To produce reliable electronic assemblies, every item on the bill of materials needs attention. While focus on critical items such as electronic components, solder paste, and wave flux, for example are crucial, the printed circuit board itself should not be overlooked. Several issues can arise in PCB fabrication and handling that can be detrimental to performance of the final assembly.

Issues and Causes

Possibly the most common issue with PCBs is oxidation build-up on pads. This impedes solderability. A number of reasons can cause this to occur:

• Improper storage (e.g. not using silver saver paper when applicable; a high humidity environment)

• Exceeding the recognized shelf life of the particular finish

• Improper handling (scratching of the surface finish). Boards in this condition may be recoverable, depending on the finish (e.g. plasma cleaning)

Cleanliness of the incoming PCBs is also important to product reliability. This is especially true in no-clean applications, where any residue or contamination present on incoming material (e.g. PCBs, electronic components and connectors) is not removed later in the process flow.

Sources of PCB contamination can include residues from improper removal of resist material, contaminants and residues present on the PCB prior to solder mask application, or the poor quality of rinse water used in cleaning. Inner layer cleanliness is important, as well. Inadequately removed etching chemicals can become entrapped within the PCB and cause inner layer failures (such as CAF) over time.

A Case Study

Recently we had a client experiencing electrochemical migration failures under power FET devices. Ion chromatography analysis revealed elevated bromide levels as the only appreciable ion present at the failure sites. Additional testing indicated that the elevated bromide levels were present on bare boards and the flux used in the HASL process was the source. SIR testing was concluded, resulting in as-received bare boards failing and cleaned bare boards passing.

Importance of PCB Quality

In order to produce a reliable product, the importance of quality inputs cannot be overstated. Inner layer cleanliness, surface cleanliness and solderability are all critical factors that can have an adverse effect on product reliability. As the case study shows, improperly cleaned HASL flux residues on bare PCBs can lead to field failures.