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: