Surface Preparation and Finger Prints. How bad are they?

In preparing a surface for paint / glue / thermal paste / soldering / whatever, there is one thing to remember. A normal finger print is of similar thickness to cling wrap (0.01mm). A thick (eg post pizza) print, where the ridges are no longer visible, is closer to the thickness of a sheet of paper (0.1mm).

Rule of thumb. If you would you would not be happy with this kind of material on the surface of whatever you are coating, remove your finger prints.

  • So a very thin coat of paint is 0.02mm, but often thicker.  So wash your hands, then it’s probably OK to touch. But don’t “Pizza and Paint”.
  • Thermal paste on your CPU is 0.07mm  to 0.12mm thick, So Pizza hands are a definite problem. A single normal finger print may well produce a small but noticeable effect to cooling. A well handled CPU with a few prints is probably not good, you would not leave scraps of plastic wrap on the CPU after all.
  • Some glues may dilute your finger print and not care (super glue). While other (silicone adhesives) will probably bond poorly.
  • Solder – The NASA soldering standards (NASA_STD_8739 .3) state: “When handling metal surfaces that are to be soldered is unavoidable, clean, lint-free gloves or finger cots shall be used. “. Many commercial soldering standards also follow this advise for handling of both solder and parts.
    • Whats interesting is that some manufactures are particularly concerned about what is on your hands (moisturisers and hand lotions cited as being particularly problematic to solder).
    • The finger print contamination of solder joints is often resolved by just keeping the joint hot until the solder takes. This extra heating burns of enough contaminate to allow solder to flow, but can damage components amongst other problems.
    • UPDATE: The new IPC standards (IPC-J-STD-001ES), which NASA, and many other companies, have adopted, don’t mention fingers or gloves specifically. They only broadly reference that handling mechanisms stall not contaminate the board or parts.