Solder Paste Problems You Never Saw Coming
Most people building hardware are familiar with using wire solder and a soldering iron (if not, a quick search will turn up a trove of helpful articles). However, modern commercial electronics are assembled largely with solder paste, not wire solder. Electronics manufacturers are well versed in using solder paste and the process nuances of storing, inspecting, and qualifying solder paste for use in their assembly process. These two groups are at far ends of a spectrum that leaves a wide middle ground of labs, startups, and advanced enthusiasts who might benefit from using solder paste in some cases.
If you're assembling small quantities of boards, prototypes, or hand-applying solder paste during rework, you might be unfamiliar with some of the problems you could encounter with your solder paste, how to identify them, and how to prevent them.
What's in solder paste?
Check out the picture at the top of this post. You can see the solder paste on the pads of a small package (maybe a SOT-5). You can see the solder paste is made up of tiny beads of metal (solder) swimming in flux. That's solder paste. (Compared to normal wire solder which has the metal solder on the outside and a core of flux in the center.)
Types of solder paste
The formulations of solder paste follow the same categories as does normal solder. It's usually broken down by details of the solder allow and of the flux:
- Typical solder alloys:
- Sn/Pb (tin/lead) (Not used much because of RoHS)
- Sn/Ag/Cu (tin/silver/copper) (Lead-Free)
- Sn/Sb (tin/antimony) (Lead-Free)
- Typical flux compositions:
- Rosin (three varieties: Non-Activated (R), Mildly Activated (RMA), Activated (RA))
As a reference point, good old-fashioned wire solder is typically 63% tin, 37% lead, with an RMA rosin flux core.
Each of the alloys and each of the fluxes is formulated for different benefits, but also has different environmental requirements for storage and for use. And operating outside of these tolerances can lead to some pretty mysterious and frustrating problems.
Solder paste related defects are often caused by high heat and/or humidity when they are stored or on the day they're used. And some solder pastes are more susceptible than others. For instance, as little as 30 minutes exposure to 80% relative humidity is enough to cause some water-soluble solder pastes to become runny and spread out (slump, as it's called) when you stencil it onto the PCB pads.[1:1]
Common Solder Paste Defects
Here's a short list of common solder paste defects associated with storing or using solder paste in high heat/humidity environment:
- Slumping. (Solder becomes runny and spreads out before it's melted, often causing solder bridges).
- Solderballing. (Solder doesn't stick to the component/PCB but rather pulls together into tiny balls).
- Voiding/Outgassing. (heat from the reflow process drives out water vapor that leads to voids and grainy joints).
There are problems associated with too low humidity as well. For instance, solder paste can become too dry and stiff if flux solvents are allowed to evaporate. This can cause insufficient solder paste to be stenciled onto the pads of the PCB.
Recommended Temperature and Humidity
You should always follow the manufacturer's recommendations on the particular solder paste product you buy, but here are some general guidlines:
The recommended environment for using most solder pastes is 21-25°C (70-77°F) and 35-65% relative humidity.
The recommended storage for solder paste is typically 0-5°C (32-40°F) although some don't require refrigeration so check with your solder paste manufacturer for directions.
Most electronics is assembled with solder paste and manufacturing facilities have the benefit of tuning their process to get good yield and high quality results. Using solder paste for occasional rework or in the lab/prototype environment brings additional uncertainty. So do yourself a favor and follow the manufacturer's environmental recommendations to help avoid unexpected problems.
What else can you do?
Prepare yourself with a familiarity of common problems and watch for imperfections in your work. ^3 is a great summary of common problems and typical causes. Also, take small steps to avoid problems. I buy my solder paste from Amazon.com in 25 gram syringes (here) and keep them in the refrigerator so they don't have a chance to get old and absorb moisture.
Image: "Solder paste" by Autopilot - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -https://commons.wikimedia.org/wiki/File:Solder_paste.jpg