Measling in PCB refers to the discreet white spots that appear on the circuit board’s base material. In this article, you will learn more about it and how best to avoid it.
The Meaning of Measling in PCB
Measling is a term used to describe the occurrence of white, discreet spots in the base epoxy-glass laminate of a PCB. The inappropriate application or resin to the circuit board is a major reason for the occurrence.
Why Does Measling Occur in a PCB?
Besides the inappropriate resin application, measling also shows up in the circuit board due to other reasons.
One of such reasons is the inconsistency in placing the layers in the board for accurate bonding. This is sometimes a “manufacturer problem” and can be mitigated if the PCB manufacturer pays an extra attention to how the layers bond together in the board’s stack-up.
The other cause of PCB measling is the hydroscopic nature of the Flame Retardant 4 (FR-4) material used for PCB production. To this end, the material sometimes, absorbs water vapor from the air. This, in turn, excess moisture will be stored only to be turned into measles when a heat source, such as soldering iron or reflow oven is used.
Is PCB Measling Destructive?
The presence of white spots in the PCB’s interior (measling) could be “destructive.” If not well-managed, it could culminate in degraded physical and electrical performance of the board.
Let us look at just how far the destruction can go on.
The Tolerance
While measling can change the exterior design of the board, it can also be tolerated. Small amounts of measling can be tolerated, but must be managed to prevent further spreading.
When Does Measling Occur in a PCB?
Although measles occurs when the resin is not properly applied, it can also occur in other cases. For example, a lowered surface finish increases the board’s exposure to measling, as it reduces the board’s chances to resist measling.
It also occurs when there is excess mechanical stress in the PCB. The fact is that measling tends to occur at least, once in the lifetime of a Printed Circuit Board (PCB).
How to Evaluate for Measle-Related Defects in a PCB?
How do you test for measling in a circuit board? Here are some tips:
Laminate Exposure to Moisture
This is a special technique to evaluate measling in the PCB. The process involves:
- Exposing the PCB’s laminate to moisture. This must be done prior to the immersion or dipping to the solder paste.
- The next step is to immerse the laminate inside a hot solder paste, which must be up to 500 degrees Fahrenheit.
Etching and Solder Processing of the Laminate
The previous method is used to evaluate the circuit board’s compatibility. The next step is to subject the laminate to etching and processing.
Note: the etching and solder processing of the board must be followed soon after the laminate’s exposure to moisture. However, you must desist from this process if the board didn’t pass the exposure to moisture. This way, you will reduce the chances of further population of measles on the PCB’s surface.
PCB Measling Preventive Measures
As much as small amounts of measling can be tolerated, it is always best to avoid it. Here are some tips on how to go about it:
Be Careful with PCB Baking
Baking a PCB is always recommended, because it involves the use of heat to facilitate the removal of excess moisture in the board.
It is always recommended to use a temperature of between 100 and 105˚C to bake the board. However, you must be careful the way you go about it.
Sometimes, PCB baking can trigger a chain of issues, such as inaugurating solderability into the circuit board.
It is therefore, recommended that the baking process should:
- Be heated between 4 and 6 hours.
- Only be done when necessary. That is, when excess moisture is trapped inside the PCB.
Here are additional suggestions on how best to avoid measling in your circuit board:
- Always work with a professional circuit board manufacturer who will use the right methods to reduce measles in the PCB.
- Follow a laid-down procedure for circuit board procedure.
- As a way of preventing the board’s exposure to moisture, consider using moisture-resistance bags for the PCB’s storage.
- An accurate temperature must be maintained on the board before it is subjected to thermal processing.
- Resin-control methods should be used to prevent measling in PCBs.
- Always store the PCB in a dry place.
PCB Measling vs. PCB Delamination
Measling and delamination are two different terms used to suggest a similar problem – the presence of defects in a PCB. While they might seem similar, some differences exist between the two.
Delamination in PCB refers to the partial separation in the circuit board’s base materials’ layers. When this occurs, blister-like bubbles or gaps are created.
The causes of delamination in a PCB include but are not limited to:
- The exposure of the base material’s laminates to humidity.
- Loosing of the resin from the laminate or foil.
- The loosing of the resin from fiberglass either from humidity or heat.
The differences between PCB measling and delamination include:
- Measling has to do with the formation of white spots on the PCB’s interior, while delamination refers to the partial separation of the board’s base materials’ layers.
- Little amounts of measling can be tolerated, but delamination can impair the board’s functions.
What Happens if Measling Cannot be Controlled?
There is also a possibility that measling might not be fully-eradicated from the Printed Circuit Board (PCB). In such a case, what do you do?
Here are some options worth exploring:
- The board may still be able to function optimally, especially if the affected (measled) areas on the board do not stretch from two traces of from through-hole to through-hole.
- Ensure the quality of the inner layers’ oxide layer by storing the boards in a dry place.
Conclusion
PCB measling doesn’t pose much of a challenge, provided the affected areas do not stretch beyond two traces. If that is the case, you can expect the PCB to short-circuit.
For excellent management, always work with a professional PCB manufacturer who will manage the process better.
