Ask Tony about Trapped Resist

My outer layer resist stripper process works fine until I run jobs with very fine lines and spaces. The dry film sometimes gets trapped between circuits causing etching problems. How can this situation be avoided?

Good question. In today’s market, it is quite common to have traces or spacing of less than 2 mils. If the bulk of your work is typically 4 mil lines and spaces, a particle size of 100 microns would most likely be sufficient. But when spacing drops to 50–75 microns or the traces are over plated and trapping the resist, the same resist strip parameters will most likely not work.

What needs to be done is to set the process up for the smallest particle size you will need, not a process that addresses the bulk of the work. The process needs to generate a particle for the most difficult of applications, or putting it simply, a particle size that is smaller than the spacing.

First of all, you need to work with a resist stripper that is designed to generate smaller particles. Many of the products on the market are old and outdated and were developed 5–10 years ago when typical spacing was 8–12 mils. Once you have identified the stripper that is compatible with today’s films and board designs, you are ready to begin testing.

Run 3–4 different concentrations and temperatures to see which parameter creates the best particle size for your needs. Once you have identified the right chemical parameters, a good horizontal spray machine will do the rest of the work. Make sure you provide adequate contact time allowing for a break point of 40–60%. Very high spray pressures of 2.5–4.0 bar are preferred in these situations. Fan shaped nozzles should be aligned properly to insure the chemistry is uniformly distributed over the panel. If this is all done well, you should have successfully eliminated any possibility of problems etching the copper from between traces.