When it comes to components, the devil’s in the details.

The interdependency of all elements of stencil printing to achieve a high-yield result can be overwhelming. When you consider just the things we’ve addressed in this column – from tooling to warped boards to stencil tension to solder paste types and everything in between – it’s clear controlling the printing process is a balancing act that takes a fair level of expertise (or at least a robust self-learning system!). Nothing underscored this reality more than a recent experience with a customer.

When printing a relatively high-mix board that contained a range of components from 01005s to large QFNs, the customer was experiencing different results from the front and rear print strokes. One direction printed relatively well. But the reverse direction left smearing on the stencil and, according to SPI results, non-optimal paste transfer efficiency. The assembler tried troubleshooting the issue, but nothing seemed to work, other than applying different print pressures on the front and rear strokes in order to get the same output from the process. This, of course, is not normal for solder paste printing.

The basics of bare board laminate selection.

Selecting the right materials for a printed circuit board assembly is essential to ensuring the robust reliability of the product and circuitry throughout the entire life of the equipment. In other words, it’s not just how you build it, but what you construct it with. A wide range of substrate materials is available, with different specific properties, and these properties are tied to the expected performance and environment for the circuit being built. Will the PCB assembly be a high-power circuit? Harsh environment capable? High-density?

Engineers specify the material for PCBs with great care and detail. TABLE 1¹ lists laminates by their industrial names and material types, and specifies material characteristics related to each type. However, to reach that level, it is necessary to understand the various characteristics such as thermal decomposition temperature (Td), glass transition temperature (Tg), dielectric constant (Dk) and dissipation factor (Df) related to PCB materials. Although such tables are important, it is easier to start with a higher-level perspective of selecting materials depending broadly on their actual use, based on a comparison of PCB materials.

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.

Is the reflow profile the problem? X-ray can help.

Looking through some recent x-ray images of what I would call “good bad” boards (at least, that is what they are for me, as they showcase “good” examples of how certain “bad” types of failure look under x-ray inspection), I came across a number of different issues that are different from “traditional” BGA/QFN problems mentioned in this space before. To wit, I noted some images showed where solder paste had not reflowed under the devices, and there was the presence of foreign object(s), such as discrete components, trapped under the package.

FIGURES 1 and 2 show how unreflowed solder paste typically looks under QFN joints in an x-ray image. In the magnified view (Figure 2), individual grains of the solder paste are seen clearly, instead of appearing as a typical single smooth continuous joint. The cause of this is probably not an insufficient reflow profile. Rather, it is more likely the board has not been reflowed at all. As it may be desired, or necessary, to x-ray inspect (representative) boards after placement but before reflow as part of a quality control process, it is worth noting this characteristic shape of the solder under the components is different from what would be expected post-reflow.