Paint by Numbers: Solder Paste Stencil Design To Reduce Shorts

I’ve never had much artistic talent, but when I was a kid I loved those paint-by-numbers kits. Up until I saw those I’d only used watercolors, but I was enthralled with the thick, slow flow of the paint. I may also have killed some brain cells from breathing all the fumes. I was completely obsessed with keeping the paint in the lines, so I didn’t use the brush they gave you. Instead, I traced the outlines with a toothpick that had barely been dipped into the paint, then dipped the toothpick again and dripped paint into the outlines, relying on surface tension to distribute the paint evenly.

Although that level of neuroticism can improve your PCB design, it's probably not healthy to apply in all areas of your life. Try and limit it to one or two places, like getting a totally perfect PCB stencil for your solder paste application.

Solder paste is a common cause of shorting in PCBs since the paste covers a large amount of your board surface and bridges can form easily. However, the solder paste itself is rarely the root cause of the shorting issues. At each stage of manufacturing, there are opportunities for defects to be introduced: starting with the stencil, during the solder paste application, or during reflow.

Solder Paste Stencil Design Guidelines for Clean PCB Layouts

Often, issues with your stencil will cause problems with the application of the solder paste. The stencil is a mask usually made from metal but sometimes out of Kapton (polyimide) for prototypes or low quantity runs. The stencil marks out where solder paste should be applied to the surface and is reused. It designates where solder paste should be applied to the surface like a spray paint stencil. No matter what it's made of, the stencil is reused as long as possible. That might be less than five times for polyimide or thousands of times for a metal stencil.

A stencil for solder paste is like a stencil for spray paint, marking out where you want your material applied.

Even a Solder Paste Stencil Is Subject to Solder Shortage Problems

While it is tempting to think that with a solder paste stencil, you do not need to worry about potential problems or interferences, that is not always the case. Here are two primary ways in which your board may encounter problems, despite using a stencil:

• Uneven solder paste: Pads that have too much solder paste applied are, unsurprisingly, more prone to forming solder bridges between neighboring pads. If too much paste is being applied consistently, either across a board or between different boards, you should consider decreasing the stencil aperture dimensions or the stencil thickness in your design.

• Stencil resolution: Another common stencil issue is having poor resolution or jagged edges. If the edges of the stencil aren’t clean, you’re more likely to have too much solder applied, uneven application, or a lack of definition around the edge of the joint. All of these situations are an open invitation to solder bridges shorting your pads together.

Stencils Aren’t All to Blame for Your PCB Design Going from Painting to Palette

Like working with paint, you can end up with solder paste in locations where you don't want it. The finished product is also affected by how the materials are handled after their application, though. Like paint cracking when it dries too fast, solder reflow temperatures have to be just right for a proper outcome.

• Smearing: A less common issue is a solder paste application that smears or slumps on the surface of the board. If you have a smear, you’ll lose clean edges no matter how sharp your stencil. Part of smearing is getting solder applied in unwanted areas. Sometimes this isn’t a major problem—just weakening the joint—but it is easy for the solder to form a short too.

• Reflow: If the temperature profile you use for solder reflow isn’t correct, the solder paste may not be adequately melted when you need it to bond to components. Initial temperature ramp rates are particularly troublesome. Too hot and too fast and your solder paste may flow away from designated pads.

Make sure to optimize your equipment to the exact specifications to avoid damages of excess.

Don’t Be Short-Sighted with Your Selective Soldering

The best way to fix solder paste shorts is to verify stencil quality and reflow recipes with manufacturers early in production. This avoids the reworking of surface areas after excess solder is used and removes the potential for unnecessarily damaging and weakening the surface or neighboring joints. It is possible to heat and remove extra solder, but it isn’t recommended as that, too, may damage nearby surfaces and joints.

When you design your PCB, the stencil is one of the layers in your Gerber file. You should choose a PCB design package that allows you great control over your design, why not consider switching to  CircuitStudioCircuitStudio? Any questions get in touch with us at software@element14.com.

[May thanks to Altium for permission to reproduce this post]