Measelling occurs on a printed board when the glass fibre strands or bundles become detached. They show up on a board as white dots or small white crosses. They will normally be visible in a consistent pattern on the board due to the consistent pattern of the glass weave.

The fault is often seen during rework where incorrect temperatures are being used. It may be seen around holes where leads or pins have been forced into holes as this will stress the laminate locally.

In the case of the printed board it can be an indication of low resin content or stress from any mechanical operation like routing, punching or scoring operations.

In the example the marks are not consistent and are due to contamination under the solder mask which has shown up after reflow.