The actual contact area depends largely on the geometry and condition ofthe probe tip. A tip which is blunt (either by design or because it has become worn or flattened during use) will make contact over a larger area than a sharp tip, resulting in lower contact pressures and reduced ability to penetrate contamination layers.
Table A and Diagram A show calculated contact pressures (spring force divided by contact area) for the 100-25 Series spear point probe contacting a flat surface. The calculations are based on nominal spring forces and a circular contact area ranging from .002 [.05] to .005 [.13] in diameter.
Note that the contact pressures shown here are significantly higher than the yield strength of solder, and will cause the solder surface to deform. As a sharp point initially bears against a solder pad, the solder will yield, the area will increase, and the contact pressure will drop until the pressure reaches the yield strength of the solder. As the solder yields, the oxide or flux which covers the solder is disrupted, and uncontaminated solder is brought into contact with the probe tip, allowing electrical contact to be made. The result is a witness mark left in the solder pad.
For multiple-tip point styles contacting flat pads, make the worst-case assumption that all tips will be touching the pad, and multiply the surface area by the number of points. For example, in the case of the triad point the contact pressure would be one third that of the spear point listed in Table A.
A chisel contacting the rim of an open via is a special case (a chisel is essentially a pyramid with a triangular base). The area of contact is easy to envision – it is spread over three regions, which are the points of contact between the rim of the hole and the three ridges formed by the intersections of the chisel faces. But the force behind the contact is actually higher than the spring force. This is because the reaction force is perpendicular to the attack angle of the ridge and increases geometrically as a function of this angle.
The vector diagram describes this, but the important concept in the case of chisels in open vias is that contact pressure will increase not only in response to sharper ridge edges and higher spring force, but also as the attack angle becomes more acute.
Table B compares the effect of attack angles on contact force for various chisel point styles. The contact force at each of the three contact points around the rim of the hole is equal to the spring force times the Spring Force Multiplier. The table shows, for example, that a 53 point style (sharp chisel) has nearly three times higher penetrating power than an 03 point style (standard chisel) with the same spring.
This attack angle principle is the same for the various blade point styles (a blade is essentially a pyramid with a diamond-shaped base), but the pressures are higher since there are two points of contact on the rim of the hole instead of three. Blades are the most aggressive point styles for use in open vias. But blades bring another key principle into play – the role of the included angle of the ridge.
The included angle is the angle formed between the faces that intersect to make the ridge. For a blade point style, the included angle is smaller (forming a sharper wedge) than for a chisel. The smaller the included angle, the more the contact surface will deform as it yields. Greater deformation means more disruption of the contamination layer, and therefore more reliable contact between the exposed uncontaminated solder and the probe tip. The end result is that even with contact area held constant, more acutely angled points make more reliable contact through contamination.
It is easier to visualize the effects of included angle with spears than chisels. Consider the case of two spears contacting a flat pad with a thick solder coating. One spear has an included angle of 90º, the other an included angle of 30º. Both have 3.5 ounces of spring force pushing behind them. Since solder yields at about 5000 psi, both spears will penetrate the solder until a conical hole of .007 [.18] diameter (at the top) is formed. At this diameter, the solder will no longer yield, since the contact pressure has been reduced to 5000 psi. This means that the 90º spear will penetrate to a depth of .004 [.10], while the 30º spear will penetrate much deeper to .014 [.36].
The greater penetration will cause more disruption of the contamination layer, and more reliable contact will result. For an extreme case, imagine a spear with a .007 [.18] diameter flat on the end. This spear would not penetrate the solder at all.
Note that sharp spears against thin solder layers can penetrate the solder layer. In such cases, the spear will bear against the substrate and stop before achieving the depth calculated.