Try this test to determine paste problems.

This month we look at solder paste slump during preheating. It is important to know how much, if any, of the paste slumps like butter on a hot day during reflow. If solder paste does slump, it can lead to shorts, solder balls or solder beads, or cause variations in solder joint volume on selected joints.

Variation in joint volume occurs when one joint acquires more solder from an adjacent joint during reflow due to the paste being linked. Testing of solder paste is well covered in IPC specifications, and equipment is available to test paste and record the results.

Alternatively, if you think you have an issue, a simple shop floor test is to use the existing profile but change the temperatures of the final reflow zones. Setting the final zones to final preheat temperatures will slow the degree of slump. Normally the maximum slump is seen earlier during reflow. Typically, as solder paste is changed or the metal particle size is reduced, slumping can be seen more often.

Tricks to eliminate exposed copper.

This month we look at solder pad coverage. Some quality engineers still want to see solder coverage, regardless of the PCB surface finish. If they see the original surface coating, they become concerned, regardless of what is stated in IPC standards.

In the case of NiAu (FIGURE 1), it is most likely the solder paste will reflow and wet the pad toe area right up to the solder mask. In the case of an organic surface protectant (FIGURE 2), the solder paste will reflow successfully, wetting the termination and pad, forming a reliable joint. However, the solder may not reflow and wet any farther than the original print area on the OSP coating on the pad.

Are parts falling incorrectly from the feeder?

Solder paste escaping gaps in underfill can cause electrical failures.

Welcome to our latest Defect of the Month. This month we illustrate what can happen with area array packages that have been underfilled during rework.

During any rework, it is important to avoid overheating components on the opposite side of the board or adjacent to the parts being reworked. If solder joints surrounded by underfill undergo reflow, or are close to reflow temperatures, the solder will expand/extrude, then flow under pressure through any openings (FIGURE 1). (We have captured solder reflow in underfill during reflow simulation in x-ray investigations for a customer at Dage.) Voids in the underfill or gaps caused by expansion of the component can result in joints becoming intermittent. Such joints can fail at any time during the product’s life.