Dry bake test results.

Printed circuit boards shall absorb moisture to the point of equilibrium. This shall occur as long as there is a source of moisture. Relative humidity provides an ample source of moisture for the absorption process. It is common for contract manufacturers to maintain a controlled environment that often relies on humidity to minimize static charge build up. Electro-static discharge (ESD) is damaging to assembled electronic components. Humidifiers are employed to maintain a 50% relative humidity environment. However, in minimizing one problem another is introduced. The humidity that is introduced to minimize ESD is absorbed into the printed circuit board and components alike.

In a production setting on a manufacturing shop floor, work is processed in stages. Production schedules are moved in and out based on customer requirements. Equipment processing capacity may form a gate. The work may accumulate and sit for days waiting to be processed. Product shall sit on shelving in a 50% controlled environment over the week-end. It is not uncommon for populated boards to be on a shelf waiting for days to be soldered.

A customer of mine had an 8 layer printed circuit board that was warping when the board was exposed to soldering temperatures. The board was built on FR4 Tg 140 deg C laminate. The warping was causing some large 100 pin smt devices to not solder at one corner of the devices. I dry baked some of the bare boards for them. The warping in their soldering process was eliminated. The components soldered without a problem.

I completed an experiment to show the affect moisture absorption has on a printed circuit board.

  1. I started by dry baking 12 boards of the 8 layer design selected at random for 8 hours at 115 deg C.
  2. The boards were then allowed to air cool to ambient temperature.
  3. The samples were weighed on a gram scale accurate to 4 decimal places.
  4. The samples were then placed in various locations with different levels of humidity for 36 hours.
  5. The samples were then weighed again.
  6. I then dry baked the boards using my standard parameters of 3 hours at 115 deg C.
  7. The boards were then allowed to air cool to ambient temperature.
  8. The samples were weighed a second time on a gram scale accurate to 4 decimal places.

The results of my test are listed in the following table…

Dry Bake Results

Dry Bake Results

The amount of moisture absorbed by the boards was slightly higher in areas with greater than 60% RH. It was slightly less in areas less than 40% RH. The 3 hour dry bake removed approximately 70% of the moisture absorbed.

Tags: , ,

3 Responses to “Dry bake test results.”

  1. fps Says:

    free product samples…

    Your topic PCB Design and Fabrication School ” Blog Archive ” Dry bake … was interesting when I found it on Friday.

  2. articles Says:

    children articles…

    Hello :) I bookmarked this site. Thanks heaps for this!… if anyone else has anything, it would be much appreciated. Great website.

  3. jake Says:


    excelent post, keep it coming…

Leave a Reply