Although modern solder pastes are much more resistant to local environmental conditions than their predecessors, significant differences in performance can still be observed when temperature and humidity are not controlled. When new products are manufactured at a variety of worldwide sites, substantial problems with product quality can occur. By identifying potential problems at an early stage, process adjustments can be made prior to implementation.

Temperature and humidity can vary significantly from location to location and from season to season. In cold climates during the winter months, relative humidity can fall to below 10 percent. In contrast, in humid climates in the summer, relative humidity can exceed 70 percent. Depending on the plant's location, temperatures, in Fahrenheit, can easily vary from the mid-60s to the low 90s. Even with the latest generation of solder pastes, these environmental extremes can substantially affect solder performance, an important consideration when setting up evaluation programs.

Evaluating Paste

A simple way to determine the susceptibility of solder paste to humidity is to look at solder balling performance. Recently, we tested four pastes for solder balling: two no-clean pastes and two water-soluble pastes. For each paste, we printed a deposit 5 millimeters in diameter on a ceramic coupon using an 8-mil stencil. The deposit was then conditioned in a temperature and humidity cabinet before being reflowed on a hot plate at 250 C for 5 seconds.

Both of the no-clean solder pastes performed well at a low relative humidity. However, Paste A had much less resistance to high humidity than Paste B. After humidity aging, Paste A produced significantly more solder balls.

A similar but more pronounced effect was seen when we repeated the test with water-soluble solder pastes. Even the best-performing product, Paste D, survived only a few hours at high humidity. Moreover, survival time will be even less with deposit sizes that are closer to those used on actual production assemblies.

This simple test can quickly and easily differentiate between solder pastes that may, at first glance, appear the same. Having identified that a paste is susceptible to humidity aging, more detailed research can determine the effect of humidity on the assembly process.

Many water-soluble solder pastes have a strong affinity for water. This hydrophilic nature makes water-soluble pastes susceptible to humidity-induced slumping. When tested at 47 percent relative humidity, the two water-soluble pastes may appear indistinguishable. However, after exposure to only 30 minutes at 80 percent relative humidity, some pastes can exhibit significant slumping that is more than sufficient to cause solder bridges.

The effect of temperature on a solder paste's rheology (the flow characteristics at low and high shear) is seldom characterized beyond its long-term effect on storage. Rheological changes caused by relatively small temperature increases can dramatically affect process yield by reducing print definition and increasing bridging, solder balling, and mid-chip beading. Such problems can easily be exacerbated by suboptimal printer settings.

The reduction in low-shear viscosity observed over a temperature range as small as 5 C or 9 F can increase the frequency of print bridges on a component with a 16-mil pitch. To maintain print quality, engineers would have to increase the frequency of wiping the underside of the stencil by 20 percent.

Tack force, abandon time, and stencil life may also be affected by the ambient temperature, so these parameters should also be considered when determining process guidelines.

These simple tests show that pastes with very similar performance under moderate temperature and humidity can show dramatically different performance when run under high humidity and temperature. Thus, before approving a solder paste for global use, electronics assemblers must consider the likely local environmental conditions at individual manufacturing sites around the world.