Defect of the Month – Sulphur Corrosion on Electronic Circuits

Hello, my name is Bob Willis and welcome to Defect of the Month. This month, I wanted to talk about sulphur attack or corrosion. Sulphur or sulphur gas in the environment can have an impact on electronic components and electronic products. This can cause intermittent failures or complete failure of an electronic product in the field. Some of the areas I have seen over the years are failures on simple chip resistors or resistor networks but there can be more

Examples above of corrosion products present on the surface of a chip resistor connection point and on and around joints on a QFP

If you have material that is included in your assembly which is loaded with sulphur for some reason it will be a problem. It may be some sort of plastics or foam. If they’re actually in a product assembly which has a printed circuit board it’s possible for corrosion to occur on surface of chip resistors or other exposed copper and silver surfaces. The corrosion occurs on resistors between the termination point and the resistive element on the surface of the exposed silver. This issue has generally been overcome with improvement in the quality of chip and resistor networks and with the plating and the coverage with tin and some of further protection during the soldering operation. However we still do see it happening 

The corrosion can be seen under conformal coatings, and I’ll talk about this in a moment. We can also see the failure mechanism on printed circuit boards and the erosion of the copper and exposed silver tracking on a printed circuit board. I’ll show you a couple of examples of this below so let’s just step back for a second. If we talk about resistors chip resistors. The corrosion can occur to such a degree that it causes first of all, a small change in resistance and then finally potentially it can lead to a complete open circuit. Now if it’s on capacitors, which is also possible, it’s not so much of an issue in terms of failure, because capacitors have multiple connection points to the termination that you are actually soldering to on the circuit board, but they’re still visible as little black spots.  Now if you look at high magnification at sulphur failure on boards or on components, it looks like a black deposit which has a flowery surface appearance, and these examples show you types. 

Examples on a chip resistor, a capacitor and around PCB via holes

We can conduct surface analysis, or have a laboratory undertake failure analysis for you to determine what is the deposit and possible source. Different techniques are available to allow you to do this, and then you can determine what the material is as shown in these examples. I have experienced this as I said, failures on resistors, but I’ve also seen it on other packages and here’s an example where you can see the exposed copper surface where you’ve got this form of corrosion present. One of the problems if you have multiple random failures is where has exposure to sulphur occurred

 

Deposits on copper exposed lead frame and inside a PCB via hole

So in determining root causes, a classic example I have experienced is with foam packing within a mobile communications assembly which was included to reduce the possibility of damage during vibration, shock or bump. We actually tested the foam and we’re able to show that certain date codes and material had very high sulphur content and some did not. This can make determining the size of the problem very difficult. Your product might be used in a location where the environment itself is completely sulphur laden in that particular case you may need to look at coatings

       

Examples of surface analysis taken from on of the authors hands on Failure Analysis Workshops

The first example is from joints on a QFP and the second from via pads on a PCB 

Conformal coatings can give a level of protection or complete protection, but you need to understand the material type you need to use and that is where you need to talk to suppliers. You need to determine the thickness of that material. Incorrect selection of coating types or thickness may cause other issues and how that will impact the reliability of the product reducing the possibility of corrosion underneath the coating 

I have seen certainly corrosion underneath conformal coating, but again it depends on the material, the quantity of sulphur in the environment. There’s a number of things you have to look out for, talk to your conformal coating suppliers, but also talk to failure analysis laboratories who can assist you with your selection process. Determining whether sulphur is present in some other part of your electronic assembly or packaging, and also look at what and why you have corrosion points on your product

We have many other Defect of The Month videos and our online articles which we hope help you solve your process and product failures. We have also listed a small selection of over 100plus videos created over many years. 

If you would like to download any of our Process Defect Guides or my three soldering books FREE just drop me an email bob@bobwillis.co.uk