QMT Features: October 2010
Testing for electronics
Force and torques testing is a low cost method to identify critical quality concerns in electronics manufacturing, both in the lab and on the shopfloor.  By Karen Davies, Mecmesin.

In the UK alone, the electronics industry is estimated to be worth £23 billion a year and is the 5th largest contributor to electronics manufacture in the world (Source: Department for Business, Innovation and Skills). An extremely diversified sector, electronics consists not only of finished products, but also  a vast variety of components, from transformers to  switches and  sensors.

The industry is growing at a rapid pace and assessing mechanical properties is becoming an increasingly important factor for manufacturers looking to perfect their design and drive down production costs. For large-scale volumes, it is vital to maintain an ongoing quality assurance program, where production run samples can be randomly selected and tested to ensure that they continue to meet established testing standards.

Force and torque measurement is a relatively low cost method of determining the physical attributes of products and materials, helping to identify critical quality concerns, which may impact on standards of manufacture. Benefits include improvements in usability and fitness-for-purpose, early detection of errors before the product reaches consumers, improvements in product safety and conformance to standards.

The following examples describe some specific applications of mechanical testing on electronic products as well as some examples of tests that can be used as part of quality control assurance both in the laboratory and on the factory floor.

Touch screen actuation force
Used within media, navigation, mobile devices and at point of sale, not to mention force and torque testers themselves, it is essential that the touch screen control panel works as intended and requires uniform application of pressure across its surface. If this is not achieved, problems, ranging from a lack of functionality to complete operational failure when a user applies an incorrect amount of pressure, will occur.

In the early stages of development, a force test can alert a manufacturer to any deficiencies or inadequacies in the design of the touch screen. Later in the process, it can be used to ensure consistent standards are being met.

A straightforward compression test will detect the exact application force required to activate the controls of the unit. To perform the test, a computerised test system is typically employed. A system such as this offers a range of benefits over simpler testing equipment. For instance, the system provides users with the ability to program tests step by step and to their precise requirement when used in conjunction with the right type of software. With the ability to detect the activation of a control signal from the test piece as a direct result of the compression test, the system will determine the exact force needed to trigger this ‘event’, and record this data.

To perform the test, a compression probe is fixed to a appropriate load cell, sensitive enough to register the low level forces used within the test accurately. The touch screen is securely positioned on an X-Y translation table, allowing fine precision alignment of the screen under the test probe without having to re-set the sample. When the sample is in the required position, the probe is lowered at a constant rate to touch the screen at a steadily increased increment of force and the software records the activation point. The X-Y table allows quick and simple repositioning of the screen to test force application uniformity across its surface at multiple points.

A computer-operated system provides a real-time graphical display of every test, which can be analysed in further detail to determine key points of interest.
PCB Component Strength Assessment

Printed Circuit Boards have a variety of small parts attached, which are soldered into position. The quality of the soldering must be of sufficient quality, considering the boards are often handled manually for preparation and then via an automated production line for the final assembly process.

Measurement of joint strength can be undertaken in 2 ways; A pull test performed at a 45º angle, and a shear test, where the board is held vertically to enable a shearing action upon the sample. As PCB’s use small components, special accessories, designed specifically for miniature tests, are introduced to accurately measure force strength.

A fastening jig fixed to the motorised test system angles the PCB at 45º for the pull strength test and subsequently, at 180º for the shear test. A special miniature test hook is fitted to the test systems load cell located above the PCB, whilst the fastening jig’s position is fine-tuned using an adjustable X-Y table. The hook is lowered to catch hold of the test component and then raised at a slow, steady speed until a specific force level is reached to pass the test or the component breaks away from the PCB to identify its peak tension strength.

Shear tests are a destructive method for evaluating solder joint strength. They work on the premise that a crack in the joint itself will affect its strength; therefore a repetitive testing program, based on acceptable force ranges, will highlight any failings in the soldering process.

For the shear test the PCB is angled at 180º and a special shear tool lowered onto the component to push it off the board. These tests are performed in accordance to IEC 62137-1-1 and –2 standards.

Smart Cards
Smart cards have revolutionised card technology, however their successful implementation, functionality and durability are dependent upon the manufacturing process used, for instance, to bond contact chips into the body of the card. If the materials used are appropriate and the set-up of manufacture is correct, the components should stay bonded and not separate, when, for example, the card is flexed.

In contactless cards, it is even more crucial to substantiate bonding between the substrate, antenna and metallic plates to guarantee longevity of the product.
Tests, during the initial design stages, are important in finding the right combination of materials, whilst durability tests are essentially performed during the production stages as they represent the final product the customer receives and can therefore highlight any problems before distribution.

A number of tests can be performed on smart cards to evaluate their quality of manufacture.

1. Peel Test: A peel test is traditionally used to quantify the adhesive strength between the film and substrate, and to check the bonding strength of adhesives. This involves holding the sample on a horizontal axis using a special floating peel jig, whilst gripping the laminate via a small wedge grip and measuring the tensile force needed to peel a layer of laminate at 90º and 180º angles in line with ISO 7810 and ISO 10373. The floating peel jig consists of two rollers positioned at the top of the fixture, through which a strip of laminate is fed and held by the wedge grip. Thus, the card can move smoothly under the rollers as the peel test gets underway. It is recommended that layers are strong enough to withstand 0.35N/mm (2lb.in) of tensile force application.
2. Dynamic Bending & Torsional Stress: Smart cards are subjected to bending and torsional stress to check their stability and endurance, as well as to establish the effects of prolonged usage upon their functionality. To conform to ISO 7816-1, cards are bent vertically and horizontally repeatedly up to 1000 times. This procedure is replicated for torsional strain tests where the card is clamped at the short side and twisted to 15º ±1º. The card is then checked for any breaks and its electrical functionality.

Switches & controls
The most common switches in use today include toggle switches, push-button switches and rotary switches. Tests include measuring force and displacement to obtain tactile feedback of buttons and keypads, assessing the actuation force profile of a rocker switch to establish suitability for its intended application, and torque measurement to guarantee a rotary switch or controller can withstand the torsion forces employed and check adjustment torque ranges.

For force and torque measurements, a force/displacement or torque/angle of rotation curve is recorded when the switch is toggled, known as the ‘switch event’. For rocker switches a simple test solution involving a compression probe, digital force gauge and test stand will be sufficient to examine its operational quality. To quantify the properties of the switch, the compression probe is lowered to trigger the switch, resulting in a positive ‘click’ action and confirmation of the force required.

For life tests performed on switches and buttons, it is important that the sample is depressed with enough force to achieve switch contact closure on every cycle. It is useful to know the actuation force of the switch in advance, as force applied during a life cycle test should ideally be set to a level 10% higher, thereby guaranteeing contact closure.

Cycle tests are often performed at a high-speed rate, requiring force application to be measured dynamically using a flat, often miniature sized, load cell. This is most significant when using a pneumatic actuator as part of the cycle test. Often employed for applications in hazardous ‘no spark’ environments where force-speed ratios are important, this mechanical device is powered by air, where the air pressure supplied is directly proportional to the probes’ force. As a low-cost solution, the pneumatic actuator offers a reliable method for performing thousands of cycles in rapid succession. Once complete, the switch can be checked visually and functionally.

Rotary controls, such as ignition switches, are mounted to a motorised torque testing system and held via 4 gripping pegs, before torque is applied at a set speed or to a specified degree of rotation. This allows measurement of switch mechanism smoothness and the degree of torsion required to enable a positive ‘click’ response.l


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