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

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
Date - 2

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
Date - 3

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
Date - 4

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.
Date - 5

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

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
Date - 7

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
Date - 8

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
Date - 9

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

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
Date - 11

12. 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
SnPb column
obtaining a smooth surface to regain a better solderability
Date - 12

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

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.
Before laser processing
Before laser processing
After laser processing
Date - 14

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: : smooth surface
Date - 15

16. 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
SnPb column
Date - 16

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
Date - 17

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
Date - 18

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
Date - 19

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

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.
Date - 21