A Question of Stability - Printed Circuit Boards Successfully Tested with Strain Gauges from HBM

Printed circuit boards of electronic components in automobiles, commercial vehicles, or smart phones must withstand harsh conditions in mobile use. Even slight cracks can cause the entire electronics system to fail. To prevent this, manufacturers test the mechanical stability of printed circuit boards. Strain gauges from HBM provide exact results.

“Strain values are the only parameters that are reliably predictive in terms of the stress loading of PCBs.” Christof Salcher, Product Manager at HBM

Cars drive over cobblestone pavement, and commercial vehicles make their way around bumpy construction sites. In the summer, they are exposed to heat, and in the winter, to freezing temperatures. All of this places strict requirements on the individual components of products such as printed circuit boards (PCBs). They are the crucial element of electronic modules. Vibration and thermal deformation can cause small cracks between the board and the component, which can lead to failure. Because of this designers measure the effect of mechanical loading very precisely during the development of a prototype. In this way, they can ensure that the PCBs will function properly up to the loading limit and also not undergo any damage in the production process.

Cars and commercial vehicles are just one field of application where measuring mechanical loading is useful. Trains and laptops are also continuously exposed to vibration. “Basically, there is a risk of breaks and cracks at every connection between the PCB and a component placed on it,” explains Christof Salcher, Product Manager at HBM. That can be expensive, for example, when the electronics in an automobile no longer work due to a small crack. Manufacturers are increasingly requiring their suppliers to prove the mechanical stability of PCBs. "Strain values are the only parameters that are reliably predictive in terms of the stress loading of PCBs. They can be measured using strain gauges, which are placed directly on the PCB to do this,” continues Salcher.

Lead-Free but More Sensitive

Strain gauges applied on a PCB

The requirements for these tests have become more stringent. One factor driving this is the conversion to lead-free soldering. A directive of the European Union (RoHS) prohibits the use of certain hazardous substances in electrical and electronic devices. This includes lead, which was previously used frequently for solder connections between the board and the component. The lead-free solution is more sensitive to mechanical influences and breaks more easily. Another factor that makes tests more stringent is the increasing use of more compact modules such as ball grid arrays (BGA). They allow for more connections than a conventional surface-mounted device (SMD). However, compared to the solder connections joining SMDs and the board, the contacts of BGAs are more rigid - mechanical stresses acting on the PCB are transferred more forcefully.

Different Standards

“Especially for the production of a new product, the entire process must therefore be accompanied by measurements made in the real production environment.” Bernd Wolf, Project Engineer at HBM

Different standards have been developed for measurements with strain gauges (SG) to meet these new requirements (i.e. IPC/JEDEG-9702). They include a description of where, how and by what means measurements are conducted. Many companies have also developed specific test procedures based on their own experience. Loading during later use and factors at work during the production process must be considered. “Especially for the production of a new product, the entire process must therefore be accompanied by measurements made in the real production environment,” explains Bernd Wolf, Project Engineer at HBM. “Strain is measured at places where there are components at risk of being destroyed.”

HBM provides the complete measurement chain for such applications, from the strain gauge through the QuantumX MX1615 amplifier and catman®AP software for data acquisition and analysis. The different measurement tasks and their constraints require the use of different strain gauges. HBM offers a wide selection for this purpose. For example, type RY rosettes have three measuring grids that can be adapted to different geometries, dimensions, and nominal (rated) resistances. Their temperature responses are adjusted for steel, aluminum, or to customer specification.

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