Measling and delamination

What is it?

Measling is the separation of glass fibres from the resin at the weave intersection in a glass fibre reinforced circuit board laminate. It usually appears in a printed circuit board in the form of discrete white spots or crosses that are visible below the surface of the base laminate. The term delamination typically refers to a failure of the copper conductor to dielectric interface in a multi-layer board and this can result in a void opening up between, for example, a copper track or pad and the dielectric above it.  

Why does it happen?

Measling is a problem that is encountered in laminate materials after exposure to high temperatures. Lower cost, lower specification laminates will be more susceptible to this problem.

Delamination is thermally induced problem in printed circuit board laminates. In a typical multilayer circuit board there will be numerous interfaces between materials that have different physical properties. These include glass fibre to resin and resin and glass fibre to copper interfaces each of which must maintain its integrity during the manufacturing process and operational life of the electronics in which they are used. Although higher processing temperatures are known to increase the likelihood of delamination occurring, the chemical composition of the laminate used also has a significant impact and so careful selection of a suitable laminate type may help in cases where delamination has been found to be a problem. Traditional dicyandiamide cured laminates are thought to be more prone to delamination than the newer hydroxyl cured materials.

What problems can it cause?

Measling and delamination may not lead to immediate catastrophic failures in a circuit board, but they do generate voids which can harbour ionic contamination and moisture. An aggressive solution can form in these voids and when coupled with the presence of a field gradient this can lead to corrosion reactions or the formation of conductive pathways between adjacent tracks, holes or vias. These could lead to long term reliability problems.

How can it be prevented?

Good control of the pre-heating and post-soldering cooling of the circuit board assembly are the two simplest techniques that can help to alleviate the occurrence of measling and delamination. Using laminates with higher Tg temperatures can also help reduce the occurrence of these problems.