Working for manufacturer engineering couple years. Sharing the manufacturing experience, skill, DFx, design, and information. Be care of this blog is personal share and content may not be 100% correctly.
Recently, our company started a new project, and the R&D team has been pushing PCB layout requirements to the extreme. As PCBs size get smaller, the solder mask (S/M) design must also shrink accordingly. However, our current PCB manufacturers struggle to meet these tighter tolerances, and those who can do it demand a price increase. The moment extra cost is mentioned, everyone hesitates and sticks with the existing PCB suppliers, resulting in solder mask misalignment that extends beyond the pads.
What Problems Can Solder Mask Misalignment Cause?
If the solder mask misalignment affects BGA pads, it can reduce the exposed pad area for BGA solder balls. This can ultimately lead to solder shorts. But how does a smaller pad increase the risk of shorts?
The key issue lies in the stencil design. The apertures of stencil are fixed, meaning the amount of solder paste deposited for each pad is theoretically the same. If every PCB batch has consistent BGA pad sizes, the aperture of stencil and paste volume can be properly optimized. However, when pad sizes vary between different PCB batches—some remaining normal while others shrink—the stencil still deposits the same amount of solder paste.
When a pad is smaller than expected, the solder paste volume may become excessive relative to the available pad area, leading to overflow. In severe cases, the excess solder can spill over to adjacent pads, causing a solder short.
Why Did the Pad Size Change If the Design Wasn’t Modified?
Think of it like wearing a ski mask that only exposes your eyes. If the mask shifts slightly, it can partially cover your eyes. In this analogy, the “eyes” represent the PCB pads, and the “solder mask” is the ski mask.
For those still unclear about what a solder mask is, let’s make it simple: the solder mask is basically the green coating on a PCB! Got it? If not, just grab a PCB and look at that large green area—that’s the solder mask! Its purpose is to cover exposed copper traces and areas that don’t need soldering, preventing short circuits and oxidation. (Note: While green is the most common color, some PCBs have black, red, or other colored solder masks.)
In Workingbear’s new PCB design, the solder mask tolerance was set to +/-1 mil (0.0254 mm), but the PCB manufacturer’s solder mask printing capability is only +/-2 mils (0.05 mm). As a result, the actual print misalignment caused the solder mask to partially cover the pads, reducing their effective size and leading to the problems mentioned earlier.
Another contributing factor is that, to prevent Head-In-Pillow (HIP) defects, we sometimes intentionally apply more solder paste to the outermost BGA balls. This leads to most of the solder bridge issues occurring around the outer ring of BGA balls.
Picture show solder mask misalignment under the BGA, resulting in inconsistent pad sizes.
Picture show solder mask misalignment affecting other components, causing varying pad sizes.
Below picture show excess solder paste due to smaller pads, ultimately leading to solder shorts in the BGA area.
Solutions & Proposal:
Ask the PCB manufacturer to adjust the solder mask opening window position and size for affected pads, ensuring all BGA pads are as uniform as possible.
Modify the stencil design to reduce the aperture size for the outer BGA pads, limiting the amount of solder paste applied and reducing the risk of shorts.
Recently, our company started a new project, and the R&D team has been pushing PCB layout requirements to the extreme. As PCBs size get smaller, the solder mask (S/M) design must also shrink accordingly. However, our current PCB manufacturers struggle to meet these tighter tolerances, and those who can do it demand a price increase. The moment extra cost is mentioned, everyone hesitates and sticks with the existing PCB suppliers, resulting in solder mask misalignment that extends beyond the pads.
Leave a Reply