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July 24, 2017

RoHS tantalum capacitors burning during functional test

We manufacturing RoHS boards with SAC305 solder paste for a customer. During the functional test, many tantalum capacitors (RoHS ready) burn and carbonize. Our reflow peak on the capacitor area is 240 deg. C, thermal profile made with KIC navigator, BTU Pyramax.

We follow paste and parts temperature specifications.To correct the problem, our customers ask us to manual soldering the tantalum capacitor on the board after the reflow because they associate the problem to the heat of tantalum caps during reflow.Does somebody see this kind of problem with RoHS tantalum caps? Do the manual soldering with tips at 750 deg C not make sense for me compare to reflow.

C.H.

Experts Comments

My product is actually for selective soldering and not reflow soldering, but... recently I've been approached because of this same issue you are having. I will be doing a special application using a selective soldering machine to simply touch the bottom side of the board to a mini-wave and use it to reflow the solder on the top side. Of course the board has nothing on the bottom so there is nothing to interfere with it.

I don't know much about your actual process or another solution but I wanted to answer your question "Does somebody else see this problem". The answer is yes, other people see this problem. I hope there will be a better answer for you to correct this. But please contact me if I can help.

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Todd O'Neil
National Sales and Marketing Manager, North America
Optical Control
Mr. O'Neil has been in the electronics manufacturing industry for over 20 years.

We have seen several situations where capacitors can fail if the internal core temperature of the cap goes above 150 - 180 C. The temperature is a function of the part itself and should be verified with the component vendor.

The key is the core temperature. So we recommend drilling a small hole in through the top of the capacitor and inserting a Thermocouple wire down into the core of the cap. Then seal it up with high temperature adhesive /epoxy. In this way you can properly measure the core temperature of the part and develop profiles accordingly.

In general, you can achieve profiles that will satisfy the solder joint temperatures of 235-240 and keep the body cool. In some cases, you may need to utilize differential temperatures between top and bottom heated zones. For example, when reflowing the cap on the top side of the board you can reduce the top side heater zone temperatures and increase the bottom side zone temperatures. The heat will conduct through the board and reflow the joints while the "cooler" air on the top helps to keep the capacitor body within spec.

Your oven supplier can work with you on the details. The best thing to do is provide the oven supplier with the profiles you are running now and the parameters you need to meet and they can typically provide guidance on changes to zone temperature and process.

But it would be best to get that capacitor outfitted with a T/C first and then run it through so the oven vendor can have that baseline info from which to operate.

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Marc Peo
President
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

You may be experiencing popcorning of your tantalum capacitors. We have observed this with several of our clients. Visually inspect and cross-section a random sample of caps post-reflow and see if you observe cracks. If yes, improve your storage conditions and the failures should go away.

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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

Yes we have seen this type of issue before.

These are very difficult to assemble in reflow depending on the size and density of the board. One trick you may try is to reduce you top side temperature and increase bottom side heating. This will help keep the capacitor from getting as hot. You may also want to play with your convection air flow on the top side to reduce the heating.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.
Reader Comment
We have seen the same kind of defects. These kind of defects are mostly due to the moisture absorption or the improper handling of the component. The component may required to be baked before assembly. The datasheet or the component manufacturer can help in the baking recommendations. Refer J-STD-20 for baking requirements if you are not able to get specifications from your component manufacturer.

You can also try reflow profile with higher temperature in bottom side and lower temperature from top side depending on your assembly complexity. Reduce the pad size for lesser solder volume and add solder preforms for good solder joint.
Subburaj, Amphenol, India
Reader Comment
I know it sounds crazy, but are you sure the polarity of the capacitors is observed during assembly? Reverse installed tantalums will do just that, burn to ashes, even carbonizing the neighboring area of the PCB.
Ioan Tempea, MDA Corporation
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