Ensuring chemical compatibility isn’t always as straightforward as avoiding throwing oil on a fire. When I was an undergrad, I worked in a lab with a heavy emphasis on etching. Processing steps use a range of chemicals, and sometimes these chemicals would not be compatible. One day, a student coworker of mine was hearing weird sounds while etching. After a few minutes, he pulled the fume hood shut and encountered a small explosion from incompatible chemical agents. Thankfully he wasn’t hurt; however, several weeks of work were ruined all because he must have left a speck of something on his wafer between processing runs.
While contamination doesn’t always have as dramatic of a consequence as an explosion, they almost always have adverse and frustrating effects. On PCBs, contamination usually is more subtle—impairing PCB functioning, damaging insulation, and causing shortages—the end result is still just as unfortunate for your hard work. Knowing what contamination is and how it comes about is the first step to avoiding it in your future processing and designing.
Effects of Contamination
Contamination just sounds bad, but what does it mean for your PCB? It depends on the contaminant, but generally, it degrades elements enough to impair functioning. Electrical issues are the most common, with corrosion damaging insulation and conductors. This might result in power supply loss, failure, or even shortages. While you can check your power delivery network for damage, it is best to do what you can to avoid contamination damages first.
Electrochemical migration is one of the biggest concerns, though. When ions are left on your board, usually by an etchant or other chemicals, an electric potential can cause “reverse plating” that pulls metal away from where you want, and redeposits new “traces”—think of them like PCB gremlins only there to cause you trouble and strife in their tiny, damaging forms. These little traces (called dendrites) create bridges that make shorts all over the PCB. They also burn off and reform frequently, so they are difficult to track down and diagnose.
Contamination on a PCB
Contamination comes from anything in the environment that is additional and should have already been cleaned off, from skin oils to remains of solutions. Depending on the type of contamination, and where it happens, a variety of issues can arise. The most common is degradation and accelerated deterioration of metalized surfaces, polymers, and masks. The areas around wire bond interconnects are especially vulnerable. Some other process steps which are common areas for contaminants are as follows:
Solder Paste Processing: The solder paste applied to the board can also get contaminated. The solder paste itself should be fine if it’s purchased from a reputable supplier, but contamination can get into the paste during the application and reflow process. The stencil printer and PCB finish are also potential sources for picking up and spreading contamination to your solder joints. Residue from flux can also be left on the PCB and function as a contaminant. Any impurity or foreign material is especially problematic for solder because it can cause tin whiskers, which will create electrical shorts between solder joints or into traces.
Etchant and Chemicals: If you look into PCB manufacturing processes, there are a variety of acids and potentially damaging chemicals that get used during processing. Etching is an obvious source of acids, but flux, electrolytic solutions, and water-soluble soldering can all leave chemical residue on the board after their application and use. After chemicals are used for any process, the board needs to be cleaned to ensure that no residue remains on the board to contaminate the next stage of manufacturing.
Some contaminants will just burn off the board, but it’s hard to tell how bad it is when your PCB is smoking.
Preventing contamination is one of the best practices for you and your PCB design. Unfortunately, humans are a common source of contamination, with skin cells and hair landing on boards throughout manufacturing, especially if they aren’t in a clean room. Human handling also leaves oil from skin and cross-contaminated sources on PCBs which are particularly difficult to remove. Taking precautions to ensure that your skin or hair does not contaminate the board, and by processing your product in a clean environment is a good first step for avoiding these contaminations.
Making sure your manufacturer also has a clean production environment with strong process controls is the second step to prevent contamination. Verify the quality of materials being used, especially solder masks, and solders that contain flux. Some manufacturers will also test for chemical residue during processing. The most common method is to submerge a sample board in solution, then measure the ionic change in the solution as chemicals dissolve off the PCB. You can learn more about the specific tolerances for contamination in the standard IPC-TM-650 Method 2-3-25 for ionic cleanliness if you are having chemical residue issues.
PCBs need to be cleaned between chemical applications, although a hose isn’t always the right approach.
Contamination on your PCB can have disastrous consequences for your work, and waste valuable time and money. But with proper care and processing management, you can take preventative measures to ensure that PCB contamination plays as little of an influencing factor in your design manufacturing as possible. Still, it’s best to avoid problems up front by using good tools and processes, starting with your design software.
For smarter communication with manufacturers regarding your component suppliers, use Altium’s CircuitStudio software and make sure your work goes down with the good kind of bang. If you want to talk avoiding contamination of your PCBs further, consider talking to an expert at Altium now.