Practical examples Industry: Electrics & Electronics
© DragonImagesBubble formation
on plastic PCB plugs
Problem definition
The term “blister effect” describes the phenomenon known in the industry of bubbles forming on plastic PCB connectors, which can occur during automated soldering. In some cases, the effect can be tolerated to a certain extent. However, if it is accompanied by a major loss of dimensional accuracy, in the worst case entire assemblies can become rejects. This example deals with such a case of serial damage, in which the blister effect meant that the housing covers of the assemblies could no longer be fitted properly.
Methodology: Structural analysis of affected plugs
- Macroscopic and microscopic defect image documentation
- Thin section preparation of previously embedded plugs
- Light microscopic examination of the thin sections in bright field
Result: Processing errors in the injection molding process
The affected plugs show bubble-shaped bulges on the surface. At higher magnifications, insufficient surface molding becomes visible, which indicates that the process temperature level is too low. In the component cross-section nest-like cavities appear within the connector, which are cavities caused by the manufacturing process. The high temperatures involved in machine soldering can cause existing molecular orientations in the component to relax, resulting in the delamination of existing surface layers and the formation of the blister effect.
In this case, the proportion of cavities in the component was so large that defective plugs could be gravimetrically identifiedduring the production process.
Christoph Zekorn
Head of the Microscopic Analysis Laboratory
Electronic components
on blanks under bending load
Problem: Product testing under realistic conditions
Electronic components on corresponding circuit boards are generally not subject to high mechanical loads. However, due to new geometries and areas of application, this can no longer be completely ruled out nowadays. Research is therefore being carried out into new material connections that can withstand high mechanical loads without any loss of electrical functionality. To verify the electrical functionality, on-line tests are carried out in which the product properties are simultaneously monitored under high mechanical loads.
Methodology: Bending tests on circuit board with electrical conductors
- 3-point bending tests on blanks using a universal testing machine
- Simultaneous monitoring of electrical conductivity
- Recording the force/deformation curve in cyclic mode
Result: No loss of electrical conductivity
The newly developed circuit boards with conductor tracks are characterised using 2 simultaneous test methods: Recording the electrical conductivity during a simultaneous mechanical bending process. This allows a wide range of deflections to be tested, including in a cyclical manner, thus simulating a successive increase in load (see image on the left).
The functionality of the components, which is checked on-line using special electronics, is always guaranteed during the bending tests. This test setup can therefore be used as part of a quality assurance or incoming goods inspection to determine whether functionality can be guaranteed under a given external mechanical load. In addition to short-term loads, the components can also be subjected to long-term loads. Dynamic cyclic fatigue tests can be used to characterise the fatigue behaviour of the electronic components.
Dipl.-Ing. (FH) Christiane Wintgens
Project engineer for dynamic testing
Investigation of
LED boards for headlights
Problem: Detachments on LED boards
As part of a research project at IKV, LED circuit boards used in automotive headlights were provided with silicone optics using an LSR injection molding process. Despite constant process parameters, detachment of the silicone component in the area of the LED chip occurred on certain PCB batches, which meant that the desired light scattering was no longer achieved. In addition, cloudy streaks were sometimes observed in the silicone optics. Appropriate investigations should help to determine the cause of the detachments.
Methodology: Microscopic examinations
- Macroscopic and microscopic defect image documentation
- Laser confocal microscopic examination of conspicuous areas
- Field emission scanning electron microscopic (FESEM) examination of cross-sections through the damaged area of affected lenses in combination with energy dispersive X-ray spectroscopy (EDX)
Result: Impurities and batch-dependent material fluctuations in the LED boards
In the area of the detachments, foreign particles were detected on exposed components both on the silicone optics and on the circuit boards. Fluid-like contamination was observed in the immediate vicinity of the LED chips. Cloudy streaks in the silicone optics obviously have their origin on the LED board due to the course of the streaks. Light microscopic examinations of other LED boards showed contamination on new parts as a result of inadequate transport protection.
The streaking probably takes place during the injection molding process and is probably due to an interaction between the silicone compound and the embedding compound of the LED chips The EDX examination confirmed the use of different materials in the area of the LED chips. The detachments could be attributed to a batch in which components made of alternative materials were used. Germanium and silicon were used in the inconspicuous batch, while the batch with detachments was made of silicon and magnesium. With the help of these investigations, the causes of the defects could be eliminated immediately.
Christoph Zekorn
Head of the Microscopic Analysis Laboratory