Improved Flux Reliability of Lead-Free Solder Alloy Solder Paste

Shantanu Joshi, Jasbir Bath, Roberto Garcia
Koki Solder America Inc., USA

Mitsuyasu Furusawa, Junichi Aoki, Manabu Itoh
Koki Company Ltd., Japan.

In recent years, a growing number of electronic devices are being incorporated into automotive and other high reliability end products where the challenge is to make these devices more reliable. The package size of the devices is largely driven by the consumer industry with their sizes getting smaller making it harder to assemble and be reliable at the same time. For automotive and other high reliability electronics product, it is of the utmost priority to secure high reliability because it directly involves human life and safety. Challenges include selecting an appropriate solder alloy and having good reliability of the solder paste flux.

For solder alloys, much development has been done and is in progress. For the solder paste flux, it is important that the flux intended for automotive and other high reliability applications should have reliable insulation resistance even in an atmosphere of high temperature and high humidity. To meet these requirements, a type of 'crack free' flux paste was developed to inhibit cracking under extreme environments making it more reliable with stringent surface insulation resistance and electro-migration criterion from automotive and other high reliability product manufacturers. Crack-free flux residues help to prevent electro-chemical migration caused by moisture entering through the flux residue cracking. In addition, crack-free residues act as a type of conformal coating providing a consideration to assemble without conformal coating use for certain applications.

Experiments were carried out to test the reliability of the flux according to various industry electro-chemical migration and dew test standards using IPC and JIS (Japan Industrial Standard) test boards. The flux residue showed no indication of cracking after pre-conditioning from-30 degrees C to 80 degrees C accelerated thermal cycling for 1,000 cycles followed by testing with no evidence of electro-chemical migration with a variety of board line widths and spacings used on the test boards. Printing, wetting, voiding and reflow tests with components were also carried out to make sure that the developed solder paste was appropriate for high volume manufacturing with results reported.

A lead-free solder paste was developed which was resistant to cracking during thermal cycling. This resistance to cracking was shown to provide reliable insulation resistance even under conditions of high temperature and high humidity using various industry test methods which helped to prevent electro-chemical migration caused by moisture entering through the flux residue cracks. The crack-free residue acted as a type of conformal coating providing a consideration to assemble without conformal coating use for certain applications.

Printing, wetting, voiding and reflow tests with components using the developed paste indicated it could be used in high volume electronics manufacturing which has been validated in production.