Putting the Screws to Test: Screw Machine Contacts
By Karl Jalbert, Bishop & Associates Inc.

This month, I’m on the trail to learn what all the screw-machine contact buzz is about. Coming from the fiber optic connector and RF/microwave connector industry, I am certainly no stranger to screw-machine technology, but as far as how screw-machine contacts fit into the test and prototype industry, I must admit I was at somewhat of a loss. So, I did what anyone would do: I called in the experts. With a little help from our friends over at Swissturn USA, Positronics, IDI, Preci-Dip SA, Multi-Contact, and Mill-Max, I now have a much better understanding of screw-machine contacts. In a nutshell, the technology of tomorrow needs to be prototyped and tested today. And when you need absolute precision and reliability, you need solid machined contacts.

First, let me explain the workings of a screw machine. A Swiss-type screw machine is an automatic lathe that has a sliding headstock that holds bar stock—typically a 12-foot-long bar of metal or plastic—and rotates it at anywhere between 1,000 and 10,000 RPM, depending upon the diameter and type of material. The sliding headstock incorporates a collet—an air-actuated part of the headstock that actually clamps onto the bar stock. Separate air-actuated tool holders hold the brazed carbide turning-tool, cut-off tool, and insert-tool. The tools move in and out of the spinning material to create the part’s diameters, as the headstock moves the material forward to create the part’s length. Just behind the tools and the collett, the clamped bar-stock is fed through fixed carbide guide-bushing. The guide-bushing is adjusted so that the bar-stock can slide through it, but tight enough to keep the material from flexing away from the cutting tool. The guide-bushing allows Swiss screw machines to hold very tight tolerances, of one ten-thousandth of an inch (+/- .0001-in.) over long lengths, in relation to the part’s diameter. For example, if you needed to turn a three-inch-long part with a diameter of .100 (± .0001), with an excellent surface finish every time, a Swiss screw machine is the only way to do this. On a conventional lathe, at that length, the material would simply bend away from the cutting tool. Newer Swiss-type screw machines are computer numerical controlled (CNC), while older models are (computer-aided manufacturing) CAM-operated. On these models, a new set of CAMs must be installed for each different part being machined. With CNC machining, you can sometimes switch parts with the push of a few buttons.

Swiss screw machines were developed around the turn-of-the-century in Switzerland to manufacture watch parts. The machines allowed the Swiss to compete with American watch manufacturers by making high-precision parts in high volume. Most CNC Swiss screw machines are made in Japan by Star, Citizen, and Tsugami.

In-Circuit Testing
The advantage of in-circuit testing (ICT) is that most printed circuit board faults arise from problems during their manufacture. For example, the use of an incorrect component, a wrong value resistor, or a diode in the wrong direction can lead to operating flaws. These problems can be easily and quickly located using ICT. ICT equipment works by measuring each component on the PCB and its correct value. When ICs intrinsically fail, one of the major reasons is from static damage. This normally manifests itself in the areas where the ICs reside close to connections to the outside world. These failures can be detected relatively easily using ICT techniques.

ICT equipment consists of two main parts. The first is the electronics. This entails a matrix of drivers and sensors that are used to set up and perform the measurements. There may be more than one thousand of these driver-sensor points. The points are normally routed to a large connector that is conveniently located on the system. The connector is the point of interaction with the second part of the tester—the fixture. Given the variety of boards, the fixture will be designed specifically for a particular board, and acts as an interface between the board and the in-circuit tester. It routes the driver-sensor points directly to the relevant points on the board, using what is known as a "bed of nails" tester—an array of spring-loaded pins, called pogo pins, which accept the circuit board.

Driver-sensors are the active circuits that are used for actually making the measurements. Drivers and sensors are always present in pairs. Drivers supply a voltage or current to enable a node in the circuit to be driven to a particular state, despite the condition of the surrounding circuitry. Drivers often need to force the output of an IC to a given state in spite of the natural output state of the device. To achieve this, the output impedance of the driver must be very low. Sensors are used to make the measurements. Like most other measuring devices they need to have high impedance so that they do not disturb the circuit being measured. This is where the precision of the pins comes into play.

The key to the success of ICT is a technique known as “guarding.” It is very easy to measure the value of a component when it is not in a circuit. For example, a resistor value can be measured by simply placing an ohm meter across it. On the other hand, when the component is in a circuit, the situation is somewhat different. Here, it is most likely that there are other paths around the component that will alter the value that is measured. To overcome this problem, and gain a far more accurate indication of the value of the component, a technique known as guarding is used. In guarding, the nodes around the component under test are earthed, and in this way any leakage paths are removed, and more accurate measurements can be made.

ICT Fixtures and Connections
The most common way to gain access to each node on the board, so that the test may be carried out, is to utilize a “bed-of-nails” fixture. The board is accurately held in place by the fixture and pulled onto spring-loaded pins (pogo pins) that make contact with connections on the board. The board can be pulled down either by vacuum or mechanically. With today’s board densities, it is very difficult to make connections directly to the component’s pads, due to solder and the type of component connection; nevertheless, it can still be attained to a high degree of reliability. To ensure that good contact is made, each spring exerts a force of between 100g and 200g. Obviously, the total combined force required to pull the board against all the pins in the fixture can be quite significant. Frequently, some type of support is required to make sure that the board does not flex, resulting in cracking of delicate surface-mounted components. Typically pins are placed on a .100-in. matrix. However, many new surface-mount IC packages require an even finer pitch. To achieve this, an adapter with focal pins is often used.

A wide range of pogo pins can be used; and the primary design differences are within the head or tip that contacts the board under test. Each type of head has a particular application for which it is best suited. Concave tips may be used to connect onto terminal posts, flat tips, or those with a spherical radius may be used to connect onto card edge fingers, while pins with a sharp point may be used to connect onto component pads. Sharp tips will penetrate any oxide layer and provide a high level of reliability on soldered areas. The three primary aspects of performance important for ICT are electrical resistance, pointing accuracy, and durability.

In an effort to reduce the fixture costs, provide additional flexibility, and enable updates to a software programs, a type of in-circuit tester known as a roving probe may be used instead of the bed of nails fixture. In this case, a simple fixture holds the board in place while a probe moves under software control to the relevant points on the board. These systems normally have a number of probes, and some can often access both sides of the board. Roving probe systems are slower than the bed-of-nails fixture because there is a wait-state between measurements as the probe moves to the next position; and this will, of course, reduce the cycle time. Nonetheless, it is a less expensive solution for the maintenance and prototyping of new boards due to the reduction in fixture costs and cost of changes.
 

Prototyping or Breadboarding
In the early days of radio, amateurs would nail copper wire or terminal strips to a wooden board—often, literally a board for cutting bread—and then solder electronic components to them. Generally, paper schematics were first glued to the board as a reference to place the components, terminals, and wires. It was rumored that a fully functional breadboard of the IC for the Polaroid SX-70 camera was first built from discrete components laid out on a 4’ x 8’ sheet of plywood. Over time, breadboards have evolved significantly, and today the term is used for all kinds of electronic prototyping. The layout of a current-day solderless breadboard is made up of two distinct types of interconnected electrical terminals, called strips. Terminal strips are the main areas that hold the electronic components. A notch runs down the middle of the breadboard, parallel to its longer edge. The notch is used to mark the centerline of the terminal strip and to provide limited airflow to cool the ICs straddling the centerline. On either side of the notch are five electrically connected clip columns, labeled A, B, C, D, and E on the left side, and F, G, H, I, and J on the right side. This allows a simple dual-in-line (DIL) IC to be plugged into column E, straddle the notch, and electrically connect to column F on the other side of the notch.

To provide power to the electronic components, a bus strip usually contains two columns, one for ground and the other for supply voltage. Some breadboards only provide a single-column power distribution bus strip on each long side. Typically, the column for a supply voltage is marked in red, while the column for ground is marked in blue or black. Perforated board (perfboard) is another base for prototyping circuit boards. Its substrate is typically made of fiberglass or phenol-based synthetic resin. Perfboard features a grid of pre-drilled holes at set intervals. Perfboard can be used for wire-wrap (a technique for constructing small numbers of complex electronics assemblies. It is an alternative technique to the use of small runs of printed circuit boards, and has the advantage of being easily changed for prototyping work) and for solder prototypes. Copper and other metals can be electroplated onto the perfboard’s substrate in pre-arranged patterns, as per the circuit’s design. Discrete components, such as resistors, capacitors, and integrated circuits, can then be attached to the prototype board. To complete the prototype, the components are soldered to the bottom of the board; and the copper or other plated-metal is scraped away in the areas where there will be no electronic connection.

Nowadays, virtual simulation and modeling software can be used in place of a real breadboard for quickly performing experiments and testing electronic and microcontroller-powered embedded applications. Below are some examples of common screw machine contacts found in manual breadboarding applications: 

Spring Probe Technology
In addition to spring probes for in-circuit testing, a battery contact is another evolution of the spring contact probes that have been testing printed circuit boards for over 20 years. Some examples of applications using battery and connector contact technology include:

• A board-to-board connection between an electronic device and its programming or docking station.

• The connection between a mobile radio or cellular phone and its battery.

• The connection between a camera and a lens.

• A switching device between the internal and the external antenna of a mobile radio.

• The connections from the motherboard to a battery in a notebook computer.

• A highly reliable battery contact for hand-held emergency radios used in military and police applications.

Why Screw Machine Technology?
In simple terms, solid machined contacts represent dependability. Screw machine contacts are utilized when tackling the most demanding of performance applications, such as in military (MIL) specifications, medical, telecom, industrial (IEC) specifications, and space (GSFC) specifications. Solid machined contacts are designed to withstand high numbers of mating cycles, often as many as 100,000, as is the case with the test and prototype industry. Additionally, they have a substantially higher degree of performance under shock and vibration than do stamp-and-formed contacts. With lower performance contacts, shock and vibration can cause discontinuity across the contact interface. Solid machined contacts with crimp terminations also provide a 360-degree, gas-tight interface between the wire and the crimp barrel. The crimp itself is also more robust and does not require additional mechanical attachment to the wire insulation in order to pass rigorous pull tests.

Solid machined “male” contacts are absolutely circular, while stamp-and-form technology produces male contacts that are hollow and are vulnerable to bending. This can result in the contact eventually breaking off. A bent male contact can also cause damage to its female counterpart, to which it mates. A solid machined male contact does not easily bend if subjected to misuse. Stamp-and-formed contacts tend to be oval-shaped or triangular in shape. These profiles can reduce the contact area inside the mated connection. When the contact profile leans toward the triangular-shape, edges are formed, and this can damage plating on the mated contact.

Solid machined “female” contacts have more material in their walls than do stamp-and-formed contacts. This allows the female contact to have a more stable insertion force over time. Stable insertion forces afford stable contact resistance. Heavier contact walls also resist unintended abuse. Machined contacts seat into connector housing by employing a perfectly square retaining feature, while stamp-and-formed contacts can have a radius on the retention feature that allows the contact to be pushed out of the housing more easily.

In addition to robustness, machined contacts have better electrical characteristics. Solid machined contacts have much greater cross-sectional area than do stamp-and-formed contacts. More cross sectional area provides lower contact resistance, and allows machined contacts more current-carrying capacity. Solid machined contacts provide uniform thermal transfer across the contact interface, and this is always the hottest area in the circuit path, assuming that conductors in the circuit are sized properly to carry its current. Efficient thermal transfer across the contact interface, toward a relatively larger circuit path, allows more current to be carried through individual contacts. Higher current density allows fewer connectors and wires to be used. This can save time and money in upfront costs, inventory costs, and manufacturing costs.

In summary, solid machined contacts mean better value. State-of-the-art manufacturing techniques nowadays have decreased the manufacturing costs of machined contacts, allowing competitive pricing against stamp-and-formed contacts in many applications. CNC-controlled tooling for both new products and for application-specific products is now dramatically lower than new tooling for stamp-and formed products. This fact negates the old “need to amortize high tooling costs into new contact designs.” And, by using high performance machined contacts, field failure can be avoided.

Karl Jalbert Director of Military & Industrial Technology, Bishop & Associates, Inc. Applications engineer, product manager, and marketing manager, Karl Jalbert’s tenure in the connector industry has spanned over 20 years. In 1985, Jalbert joined the applications engineering team and later became western regional sales manager for Optical Fiber Technologies Inc. (OFTI), which became part of the AMP Inc. product line in 1992. Throughout the 1990s, Jalbert was a senior applications engineer for M/A-Com in Boston, and senior product management specialist for AMP in Harrisburg, PA. Prior to joining Bishop & Associates, Jalbert was a market development director for FCI Electronics USA, focusing on the industrial and embedded computing market sectors.   Jalbert holds a BSME from Central New England College.