Challenges of Ultra-Fine Feature Printing Optimization of Pb-Free Solder Paste

Tony Lentz
FCT Solder
CO, USA

The trend of miniaturization of electronics requires the creation of ultra-fine solder joints. Semiconductor manufacturing, flip chip, package on package (PoP), system in package (SiP), and miniature components like 0201M (008004 Imperial) may require printing through stencil apertures of 50-100 μm (2-4 mils) in size. Creating these miniature solder joints requires solder pastes optimized for use with IPC Type 6 (5-15 μm) or smaller solder powder sizes.

Type 6 solder powder size has a much higher surface area per Kg than Type 3, 4, and 5 solder powder sizes. The high surface area of Type 6 solder powder affects the performance of the solder paste including: shelf life, stencil life, printability, reflow performance, voiding, and reactivity or stability. Testing was conducted to measure solder paste performance for both water soluble and no clean solder pastes with Type 6 SAC305 solder powder. The performance of older generation solder pastes was compared to newly optimized solder pastes. The solder paste data was compared, and recommendations made for successful print and reflow in these miniaturized soldering applications.

There are many challenges for solder paste to overcome when using Type 6 or smaller solder powder sizes. The five solder pastes tested with SAC305 Type 6 solder powder showed performance differences. Here is a summary of the results.

• The NC New 2 and NC Old solder pastes were able to print with acceptable TE% through 0.60 AR apertures.
• In print and pause testing, each of the solder pastes showed a decrease in TE% over time, except for the WS Old solder paste, which is particularly stable over time on the stencil.
• The NC New and NC New 2 solder pastes showed a slower rate of TE% decrease than the NC Old solder paste.
• The NC New and NC New 2 solder pastes showed acceptable wetting, and the best possible solder balling and graping performance. They both outperformed the NC Old solder paste in reflow.
• The WS New solder paste showed good wetting and solder balling performance, and the best possible graping performance. This solder paste outperformed the WS Old solder paste in reflow.
• 24 hours of air exposure of the printed solder paste improved the reflow performance of the NC New and NC New 2 solder pastes. The NC Old solder paste showed worse reflow performance after 24 hours of air exposure.
• The WS New solder paste gave similar reflow performance with 24 hours of air exposure, as opposed to the WS Old solder paste which declined in reflow performance.
• Voiding performance was optimal for the NC New solder paste, with the NC New 2 performing 2nd best. The NC Old showed mid-range voiding performance. The WS New solder paste outperformed the WS Old solder paste in terms of voiding. Reflow profiling can be used to minimize voiding for each of these solder pastes.
• The NC New solder paste showed a significant decrease in tack force over 3 days, which indicates instability in this test. The NC New 2 and WS Old solder pastes gave a small increase in tack force over time, which is not unusual. The NC Old and WS New solder pastes were stable in tack force over time.
• Heat aging showed instability in the NC New solder paste, but NC Old solder paste showed stable viscosity and tack force. The NC New 2 solder paste showed an increase in viscosity and tack with heat aging. The WS New solder paste showed a slight increase in viscosity but stable tack force with heat aging. The WS Old solder paste showed an increase in both viscosity and tack force with heat aging.

Overall, the NC New 2 and WS New solder pastes outperformed their older technology counterparts with SAC305 Type 6 solder powder. These solder pastes may be acceptable for use with ultra fine feature printing and reflow for the creation of miniature solder joints.