Removing Flux from Low Standoff Heights

Michael C. Savidakis, Ph.D., Robert Sell, and Christine Fouts, Ph.D.
Petroferm Inc., Gurnee, IL USA

Our insatiable desire for smaller, faster and highly functional electronic devices presents numerous challenges for package designers and manufacturers. Current day popular approaches include stacked components and boards, high I/O density, and short interconnection distances. Unfortunately, these solutions make flux residue removal from underneath components increasingly difficult.

Adding to the challenges are the changing global environmental and safety regulations which make the cleaning task even more challenging. The objectives of this study are to 1) evaluate cleaning effectiveness of several currently available cleaning chemistries/processes in removing flux residues from underneath low standoff height components, 2) determine differences in the effectiveness of these individual cleaning processes on Sn/Pb and Pb-free solder paste residues, and 3) evaluate inherent advantages or limitations for each type of cleaning chemistry and process.

With the increased use of high density and miniature components in modern electronic devices, the electronics assembly industry continues to deal with the challenge of finding the right cleaning process for removing residues from low standoff components. The advent of new flux chemistries along with the changing environmental regulations add to the complexity of finding the right cleaning process. Moreover, economic considerations, waste disposal and space requirements also play an important role in choosing a cleaning process.

this study, four different cleaning processes were evaluated for their performance in removing flux residues from underneath 5 mil and 1 mil clearances. The processes included Aqueous Inline Spray-in-Air, Aqueous Batch Ultrasonic, Monosolvent Vapor Degreasing and Cosolvent Vapor Degreasing. Three Sn/Pb and two Pb-free solder pastes were tested in each of the four processes.

Each process demonstrated the increased difficulty in cleaning as the standoff height was reduced. While some reduction in cleaning efficiency was observed under 5 mil spacing as compared to cleaning in unobstructed areas, complete cleaning is achievable under 5 mil spacing with the selection of the right chemistry/process and optimization of process parameters.

However, cleaning underneath 1 mil spacing remains very difficult as evidenced in this study. While none of the processes tested were able to clean all of the paste residues underneath 1 mil spacing; the Aqueous Batch Ultrasonic process was most successful. This can be attributed to the apparent reduction in the surface tension of the aqueous cleaner due to the ultrasonic energy introduced into the system. The results of this study also demonstrate that under similar operating conditions, Pb-free pastes continue to be more difficult to clean than Sn/Pb pastes.
This suggests that solder paste manufacturers should consider the need for cleaning when designing Pb-free and Sn/Pb solder pastes for low standoff height applications. Additionally, novel approaches may be required to meet the cleaning needs as new manufacturing trends continue.