No-Clean Flux Applications

Reductions in the use of ozone depleting solvents (CFCs) for board cleaning have led to increased use of no-clean fluxes. Properly tuned fluxing processes with modern low-solids fluxes result in boards that are readily testable. However, the real world often presents Test Engineers with no-clean boards coated with layers of contamination ranging in texture from hard and brittle to soft and gummy. The following summarizes recommendations for probe selection to make reliable contact through contamination layers. This information is drawn from industry studies and from customer feedback about probes in production environments.

The principle behind making electrical contact through contamination is that higher contact pressures better displace and penetrate contamination, resulting in higher reliability. With spring probes, contact pressure is affected by both spring force and contact area. Sharper points will reduce the contact area, thereby increasing the contact pressure; and higher spring forces will increase contact pressure as well. But simply putting the strongest spring behind the sharpest point is not always the solution – there are other factors to consider:

  • Although using higher spring forces will improve contact reliability, the ability of the test fixture to overcome the spring force and actuate fully must be considered.
  • The tip style chosen must be physically stable on the surface being contacted. For example, although a sharp chisel point may be ideal for a via or pad, using it for a through-hole component lead will result in glancing and side loading.
  • Ultimately, the selection of tip styles is a subjective decision – experienced Test Engineers will often have different preferences for the best tip style to use on a given contact surface. Testing and field use have shown a particular group of tip styles to be well-suited for contacting heavily contaminated contact surfaces.
  • Steel tips are harder and will remain sharp longer than beryllium copper, so steel is recommended for applications requiring greater durability. Many of the tip styles recommended are available in various combinations of beryllium copper or hardened steel.
image of various tip styles