Defect of the Month – Conformal Coating

Hello, my name’s Bob Willis and welcome to Defect of the Month. Every month, I try and provide an example of a particular process defect and hopefully some solutions. I’ve made over a hundred different defect videos over the years. Hopefully one will solve one of your problems. 

One of the important characteristics to consider when selecting a conformal coat process and materials is the effect on expansion and contraction of the material and the impact potentially on solder joints and joint connections.

Whenever you’re looking at conformal coat to many companies, it’s a new technology, it’s a new aspect, a new material to consider. So please work and talk to the suppliers!! So they fully understand the process you require and the requirements of the product, but more importantly, the characteristics of the materials you’re going to be using. And any other design criteria that you need to adopt coating expansion and contraction can elongate solder joints. 

If the coating is between the component and the surface of the PCB, just think about the expansion characteristics of these three or four different combination of materials. Again, the supplier will be able to give you information specifically on the expansion coefficient of the material he’s going to be using before and after curing. But it’s up to you to make that decision and evaluate to see that it’s not actually going to cause you a problem during manufacture. We have seen some excellent examples and I say excellent examples of defects, as that’s what my business at looking at process defects like expansion of solder joints during temperature cycling of products with BGAs and other parts that may be effected

So it’s something to look out for. It may not necessarily cause a failure, but it’s certainly going to go a long way to contributing to a failure in the future. And I’ve seen packages separate. I’ve seen packages actually lift the pads from the surface of the board, not just crack the solar joints where you’ve got the expansion coefficient incorrect.  

It’s all down to selection of the materials and making sure that decision is correctly made. Now you can possibly prevent the materials entering underneath packages in very small spaces. So if you’re looking at QFN’s traditional TSOP’s, BGAs any parts of these configurations where capillary reaction can draw in coating, even though you didn’t mean it to go underneath it may then contribute to a problem. So you might decide to have a no go coating area around specific parts. Everything else on the board is coated, but not particular areas, knowing that capillary reaction could take place

So again, talk to your suppliers it’s possible to put a dam around packages to stop material flooding underneath and then causing your problem in the future. A lot of materials when you’re applying them perhaps by spray, you are unlikely to get the volume of material under many of these parts that then will expand and contract against the body and the PCB. So again, the process you use is also important. 

This is not a new problem because people were having problems back in the day with the introduction of coating materials on glass diodes, certainly we’ve seen the issues on small LEDs and other electromechanical parts. So find out all the information, make sure you select a supplier and the correct materials and think about thermal expansion and solder joint failure because of the use of conformal coat.