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May 5, 2021

Baking Concerns for Stacked Trays of Components

When we are baking multiple trays of parts stacked on top of each other in our oven, will this lessen the effect of baking on the components in the middle trays.

Do the trays need to be separated? We bake our components according to IPC specifications.

T.G.

Expert Panel Responses

My suggestion would be to stack the trays, so air can circulate over the components of each tray to insure all the components are exposed to the same temperature and air flow.

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Michael Kaminsky
Sr Field Applications Support Engineer
Kester Inc.
Mr. Kaminsky has 30+ years of circuit board soldering assembly experience along with a patent for wave solder VOC flux process.

All moisture sensitive devices (MSDs) such as QFPs, TQFPs, TSOPs, etc. need to be baked before soldering based on their moisture sensitive level (MSL) number in accordance with IPC/JEDEC standard J-STD-003A. The time and temperature of the baking cycle is generally dependent upon the device thickness and the distance between the lead-frame and the plastic body.

Most baking ovens allow multiple trays of devices to be stacked inside the oven on slots that leave a gap between each tray. As long as the vertical distance between trays does not impede the airflow within the oven, stacking multiple trays should not impact the baking process.

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Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.

We also bake parts out on a regular basis and I assume you are using standard trays which stack and inter-nest. We have seen no negative impacts from parts baked out in the inside trays or the bottom trays.

We performed a test to see or test for impact and we found no negative trends. I will advise the volume of trays should be considered on when the parts all reach temperature which will be based on load and volume of trays and the official bake time only starts when all parts or trays reach temperature.

We take that into consideration to insure the bake out times reach peak temperature before the clock starts. No knowing the part types or tray style - I would recommend performing a DOE test and track the parts for bake out yield efficiency. We did that DOE so that we would have objective evidence for our end customer to insure our bake out procedure did not introduce any negative impacts on the end product.

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Mark McMeen
VP Engineering Services
STI Electronics Inc.
Mark T. McMeen is STI Electronics Inc.ʼs Vice President of Engineering Services. He oversees the daily operations of the Engineering Services division of STI. He has over 18 years experience in the manufacturing and engineering of PCBs.

I would think the air circulation in the oven is critical so it moves over all the products being baked in the process of removing the moisture.

The second point I would consider, is the thermal mass of all the boards in the oven and the time it will take the oven to stabilize the temperature for that particular load. I would make sure my time count starts when all the product is at temperature, not when they are put into the oven.

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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

You need to ensure that there is enough heat flowing through the trays. If they are too close to each other you might not get the desired effect in the middle trays.

Perform a small experiment comparing the pin-hole or void occurrences between top, bottom and middle trays. The results will help you determine the optimal spacing between trays.

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Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Space constraints might require stacking the trays on top of each other. As the trays geometry differ, assuring that air flow happens on each trays stack (there are spaces for the air to flow in between components) than you should be ok.

However, if the tray design does not have any openings in between components/components individual locations, then you are running some risk that the parts in the middle might not see all the heat needed. You can compensate that with baking time but there is no magic formula for that. Assuring that part exposure to heat is done is the best way to set for a successful process.

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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

The parts in the middle will not see the same air flow like the ones that are towards the outside. Baking the parts to the IPC specification should suffice. One recommendation for this process is to leave space in between the different tray stacks to assure air flow from the sides of the trays.

When we are baking multiple trays of parts stacked on top of each other in our oven, will this lessen the effect of baking on the components in the middle trays.

Do the trays need to be separated? We bake our components according to IPC specifications.

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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

The correct standard to reference when performing bakeout of components is J-STD-033D, not J-STD-003A as was mentioned, because the question pertains to components, not PWBs or CCAs. Mark McMean is dead on in his recommendation to "perform a DOE test and track the parts for bake out yield efficiency." Why is this so critical? Because MSD Level 5, 5A, and especially Level 6 often require nearly a full bakeout of all moisture in order to not be damaged during subsequent reflow.

But in addition to the methods suggested in this column for qualifying the bakeout process (which are all good) and to ensure even the components in the center of the middle trays are baked sufficiently, I would direct you to a different IPC specification that is also very helpful, IPC-1601D, typically used to determine the minimum bake time required for PWBs. Within IPC-1601D are "bake and weigh" methods that can be used for components as well as for PWBs. Using a precision scale, you leave a small quantity of scrap parts exposed to relatively high humidity levels for perhaps a week or so to ensure they reach their completely saturated moisture levels (worst-case condition). Then move them into your baking oven loaded with either "real" production parts or some other dummy load in the stacked trays.

Place your scrap samples in the center position but also put a few samples in other positions near the edge of the trays and in top and bottom trays for comparison. Remove the samples every 30 minutes and quickly weigh them on scale and document the initial saturated weight, and the weight after 30 minutes, after 60 minutes, etc. until the rate of weight loss is so insignificant as to be negligible. Note the amount of weight loss after every 30 minute interval and quickly return them to the oven each time. Document the point at which at least 90% of the saturated weight has been removed, and you now have the optimum bake time for the level of dryness desired.

This optimal bake time is good to know, because extended baking past the time required to achieve 90% dryness is both wasteful and can greatly reduce solderability of the component leads. Next, take the parts out of the oven and set them back in what you consider to be your "normal" working environment (40 to 70%RH for most factories). Note the humidity level of the area, and again weight the components every 30 minutes while leaving them out, exposed to humidity. Document those weights as well, until the parts are no longer measuring any significant weight due to moisture from humidity. This will give you some indication of just how long you have to get those parts onto boards and through reflow.

I would try to limit that time to less than two hours, because I have seen many Level 6 parts fail if they even reabsorb as little as 15% of their saturated weight. Using this method will give you a very definite rate of moisture loss while baking at different locations in the oven (called oven characterization) and compare the weight loss over time at the various trays and locations within the trays, indicating the stackup is not hurting or not hurting your bake process, as well as a firm rate of re-absorption which can lead to failure. I recommend a bake temperature of 105 deg. C, just above the boiling point of water. Now get that oven fired up and start baking and weighing, and have fun with that!

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Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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