Dry baking printed circuit boards.
What most people don’t realise is that printed circuit boards are hygroscopic. They absorb moisture from the atmosphere like a sponge does water. Although the absorption process is much slower it does happen. The amount of moisture absorbed is measured and provided on material data sheets provided by most laminate manufacturers. For example the moisture absorption limit listed on the Isola FR406 data sheet is 0.20 % measured on a .028 inch thick piece of stock laminate. Lead free assembly compliant laminates are even more hygroscopic. For example, Isola IS410 is 0.45 % measured on the same thickness material per the data sheet. Different grades of material from different manufacturers have different moisture absorption ratings. The percentage of moisture absorption increases when you add inner layer circuity and through holes. A low humidity environment results in a slow absorption rate. A high humidity environment results in a fast absorption rate. Moisture shall be absorbed to the point of equilibrium. Once equilibrium is achieved the absorption stops.
Printed circuit boards that have moisture in them can become damaged in the assembly process. In a uniform sample the moisture content may be spread out evenly. In a sample with through holes and inner layer circuitry the moisture content may be spread out unevenly across the sample. The soldering proccess is done at high temperatures above the boiling point of water. When the board is subject to wave and reflow temperatures the absorbed water heats and expands quickly and violently. Vaporised water needs to go somewhere. If the moisture content is high near the through holes it may result in blown holes or delamination. The moisture may even force the bare board to warp and twist. This can happen since the assembly temperatures are higher than the material’s Tg rating. Above the Tg rating the laminate becomes soft and pliable. Unbalanced constructions or copper distribution over a plane layer shall warp and twist if there is excess moisture in the laminate. Fine pitch smt or BGA devices may not rest evenly on the board surface resulting in partial or no solder joints in some areas of the device. Typically at a corner.
Unfortunately there is no magic meter to determine if the board has too much moisture in it or not. When considering the cost of all the value added materials and labor put into the assembly, dry baking is cheap insurance. Most bare board manufacutrers do not dry bake prior to shipping product. Contract manufacturers or assembly groups may have the bare board open to room humidity as it awaits the reflow process. To ensure success at assembly the moisture must be removed from the pcb prior to assembly. Leaving the board in a dry room will not remove the moisture. Forcing the moisture out through dry baking is the only practicle method.
When dry baking printed circuit boards the following considerations must be taken into account…
- Use a clean and well ventilated oven. A clean oven is inportant since we do not want contaminants contained in fumes to be deposited onto solderable surfaces. Ventilation is important since we want to evacuate harmful fumes and moisture forced from the board out of the oven. A calibrated over with good temperature control is a must.
- The bake temperature used must be above the boiling point of water (100 deg C). This provides the driving force that forces the moisture from the printed circuit board. It is not a good idea to exceed the MOT of the bare board. Exceeding the MOT shall result in damage to the bare board and may result in field failures if exceeded for long periods of time. MOT is Maximum continuous Operating Temperature. This is a value defined through Underwritters Laboratories (www.ul.com) on product submitted for testing. The MOT for a typical printed circuit board is 120 deg C. It is best to check with the board supplier to verify what this value is prior to dry baking. When dry baking with components on the board the bake temperature used may not exceed the component with the lowest MOT.
- Do not stack the boards one on top of another. This prevents the moisture in the center of the stack from being forced out of the board. Dry baking boards on a flat perforated or wire shelving is prefered. The more exposed surface area the better. Open surfaces provide more avenues for the moisture to escape. Dry baking at an angle on racks or product holders may result in rack marks or similar damage.
- Once an oven, temperature and placement have been determined, a time of baking needs to be calculated. To calculate the time required for a given system we must start by taking a weight measurement. Weigh a board on a gram scale accurate to at least three decimal places. Bake the board for two hours at temperature. Weigh the board again. Bake the board for another hour at temperature. Weigh the board again. If there is no change from the second mmeasurement then the time required shall be 2 hours. If the weight is less bake for another hour at temperature and then measure the weight again. Repeat the process till the weight doesn’t change.
Low temperatures for long periods of time work the best. For the average printed circuit board 115+/-2 deg C for 3 hours at temperature shall be more than enough to do the job. Dry baking a populated assembly prior to reflow 105+/-2 deg C for 6 hours should work well enough. Some components are very sensitive to extended periods of heat hence the lower temperature for a longer length of time.