not as good as the tested tin-lead pastes. The lead-free print volume and paste transfer rate were lower. Consistency of paste print was reduced with lead-free, but within acceptable limits.
2. Assembled six surface-mount reliability test board designs (produced four new designs and re-used two others). Universal Instruments built the boards, assembling with standard manufacturing equipment. Used the evaluated tin-lead and lead-free pastes plus components for comparison.
   Results: No problems with printing, placement or reflow (higher temperature reflow used for lead-free paste and nitrogen reflow atmosphere used for both). Upon visual inspection, the solder joint appearance is different between tin-lead and lead-free.
3. Assembled boards were submitted to multiple inspections: visual inspection

• automated optical inspection (AOI)
• x-ray
• C-SAM (scanning acoustic microscopy

Results: Spreading of lead-free solder is somewhat reduced compared with tin-lead; increased voids were seen with lead-free solder paste on components with tin-lead coated leads.

Process Group Conclusions and Recommendations:

• Lead-free solder assemblies can be successfully manufactured with existing assembly process equipment; however, printability is reduced, solderability is reduced and voiding increases. This should improve with more experience and succeeding generations of lead-free solder pastes.
• Inspection criteria may need to be modified and inspection equipment re-programmed for lead-free assemblies.
• Full-scale production trials will provide the best test for lead-free solder processing.

Component Group

1. Initially targeted a reflow peak of 260 C as an evaluation metric to test components.
   (Melting point is 217 C +42 C = 259 C Ò 260 C peak.)
2. Profiling studies conducted:
   • Determine temperature spectrum
   • Monitor energy consumption
   • Identify process tolerances
3. Larger and most moisture MSL-sensitive components tend to have lower peak temperatures than smaller parts and will not reach the peak reflow temperature of 260 C. (MSL = moisture sensitive level.)
4. Attempted to demonstrate use of a lower temperature (240 C), but this target was too low (see conclusions and recommendations, below).

Component Group Conclusions and Recommendations:

• 250 C (+/-5) is an acceptable short term target for maximum peak reflow temperature for testing smaller packaged components.
• 245 C (+/-5) is an acceptable short term target for the maximum peak reflow temperature for testing large, high thermal mass packaged components.
• The economics of the market will drive the implementation of high temperature capability.
• New standards must be developed, or existing standards revised, to reflect new soldering temperatures for lead-free boards.
• Additional work that must be done:
  Develop reliability data for solder joints formed at lower peak temperatures (~228 C to 235 C)
  Gather data to define impact to first pass process yields resulting from lower processing headroom.

Reliability Group

Performed reliability testing for selected solders, components and board finishes. Combinations of lead-free solder and lead-free components were contrasted to tin-lead solder and tin-lead coated components. Additional cells had lead-free solder with tin-lead coated components.

1. Accelerated thermal cycling (ATC), was done under two temperature conditions:
2. -40 C to 125 C
3. 0 C to 100 C
4. Bend testing
5. Electromechanical migration testing

Reliability Group Conclusions and Recommendations:

• ATC testing: lead-free/lead-free combinations performed equivalent to or better than tin-lead/tin-lead benchmark; tin-lead/lead-free combinations were not always better than benchmark.
• Three-point bend testing: no differences observed between different combinations.
• Electromigration: no issues seen with lead-free alloy.
• Need to complete failure analysis, and data analysis.
• Final report targeted for April 2002.

 

Alloy Group

Much of the Alloy Group's work was completed early in the project and was reported at APEX 2001.

1. Determined phase diagram of SnAgCu system.
2. Completed review of existing solder alloy information.
3. Identified best practices for solder alloy property data generation.
4. Currently collecting and reviewing existing material property information on SnAgCu.

Alloy Group Conclusions and Recommendations:

• Recommended Sn3.9Ag0.6Cu for industry to use in reflow soldering
• Determined and published details on eutectic for SnAgCu solder
• Compiling data base on solder characteristics
• Work underway to define solder joint reliability model

See related press release

---

For further information:
Cynthia Williams
cwilliams@nemi.org
phone: 207-871-1260