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moisture in pcb's how is it measured



moisture in pcb's how is it measured | 19 October, 1999

I would like to know how to measure the moisture in pcb's. I have experienced blowholes and am sure it is the board manufacturer. What is the limit and is there any way to correct the problem when moisture rich boards are already in your process.

Thanking you in advance for your answers


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Stu Leech


Re: moisture in pcb's how is it measured | 19 October, 1999

We are deeply involved with this question because we have developed a much faster way to remove moisture from PCB's.

PCB's absorb varying amounts of moisture at differing rates. The rate is influenced by the laminate type board configuration , location of ground and power planes and much more.

Here is how we test for moisture content.

1) weigh a "factory board."

2) place it in a convection oven at 125 deg C. or other acceptable temperature.

3)weigh the board at intervals (every four or eight hours). CAUTION: Let the board return to ambient temperature before weighing. Hot boards have not reacquired their monolayer of moisture and will weigh differently depending on temperature. This will drive your data all over the map and you nuts.

4) Weigh your suspect boards.

5) The board is "dry" when the weight does not drop for two successive bake cycles.An standard measurement of moisture in a board is the amount of moisture weight expressed as a precentage of the board weight.

There are a number of hotly contested points of view on what the safe level of moisture is for PCB's. I won't get into that as all we do is cut peoples bake-out cycle times in half or less. We do not fabricate of populate boards. I try not to "bring a baseball bat to a gun fight." Hope this helps.


Stu Leech

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Re: moisture in pcb's how is it measured | 20 October, 1999


Stu's method is a half-truth. What it measures is the aggregate of all volatile material with a significant vapour pressure at the baking temperature, not just H2O.

Short of very complex methods, it could be complemented by a comparitive measurement of the dielectric constant. Water has a dielectric constant of about 80, compared with about 2 for most of the other volatile matter likely to be present. Most cross-linked laminates have dielectric constants in the range of 3.5 to 5.2 (linear polymers somewhat less). If the laminate dielectric constant goes down significantly (say by 0.05) after the baking process, you can bet that the volatiles were water. If it stays constant or rises, then it is not water. Always position the electrodes at the same position before and after, because small changes of thickness or density will introduce errors. Ideally, use etched electrodes on a test coupon.


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