Fine Feature Assembly Issues vis a vis Lead-Free

Folks,

We are all aware that the miniaturization of electronics continues unabated. As consumers we benefit from this phenomenon, as electronic assemblers we struggle with it. The smaller stencil aperture openings make stencil printing more of a challenge, while at the same time reflow is more difficult because the small solder paste deposits have a higher surface area to volume ratio (SAVR)*. This greater SAVR makes more work for the solder paste flux to remove and prevent oxidation.

As many in the industry go halogen-free, the larger SAVR will be an even greater challenge. So look for continued improvements in halogen-free solder pastes Re their fluxing ability.

Another interesting effect of smaller solder joints is that it is possible, especially if cooling is slow coming out of the reflow oven, for the small solder joint for be a single solder grain. If the orientation of this single grain is such that its weakest direction is the same as that of the maximum stress on the solder joint, mechanical failure can occur.

A short time ago I had a posting on the trend that SAC305 appears to be fading in favor of SAC105. I mentioned that before SAC105 can be dominate, sister alloys (similar to SAC105 but with additional additives) will need to be developed that lower SAC105’s melting temperature from 227C closer to SAC305’s 217 and improve SAC105s fatigue life and further improve its shock resistance. In addition to these needs, this new, as yet to be defined alloy, (often called SACY) will benefit if it resists forming single grains upon cooling.

It is an interesting and challenging time to be a solder metallurgist!

Cheers,

Dr. Ron

*
To understand why SAVR becomes so great as solder deposits become smaller, consider a sphere. Its surface area is given by 4 Pi r^2, whereas its volume is 4/3 Pi r^3. The SAVR is then 4 Pi r^2/(4/3 Pi r^3) = 3/r. Hence, SAVR becomes infinite as r=>0!

The photo of passives is from a paper I co-authored with Rita Mohanty of Speedline, etal. (Mohanty, Rita, Lasky, Ronald, etal, Process Development for 01005 Lead-Free Passive Assembly: Stencil Printing, APEX, Los Angeles, February 2007).