3. Differences in Inch and Metric Spring Probes

3.1 Imperially sized test probes

Grid spacing (pitch) in printed circuit board design – and therefore also in test engineering – is widely dominated by the inch based imperial measurement unit. Almost all semiconductor devices feature pitch values of 100 mils, 50 mils, 20 mils, etc. And the entire board industry is talking about 100 mil pitch or fractions of it. Irrespective of the fact that – besides the United States – meanwhile only Myanmar and Liberia are using the imperial measurement system (inches, feet, yards, miles), this standard will still dominate the world of electronics for a very long time.

So US manufacturers have developed sizes for contact series which are oriented towards common standard sizes of drills and other tools in the United States. Many of these designs have been globally introduced and adopted. Nevertheless, the metrically thinking world faces certain difficulties when it comes to purchasing articles with partly unusual sounding dimensions. Frequently, data sheets of our American competitors indicate dimensions like “drill size #50”. The average European will at first only shrug his or her shoulder. Inch based dimensions can be easily converted. But what on earth is a “drill bit #50”? 

Every America expert has the answer: in the days of massive immigration to the United States – also known as the time of economic miracle – countless people from different countries speaking different languages found jobs in America’s factories as semiskilled workers. Communication problems were a daily business, the error rate increased. Pretty soon each area of tools and working materials got its own naming system. Drill bits, e.g. simply have been identified using numbers and not – as is usual in Europe – by their exact size. These numbers  (“gauges”) provided a unique identification and have been understood by everybody. So e.g. a drill #50 is a drill bit with a diameter of .070 in (1.778 mm). Stating “#50” in a work instruction was much easier to handle. Similar gauging systems can be found for cables and wires which are simply numbered according to the AWG system (American Wire Gauge). The “metrical world” again prefers to indicate the actual cross-sectional area throughout its standards. But now back to spring contact probes ...

Due to the above mentioned definition of drill bits by their gauge numbers (let’s stick to #50 to continue the story), bit #50 will produce holes of approx. 1.778 mm. Well, not exactly 1.778 mm. Depending on used material, machine speed, machine type and quality, wear of the drill bit and some other factors, the resulting hole will vary between 1.735 and 1.782 mm. And that is too much allowance for a press fit. What’s more, the receptacle itself has a certain diameter tolerance which has not yet been taken up.

Ingenious inventors in the United States decades ago found a smart solution and provided the receptacle with a bulge called “press ring”. That press ring provided the necessary tolerance counterbalance required for the insertion of the receptacle into the drilled base plate. Additionally, the press-in insertion depth can be varied within a relatively large range and thus provides a certain flexibility in selecting plate depth and concept of fixture construction.

Based on that principle decades of further development brought up spring contact probes and receptacles of different sizes that all shared a large similarity with each other: 

  • The receptacle features one or more press rings
  • The spring contact probe has an overall cylindrical housing, i.e. no collar or other limit
  • When the contact probe is inserted into the receptacle, it slides down to the ground of the receptacle

Only a few exceptions prove the rule. What are the main differences compared with metric designs?

3.2 Metric Test Probes

The basic functional principle of metric and imperial probes is identical for both types:

The receptacle is solidly attached and wired and stays unchanged – even if spring probes are replaced. The spring contact probe consists of a barrel as housing, a pressure spring and the contacting plunger. The metric world doesn’t know about “drill bit no. 50”, and even the dimension of 1.778 mm would pose us problems and provoke questions like: “why not 1.78 mm, or better 1.80 mm, or 2.00 mm at best?” And that was exactly what happened then. While in the United States several manufacturers supplied the growing market with the usual styles, at the same time a different system developed in Europe which was based on metric standards. To be precise, it’s not perfectly correct to speak of a “metric contact”. It is rather a “spring contact probe for metric installation standards.” It doesn’t matter, whether the pitch reads 2.50 mm or 2.54 mm (the aforementioned 100 mils US standard). As long as the probe fits and the clearance between contacts is sufficient to prevent shorts, the exact pitch doesn’t matter at all. For the sake of simplicity we will stick to the term “metric test contacts”. Let’s have a closer look:

First obvious difference: Instead of a press ring the receptacle features a limit collar at its top end. Clear statement: The receptacle shall be completely pressed-in to its limit collar. The spring contact also features a collar. Again: Insert the probe into the receptacle to its limit collar. Hence the mounting height of the contact tip above the base plate is exactly defined. Okay, but the imperial system from the American colleagues shows that practical installation flexibility in defining the resulting height. How does it work here? It’s that simple: The spring contacts optionally feature different collar heights, and additional spacer tubes are available, typically in incremental steps of 1 mm. And there are several series which have the same diameter but differ in length and travel. So height adjustments are accomplished easily and reliably.

3.3 Effects on the application, examples of application

Having briefly introduced both “worlds” including their histories, we will proceed to a comparison of advantages and disadvantages of both “counterparties”.  In fact, they aren’t exactly counterparties, but complement each other.

1. The receptacle with press ring shows higher flexibility during insertion but suffers from two disadvantages:

a) Over time, after many agitations it may slide down a bit in the drilled hole. Installation height changes and the adapter may lose its function.

b) The diameter of the drilled hole must be slightly larger than the outer diameter of the receptacle. The receptacle is finally fixated by the press ring. That way a tiny little  wobbling may arise and at the tip of the inserted spring probe – after a lever arm length of 10 to 16 millimetres – it would show up clearly visible as an offset from the centre axis amounting to several tenths of a millimetre. In other words: The pointing accuracy of the probe is questioned.

2. Receptacles arranged in a pitch of 100 mils (2.54 mm) could use drill diameters in excess of 1.78 mm without running the risk of generating shorts with adjacent pins. Indeed, the diameter of the drilled hole for the metric type receptacle is 14 percent larger compared with the imperial standard type. And the difference for spring probes reaches 20 percent. For current carrying contacts that difference is substantial (larger cross section) and for mechanical stability the distinction is equally obvious: The plunger of a 100 mils imperial standard pin has a diameter of approx. 1.0 mm. The comparable metric type features a diameter of 1.34 mm. The larger diameter of the barrel not only allows for stronger and more durable pressure springs. It is much more difficult for lateral forces to break a 1.34 mm plunger instead of a 1.0 mm type.

3. By the way, when breakage is discussed – and breakage can’t be avoided with certainty – another small but subtle difference comes up: Whereas the remaining part of a broken imperial-type contact sits deep inside the receptacle and is hard to remove (cork-screw principle, but only applicable with 1 mm diameter types) you can easily grip the collar of a metric contact barrel and pull it out using a pair of small tweezers or pliers.

4. The metric receptacle requires very precise drilling as it has no press ring to compensate for tolerances. However, we have added so called “press-fit zones” to our standard receptacles S 30.00-xx and S 50.00-xx. The press-fit zone is an area with 6 to 7 millimetres in length below the limit collar which shows higher flexibility than the remaining cylindrical part of the receptacle. Particularly when hard FR4 base plates are used, this solution can demonstrate its strengths. You can safely use a 2.0 mm standard drill bit – the receptacle will perfectly fit. Some products on the market require special sizes of 2.01 or 2.02 mm diameter drills. Purchase is unnecessarily hampered.

If you now got the impression that we – for any reason – have favourably described metrical types compared with the imperial types: this is deceptive. We are not obliged to any system. It is just an objective description of things as they are. Finally, you decide which system to use for your application.