Solder Paste

Flux: No-clean pastes tend to have a wider print process window in terms of print speed and pressure. More importantly, no-clean pastes are more tolerant to changes in temperature and humidity. They also exhibit longer stencil life than water-soluble pastes.

Particle Size/Distribution: The majority of published research that prescribes solder paste compatible for 0201 assembly recommend the use of a Type 4 powder distribution. I endorse this advice as insurance to achieve the best results, but that does not mean the process will not work using a Type 3. The solder ball diameter range in a Type 3 (-325/+500) paste is 1.0 to 1.8 mils, compared to 0.8 to 1.5 mils solder ball diameter range for a Type 4 (-400/+635). As long as the printing process is monitored and proper stencil maintenance procedures are followed at regular intervals, printing with Type 3 paste should be ok.

Metal Content: We have all been accustomed to using a standard 90%metal content, or thereabouts. Now, with the imminent phase-out of Pb bearing soldering alloys, we must realize that this �standard� is changing. Typically, Pb-free solders will exhibit slightly reduced metal content. Although the mass ratio has changed, the alloy volume ratio between eutectic Sn/Pb and Pb-free pastes is nearly the same to achieve similar rheological conditions. Apart from this, steer clear of elevated metal content pastes, as these stiffer and higher viscosity formulations to not transfer well into and out of small 0201 apertures.

Viscosity: 900 to 1000 kcps solder paste prints well for 0201s and is recommended.

Alloy: The printability of several commercially available Pb-free solder pastes print just as well as the best eutectic Sn/Pb solder pastes. There is little, if any, changes to be made in the operation of the printer between using Pb-free and Sn/Pb pastes.

- Jeff

This message was posted the OnBoard Forums

reply »