|Ask the Experts|
May 8, 2020
Question About Dry Storage of PCBA's
One of our products uses 3 PCBA's made on our own internal surface mount line. After the PCBA's are assembled, they are hand soldered together with through hole components and then put into a housing which is then filled with an Epoxy resin (Stycast 2075).
Should we put the PBCA's in a dry cabinet before we solder them together and cover them in the resin to minimize the chance of product failures long term? This dry storage process is costly and requires more capital equipment which we are trying to avoid. If we should dry store them, what storage conditions should we follow?
|Expert Panel Responses|
If your hand solder and potting operations are performed soon after your SMT processes, you should not need special dry storage.
Principal Product Engineer
Benchmark Electronics, Inc.
It sure sounds like you are concerned about unwanted moisture entrapment within your potted assemblies, a legitimate concern relative to long term reliability.
If the dry storage option is causing you unbearable cost in capital equipment and time an oven moisture bake out might be a more practical solution for you. We process a large volume of BGA components utilizing conventional "Hot-Gas" equipment. We are always concerned about moisture content prior to hot gas processing.
If we don't bake out the PCBA's prior to processing we run the risk of causing damage like PCB/Device delamination or pop-corning. For this reason we perform a moisture bake out as a standard part of our processing. Obviously I know very little about your particular application but ovens come in a wide variety of shapes, sizes, controls and price. Ovens are available at very reasonable prices (especially used) and they allow you to accelerate the drying process.
However, very important... MAKE SURE YOU MATCH YOUR OVEN TEMPERATURE AND BAKE TIME PROPERLY FOR YOUR PCBA'S AND THEIR COMPONENTS.
Circuit Technology Center
Since you’re hand soldering the through-hole components it would appear your production volumes are relatively low. To avoid using a costly dry cabinet you may want to consider placing your PCBAs in a drying oven but you will have to experiment with the temperature and duration since in general dryness is guaranteed at 100°C.
As an example one of our customers dried their packages for three hours at close to 100°C before underfilling. This was to avoid moisture coming out of a BTS substrate the flip chip was mounted on before underfill.
Just to clarify – dry cabinets will not actively dry out a component which has already absorbed moisture, unless they include a baking function. Dry cabinets prevent moisture from reentering the components or bare boards. That is why there is no "standard" schedule for how long it will take to dry products in a low RH environment, only for drying product at elevated temperatures, via baking.
IPC-1601, section 3.4, is a drying schedule for bare boards, to drive out moisture from within the laminate of the bare board before soldering. This is a good recommendation to dry out your assemblies before potting, to make sure your boards (with components) are free from moisture. However make sure to check your components to confirm they are rated for the temperature used in this bake schedule prior to baking. If your components may be at risk, you might consider the bake schedules outlined in J-STD-033, Table 4-1, which specifies several different temperature and time schedules for driving out moisture from components, and will also dry out the bare board as well.
SmartDRY Product Manager
Thank you for asking, however I would want to know the exact failure mechanism being experienced before submitting the products to dry box environment. It mentions in the email that the boards are manually soldered together, so the temperature excursion is minimal as opposed to utilizing a mass soldering system.
The need for a risk analysis of product failures needs to be investigated before investing in a system which may or may not prevent any problems.
To fully answer this question more information is necessary, and once the failure mechanism is defined, then the appropriate action can be taken to address the issue.
Vice President, Technical Director
Dry Storage is a great way to keep the assemblies dry after mass solder (reflow). Moving them from reflow, after they cool, and while they are very dry, will keep them from gather moisture while they await hand soldering and potting. Keeping them dry is far less costly than baking them dry before hand soldering or potting. The cost to bake (1000s of Watts for hours or days), the lost production time, not to mention the risk baking can present to the components, is eliminated if you keep the assemblies in dry storage, whose average energy use is typically < 100 Watts. Investing in a dry storage cabinet is thus a good idea, something anyone handling SMT components should have available. As for the storage conditions, anything 5% or lower will keep components dry per J-STD-033.
Senior Project Engineer
Electronic Controls Design Inc
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