Products

There is generally more than one suitable tip style for a given contact surface, and experience is the best way to choose which tip to use. The choices can be narrowed by reviewing the following list of recommended point styles and accompanying notes.

The points shown are simplified versions of each style - a given style may be available in headless or headed; beryllium, copper or steel, or with variations in geometry and sharpness.

Self-cleaning crowns are a good choice for contacting flat solder pads. Serrated and crown points distribute the force over a greater area (reducing the likelihood of marking the pad) but require additional maintenance.

Use a spear, chisel, sharp needle or triad to increase tip contact pressure for reliable testing of heavily oxidized or flux-coated solder pads.

When contacting gold plated pads, spherical points are recommended so that marking is minimized.

Choose the point style which will be the most stable on the lead in order to minimize sideloading. By minimizing sideloading, probe life will be increased.s Point styles with fewer tips and/or sharper internal geometry (such as the triad) are best used on contaminated leads.

Serrated point styles are generally the most stable on long leads.
Standard crowns (valleys cut straight across the point) are more stable than self-cleaning styles (valleys sloping down and out).

Cups can be used on extremely long leads but are likely to require more maintenance. (See also "Terminals and Posts").

Use a three-sided chisel or six-fluted star to contact contaminated holes reliably.
Chisels have fewer contact edges than stars and therefore higher contact pressure, so chisels are preferred when penetrating contamination.

For heavily contaminated or oxidized holes the blade point will provide increased penetrating pressure since contact is made on just two sharp cutting edges.

The chisel triad point style is a good choice for contacting both filled and unfilled vias.
The spear point style is used when the rim of the hole must remain free of marks. Contact is made on a circle rather than on sharp edges.

Since cups can easily collect dust and contamination from the Unit Under Test, they are best used in a horizontal or downward-pointing orientation.

Self-cleaning crowns or serrated points will require less maintenance than a cup if used in an upward-pointing orientation, but are more likely than a cup to glance off the post.

Flats are stable on this type of contact, but have little ability to penetrate contamination. Use flats in clean conditions and with higher spring forces.

Reductions in the use of 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 which 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 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 force will increase contact pressure as well (for more information, see the following pages). But simply putting the strongest spring behind the sharpest point is not always the solution - there are other factors to consider:

1. 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. (For assistance in these calculations, request the Applications Note Spring Force Considerations).

2. 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 hole or pad, using it for a through-hole component lead will result in glancing and side loading.

3. Ultimately, the selection of point styles is a subjective decision - experienced test engineers will often have different preferences for the best point style to use on a given contact surface. Testing and field use have shown a particular group of point styles to be well-suited for contacting heavily contaminated contact surfaces:

   • For flat pads, use Sharp Chisels (QA point styles 53 and 63), Sharp Triads (08), and Spears (31 and 41).

   • For leads, use Self-Cleaning Crowns (34 and 24), Tulips (17) and Sharp Triads (08).

   • For holes, use Sharp Chisels (53 and 63), Chisel Triads (18) and Blades (51 and 61).

Steel plungers are harder and will remain sharp longer than beryllium copper, so steel is recommended for applications requiring greater durability. QA's sharpest point style, the needle (31), is available only in steel. Many of the point styles listed above are available in various combinations of beryllium copper or hardened steel.