PCB Measling is a phenomenon that doesn’t happen often but when it does, the root of the problem must be tracked down in order to prevent possible problems in the field. Measling is defined as a visible inter-layer delamination of a Printed Circuit Board, appearing as lighter-colored spots (measles) on an inner PCB layer of a multi-layer PCB (usually 4-layers or more). These are usually located at the boundaries of the prepreg adhesive or more rarely, within a layer of fiberglass weave that is unrelated to the prepreg. The lighter-color measles may be visible through a semi-transparent portion of the top layer, or through a semi-transparent portion of the bottom layer, possibly both.
PCB Measle Cross-Section View
The figure below shows a cross-section cutaway of a PCB including its vias or component mounting through holes to the left and right. If the area of measle delamination were to grow to encompass both of the copper plated through holes, an electrical pathway could be presented where moisture in the cavity could cause metallic dendrite growth, or more likely there could be a conductive ohmic resistance formed between the two vertical copper conductors causing circuit damage or circuit malfunction to occur. Although, such occurrences are rare.
Why Does Measling Occur?
Printed Circuit boards are built from FR4 epoxy-fiber glass which is known to be hydroscopic. This means that it may absorb water vapor from the air. When bare PCBs are not stored in the right environment, it may take in moisture and then when a soldering iron, reflow oven, or other source of heat reaches that area, the water will evaporate thus creating an air pocket which is seen as a measle. This is usually in the prepreg adhesive inter-layer but also happens within the fiberglass weave of layers less frequently.
Another reason PCB Measling may happen lies with the manufacturer. Sometimes, lower quality PCB fabricators will not have the controls in place to consistently bond the layers of the PCB. Each layer is bonded together with the next layer in a stack build-up by using an inter-layer of prepreg bonding adhesive material. PCB measling may occur when multiple layers of circuitry and prepreg are stacked together and not enough resin is applied during the lamination process. This stacking process could also entrap pockets of moisture within the stack-up as the layers are bonded together under high compression and plus heat. If the heat temperature and duration of the bonding process are not enough to purge the entrapped moisture from the PCB stack-up moisture could be entrapped within the PCB materials.
How to avoid PCB Measling
The best way to avoid measling is by using a quality PCB fabricator and storing the boards in ESD, moisture resistant bags in a dry area. During the assembly process, bare boards shouldn’t be removed until they need to be used. If the packaging is opened but not all of the boards are used, they should be resealed. If you have boards from an order that previously exhibited measling, you may want to bake the boards prior to use and have a test board before stuffing or running the rest the batch. IPC outlines these standards but in general it is 100-105 degrees Celsius (sometimes higher) for 4-6hr. The heat rids the boards of moisture and makes them safe to use. Baking boards may introduce solderability and other problems so it is important not to bake them if it is not necessary.
Are Boards with Measles Safe to Use
This really depends on the severity of the measle. IPC recommendations are very vague and not overly helpful. Whats important to think about here is if there is a potential for a short circuit. If the area affected stretches from through hole to through hole or to two traces. This will likely lead to failures in the field. If they are smaller than that, in general they do not get worse and should be able to function as long as they pass your other testing.
If you have further questions on measling or other PCBA quality concern, please reach out to us. We also have a network of vetted PCB fabricators from across the world and are happy to get your project quoted with whatever budget and lead time you need. Our knowledgeable staff is happy to find a solution for you.