|Ask the Experts|
May 25, 2011
Spec for Baking Assemblies Prior to Rework
I see the expert panel has touched on this subject in the past, but I hope you can clarify. We often rework circuit board assemblies that may have disk drives, batteries, plastics or other heat sensitive components. What is the industry consensus for baking out moisture in circuit board assemblies prior to convection rework, localized mini-wave rework, or hand soldering rework? In each case we are scraping the removed components so that has no impact on the baking time or temperature. In addition we are masking and shielding the adjacent area to isolate the extreme temperature to the rework site. Should we be baking the board per the IPC/JEDEC J-STD-033, or can we utilize the IPC-1601 Handling and Storage guidelines which focus on the board material?
|Expert Panel Responses|
There are a number of factors which determine the susceptibility of any particular PEC assembly to moisture absorption and the time and temperature required to remove it before soldering processes. These include the type of resin within the PCB and the layer count and complexity. Also, you must consider the components and any which may have a high MSL. The best approach is a catch all which is a policy of baking every assembly prior to any soldering operations and I would suggest that 110 C for 12 hours will ensure that you do not experience any moisture related problems. Just be careful of any particular exceptions due to highly sensitive components and avoid too many bakes since you may degrade solderability.
Principal Engineer - CMA Lab
You can completely dry any fully populated CCA at room temperature if you choose to, but there is an inverse relationship to the amount of time versus the baking temperature used. This is easily qualified. First, refer to IPC-1601, and read and understand the methods for baking moisture out of an assembly while weighing the CCA over time. You can accomplish the same moisture removal process using a good desiccator box such as a McDry, Dr. Storage, or equivalent that is capable of obtaining a 3% level of humidity or less. You just end up with a longer curve over time. However, most CCAs can be fully dried to less than 20% of their maximum saturated moisture content within three weeks at 23 deg. C. I have never seen or heard of a CCA that was damaged by "normal" rework profiles after baking/desiccation to that level. Keep in mind you cannot weigh a given CCA to determine if it is dry (too much variation due to amounts of solder, resin, etc.), but you can weigh a given CCA at a certain frequency (remove, weigh with a precision balance, and quickly return to the oven or drybox every half hour or every hour) while at a fixed temperature (or humidity level to determine the RATE of moisture removal, record the desiccation curve, and then determine the point in time where further baking or desiccation results in very little additional moisture removal (dt/dM-->0) or you have reached a moisture content of 20% of saturation or less. The saturation weight is determined by placing the CCA (or a scrap equivalent) in a condensation chamber for 5-7 days at 85% RH, then you take it out, bake for 30 minutes at 70 deg. C to remove the surface moisture, and weigh. While desiccation at room temperature takes awhile, it may be worth the wait if you can avoid having to remove and scrap out heat-sensitive parts, as well as the risk involved in doing that. Or you can determine a "happy medium" between the bake time and temperature that best meets your needs, by baking at temperatures somewhere between room temp and 150 deg. C. Remember, J-STD-033 is a guideline, and using the methods outlined in IPC-1601 can determine the exact time/temperature to accomplish the same goal.
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