原创 Nasty causes of PCB failures

Heat, humidity, and cleanliness should be strictly controlled during the assembly process. Depending on the product, a controlled clean room must be used to ensure cleanliness. For standard fine-pitch assembly and for consumer or non-critical business use, more standard conditions can be used. However, the cleanliness should exclude large contamination. Some dust might not matter, but large pieces of material can bridge conductors and cause problems. In this article I am going to spotlight two cleanliness issues, the first example shows an assembly area that does not have the expected cleanliness. The second illustrates an environmental issue that caused the product to fail.

Figures 1 and 2 show the inside of a failing product. The product has a sealed enclosure, butÿthere are ants beneath the polyimide tape that was used as a protective layer as well as scattered on the PCB assembly (PCBA). You can only imagine the number of ants that must have been crawling around the assembly area to have two crawling on the PCBA surface so that they could be trapped with the small piece of tape. This manufacturing plant certainly needed an exterminator!

Figure 1: Ant on the PCBA surface

Figure 2: Ants beneath the polyimide tape.

The second example comes from a product that failed in the field after two years of operation. This product was used in a janitorial closet (wardrobe?) in a school building. The outside environment was hot and humid, and it was located near a seacoast. Figures 3, 4 and 5 show some of the dust and debris found within the cabinet. Look at the number of mosquitoes (Figure 5) that had collected in the chassis over the two years!

Figure 4: Dust and debris on headers. Header pins were also corroded

Figure 5: Mosquitoes found in the chassis

In the second example, the product was thoroughly cleaned and tested. It was then fully operational, showing that it was the buildup of dust and debris that induced the failure. The dust and debris had absorbed enough humidity to become conductive. This conduction was enough to disrupt the signals to and from the memory devices. In this case, the corrective action was to install additional filtration to filter out some of the dust and to suggest that a janitorial closet was not the best place to house the equipment.

What kind of gross contamination have you found in your products returned from the field? If you have some good PCB contamination photos, share them in the comments section with a brief explanation/caption.

Michelle Woolley