Posts Tagged ‘dry’

Simple Yield Improvement. (Part 2)

Sunday, January 6th, 2013

In the previous post we discussed what happens to the printed circuit board structure when it is heated. We discussed how different materials expand at different rates and the effect the expansion rates have on the warp and twist of a Printed Circuit Board (PCB). We also discussed how moisture absorption can increase the degree of warp and twist and the adverse effect it has on assembly yields and potential field failures. We also discussed how dry baking removes moisture from the PCB and how it minimizes if not eliminates potential problems.

In this post we shall discuss the stabilizing effect that (more…)

What damage does the assembly process do to a pcb? (part 5)

Sunday, August 28th, 2011

In part 4 of this series of posts I discussed the effect moisture has on the printed circuit board at soldering temperatures. I explained the material properties of FR-4 laminate and how they are hygroscopic. We also covered an acceptable practice known as dry baking used to force moisture from the product just prior to being exposed to soldering temperatures. I received some very good comments and feedback regarding part 4 of this blog series. I thought it appropriate to share this feedback in another post.

With regards to sources of moisture, in part 4 of this series I comment upon a practice where some printed circuit board manufacturers shall dry bake the printed circuit boards prior to shipping them. The important thing to add here is that common packaging materials used by the industry do not act as a 100% vapor barrier. The shrink wrap film used only slows down the process of moisture absorption by the product. Moisture in the environment outside the bag shall migrate through the protective film but at a much slower rate. Things to consider are as follows… (more…)

What damage does the assembly process do to a pcb? (part 4)

Tuesday, August 23rd, 2011

In part 3 of this series of posts I discussed how phenolic cured laminates are mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on the material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success.

On this post I would like to discuss the effect moisture has on the printed circuit board. What most people don’t realize is that printed circuit boards are hygroscopic. Boards shall absorb available moisture from the surrounding environment to the point of equilibrium. The various FR-4 laminate data sheets list a Moisture Absorption value. The values are calculated in accordance with the IPC-TM-650 specification.

The specification basically tests a solid rectangular piece of FR-4 free of copper, no holes and the edges sanded smooth. First the sample is preconditioned, aka dry baked. Immediately after dry baking, the sample is weighed. This is the dry weight. The sample is then submerged for 24 hours in Distilled Water. The sample is removed, towel dried and then weighed. This is the wet weight. The wet and dry weights are then plugged into a formula listed in the specification to produce the Moisture Absorption value.

Dry Bake Oxidation

Saturday, August 8th, 2009

As with any baking operation it is important to realize that the surface finish shall start to oxidize. There are many bake operations employed in printed circuit board manufacturing. When bare copper is baked at 300  deg F for an hour the copper turns color. The copper has become oxidized and must be cleaned prior to continued processing. The final solderable finish shall oxidize as well when exposed to heat. At the time of this posting the IPC and its members are currently working on a specification that deals directly with baking of printed circuit boards. The preliminary specification can be found posted on the IPC web site at… (more…)

Dry baking printed circuit boards.

Sunday, July 19th, 2009

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 (more…)