Effects of cavities under components on the system reliability of electronics and microsystems
Electronic assemblies are often coated with protective lacquers to protect them from harsh environmental conditions. However, cavities under components and in the form of bubbles in the protective coating layer cannot be completely prevented during production. In the subsequent process control, this leads to a general negative classification of the affected components and thus to increased rejects.
Coating defects and the risks involved
Coating printed circuit boards with protective lacquers can increase the reliability of electronic assemblies in humid and aggressive environments. Small cavities, which are unavoidable during coating, often result in bubbles in the coating layer, as the trapped air expands during the curing process in the oven. The most critical example of cavity formation is bridging bubbles, where the gas bubbles remain trapped between the miniaturized components or contact legs with different voltage potentials.
Microclimates can form in the cavities due to diffusing moisture. This moisture in combination with the electrical potential differences and the presence of process residues can lead to electrolyte formation between the electrical contacts of the components and initiate electrochemical corrosion processes. This significantly increases the risk of failure of the electronic assembly.
Current assessment methods for protective coatings and their consequences
Despite this, the risk of failure due to bubbles / cavities in painted electronic systems has so far only been assessed in general terms and without scientifically sound evidence. This assessment, combined with the very strict specifications of the industry, leads to the direct exclusion of coated assemblies with optically visible cavities or bubbles, although no well-founded data on the actual risk assessment is available to date.
As the protective coating is applied at the very end of the production process and reworking the faulty assemblies is very costly, often technically impossible and even forbidden for many applications, this blanket assessment leads to considerable economic and ecological disadvantages.
The approach and aim of the AHBSEM project
In order to investigate the effects of cavities and bubbles under components, these are initially generated on test substrates and real components as part of the project. The components are exposed to climatic stress and at the same time checked for failures by means of electrical wiring. Subsequently, the exposed samples are examined both in terms of surface analysis and microstructure. In addition, moisture diffusion through the paint into cavities is examined more closely using simulations.
The results of the research work will be translated into a scientifically sound risk assessment of the damage potential of cavities and bubbles in varnished electronic systems. This also results in a recommendation for action on electrochemical component reliability. The associated reduction in rejects, improvement in risk assessment and communication along the value chain will have an enormous economic leverage effect.