Tip Style Selection

Printed Circuit Boards (PCB) are an essential part of the many electronic products we rely on every day. Both in-circuit and functional testing after manufacturing are critical steps to ensuring their dependability. QA Technology’s test probes are an important part of this process, offering the best possible electrical contact with each board being tested.

While there are many probe factors that affect its ability to make reliable electrical contact, proper tip style selection is likely the most important one. Incorrect tip selection could cause false failures where no defects are found (NDF), lower your first pass yields (FPY), or in extreme cases, damage the unit under test (UUT). Selecting the right tip can reduce the overall cost of test by increasing the output of units being tested and extending the life of your test probes.

Test engineers often have very different experiences as to which is the best probe tip style to use for any given contact surface. QA offers a wide range of styles to support the various board test applications known in the industry. The following considerations will help you select the right tip style for many of today’s common test targets.

It is important to consider the size, shape and other features of the test targets. There are various PCB targets such as pads, vias, leads, posts, and solder bumps, which range in probe-ability from easy to difficult.

QA’s tips are made from either beryllium copper (BeCu) or steel. Both are gold plated over nickel. The hardness of steel ranges between 58 and 60 on the Rockwell C scale, 38-42 for BeCu. As such, steel will have reduced wear and remain sharp longer.

While there is a difference in the average resistance of less than 10 mOhms between the two base materials, it will not affect the vast majority of test applications.

QA offers many different tip styles, each with their unique geometry. The large number of headed or headless choices supports the wide variety of test applications

Geometry and sharpness will determine the ability to penetrate contamination layers. A tip that is blunt or has dull edges will make contact over a larger area thus reducing its ability to penetrate contamination layers on the test targets.

A tip that has a sharp point or steep cutting edges applies a higher pressure against the solder causing it to yield. As the solder yields, any oxides or remaining flux residues are disrupted, allowing better electrical contact.

TEST PADS, VIAS and THROUGH-HOLES come in a wide range of sizes, surfaces, and shapes. They can be solder pasted, unpasted, concave, flat, or convex (dome).

Vias and through-holes in a PCB are typically used to pass a signal from one layer to another. When these are to be used as test points, it is necessary that they are not solder-masked so that they can be easily probed.

A lead is a terminal on a component. They could be different lengths, straight or bent. Smaller leads will require tip Styles with closely spaced cutting edges to trap the leads. A post is larger in diameter and more rigid than leads.

Multi-point tips are designed to capture a target, such as a lead or a post. Some styles have steeper valleys between the tips, creating a self-cleaning feature where it allows contamination to escape.

Technology provides test point accessibility, using a probe to contact a variety of solder beads or bumps that are placed on a trace where the solder mask has been removed. In addition, by placing an excessive amount of solder on vias or test pads, a round dome of solder may also need to be probed.

In addition, QA offers many specialty tip styles for other important, but less common applications.

• Our CHISEL TRIAD “38” tip style is designed specifically to be a multi-purpose solution for boards that may or may not have leads present during testing. When a component is present, the 6 peaks aggressively capture the lead within the cutting edges. When the component is absent, the point of contact varies based on the diameter of the through-hole. With smaller holes, the three sharp points will hit the face of the via ring. When contacting a larger through-hole, the three sharp cutting edges will contact the inside of the hole, giving optimal electrical points of contact.
• Our INSULATOR “89” and “99” tip styles are used to test the presence or absence of a component on a PCB. An insulating sleeve around the probe keeps the probe electrically isolated from a flat contact surface, but allows the probe to make electrical contact with a lead or post.
• Our CONNECTOR “70” tip style is used to test a female pin connector where the rounded tip makes contact with the inner beams. A shoulder of the tip acts as a stop to prevent the tip from sliding completely through.
• QA offers the various tip styles for probes used in tester systems known throughout the industry. In our catalog and website, QA provides complete probe details that match tester manufacturers’ series

• When new fixtures are built, we recommend verification of the selected tip styles once all board elements are identified. Initial processes and solder types can vary from the prototype PCB design to the production boards. In preparation for the launch of production UUT’s, tip styles may need to be changed to a different geometry or a more aggressive style based on the cleanliness of the board.
• If the tip style recommendations do not generate good electrical contact, look closely at the witness mark that the probe tip should have made on the test target. Poor pointing accuracy, TIR of the probe assembly, as well as the diameter of the target can be attributed to missing the intended target. While certain tip styles may work well on flat or concave vias, when a dome is present, and pointing accuracy is not good, they may glance off the target and hit the solder mask.
• If your board process causes the solder mask to be below the test point, a multi-point tip may work even if you do not have good pointing accuracy. If your solder mask is higher than your target, one or more of these multipoint tips could prevent the others from making electrical contact. In this scenario, change to a single point probe to hit the target.
• In cases where you cannot improve the pointing accuracy on leads or posts, select a larger diameter tip style to maximize the ability to hit the test target reliably.

In summary, it is critical to select a tip style with the right number of points of contact, the correct angles, and sharpness. Improving the electrical contact to your test targets will help increase probe life and improve your FPY. Higher yields mean less time troubleshooting false failures, faster through-put, and ultimately, lower overall cost of test.