Experiments on Solder Column Interposer: cryogenic cleaning and local laser reflow

1. Experiments on Solder Column Interposer:cryogenic cleaning and local laser reflow Irving HAMON, Electronic Materials and Assembly Processes for Space (EMPS) Workshop: 4.05.2011

2. Purpose of the experiments Experiments conducted to improve properties (solderability, cleanliness) of the connections of an electronic package. This package integrates in a single component every functions of the satellite calculator. Solder columns have been chosen as an example: some of the following results derived from the experiments could apply to other types of connections (solder balls, leads…) or even boards ceramic Electronic package Solder columns ensuring the connection from the die to the substrate

3. Solder Column Interposer (SCI) • the Solder Column Interposer (SCI) is made of a ceramic plate in which the array of solder columns are inserted. Basically, the attachment of SCI on the CLGA is made by reflowing eutectic solder bumps at the top of the SCI to form the package ready to be assembled in the end (CCGA for Ceramic Column Grid Array). Sn10Pb90

4. Assembly qualification • Previous assembly qualification conducted in 2007 showed a poor reliability of solder columns. During the qualification test programme (vibration + shock + thermal cycling), large voids and cracks within the solder joints were noticed on external rows of the array of columns. • The occurrence of such defects lead to end the qualification earlier than expected. Cross sections of solder columns: left: defect-free, right: voids within the solder joint on an external row

5. Defects observed after investigations • Investigations carried out proved that columns were porous and that porosities may tend to propagate during tests within the solder joints to create the voids observed. • Visual inspection also showed the presence of organic contaminants at the end of the columns (green spots on the picture). The following goes through the different improvements made to recover correct results.

6. The study was broken down into 4 main steps: • SCI assembly on the CLGA • Improvement of SCI reliability • Package board assembly • Thermal cycling

7. Improvement of SCI reliability • 2 major actions were performed to improve SCI reliability: • pollution removal by using a cryogenic cleaning method • solderability improvement and porosities mitigation by local laser reflow

8. Improvement of SCI reliability • Pollution removal by cryogenic cleaning (technique developped by Air Liquide) principle of cryogenic cleaning projection of particles of dry ice on the surfaces to be cleaned • infra red thermocouple + electrostatic fieldmeter utilized during the experiment nozzle

9. Improvement of SCI reliability • Results after cryogenic cleaning • temperature ranged between -1 to 8°C • only 30 s is sufficient for cleaning • electrostatic charges can be generated during the process • surface roughness can be modified • Inspection under UV light: green spots indicates the presence of contaminants. More than 95% removed after cryogenic cleaning. • Good method to remove contaminants before cleaning after cleaning

10. Improvement of SCI reliability • Results after cryogenic cleaning • left: before cleaning, right: after cleaning surface roughness modified after processing

11. Improvement of SCI reliability • Results after cryogenic cleaning good results obtained, some improvements are necessary to be compliant with the electronic industry: • charge generation: air deionizer necessary to prevent charge generation • visual aspect after cleaning: impact of dry ice particles diameter on surface roughness to be optimized

12. SnPb column Improvement of SCI reliability • Aim: • forming a protective layer to prevent voids from coming out of the solder columns • Local reflow of the column tip by Laser reflow • obtaining a smooth surface to regain a better solderability

13. Improvement of SCI reliability • Local reflow of the end of the column by Laser reflow Laser beam package N2 flux

14. Improvement of SCI reliability • Results after Laser reflow • smooth surface obtained after Laser reflow. • Controlled reflow that keeps the geometry of the columns after process. After laser processing Before laser processing Before laser processing

15. Improvement of SCI reliability • Results after Laser reflow • smooth surface obtained after Laser reflow. • Controlled reflow that keeps the geometry of the columns after process. Before laser processing : Rough surface After laser processing: : smoothsurface

16. SnPb column Improvement of SCI reliability • Solderability test • 1 Part subjected to solderability testing after cryogenic + Laser reflow. • Reference standard: NF-A-89400 • dip test • flux used: C25D (RMA) • immersion time = 5s • temperature of solder bath = 235°C measurement of the contact angle  • measurement of the force F • visual inspection

17. Improvement of SCI reliability • Results after solderability test • not good according to the standard (slow wetting) but correct from an optical point of view (height of wetting area greater than the immersion depth) and better than previous solderability tests performed in the past (with regards to the height of wetting area) Usually, with no treatment After laser reflow

18. Board assembly 3 parts were assembled on board: • 1 part not processed • 1 part subjected to cryogenic cleaning and Laser reflow • 1 part subjected to Laser reflow (not polluted by contaminants) Parts subjected to 500 Thermal Cycles (-55/100°C) • assembly after Laser reflow Laser marking

19. Results after 500 Thermal cycles • same behaviour for the 3 parts • cracks within the solder joints (left picture) & solder shrinkage (right) Laser marking crack

20. Cross section after 500 Thermal cycles • part not processed (left). Laser processed (right)

21. Conclusion • The attempt to improve the SCI reliability failed; the same behaviour as during the first assembly qualification was observed (void, cracks within the solder joints). • Cleaning method (cryogenic cleaning) and Laser reflow: good techniques to regain better physical aspects and physical properties (solderability), but not enough in this case to counterbalance structural defects within the SCI.