Defect of the Month – PCB Solder Finish

Hello, my name is Bob Willis and welcome to Defect of the Month with WNIE. Printed circuit board solderability, it’s something that I’ve been looking at pretty much all my career, we started off with tin/lead as a coating on a printed circuit board. It was used as a etch resist as part of the PCB fabrication process. This is when I and other engineers in PCB fab wore wellie boots to work. The plating covered the copper areas of the circuitry were needed to retain the copper during the etching process steps

We then introduced solder levelling to allow us to apply solder just to the pads that we wanted to solder on the assembled boards. In this process there was no solder underneath the solder mask which would have wrinkle during reflow and wave soldering. Depending on the thickness and quality of the solder mask and the width of the circuit pattern the green mask could flake off due to movement of the solder coating

Image above shows what originally happens with solder under mask back

in the day, not necessarily a failure but close to it

We then had the option of gold over nickel. Today there are different specification for solderable gold all defined by IPC. Then we also had copper OSP (Organic Solderability Preservative), a coating treatment that protects the copper surface during heat exposure. More recently we also have something that’s very popular within the automotive industry, tin finish. Tin has been popular with many customers but not one of my favourite surface coatings. In addition we have silver from some suppliers as a process finish 

Example of poor solder wetting due to the initial surface

preparation of the copper surface for OSP

Engineers need to evaluate each of the different finishes and determine what’s right for their particular product and application. I’m a great fan of OSP used in the right applications. It can be a very good and very reliable surface finish, but again you need to check and see the quality of the coating and the surface preparation from your PCB supplier during a factory process audit. Surface coatings can deteriorate quite quickly if the assembly process is not run correctly, and it will impact on the quality of soldering. 

Our guide to PCB Surface Finish Defects is available FREE to download

If we do consider Tin, initially solderability it is very, very good. However, it doesn’t necessarily stand up to PCB moisture bake out. It doesn’t necessarily stand up to multiple reflow operations before it starts to deteriorate. In terms of solderability if we look at nickel gold, it’s pretty flexible because it will stand up to multiple reflow and secondary assembly operations provided that it’s a good quality finish from the supplier. Like all of finishes, the finish needs to be assessed and determined to be correct for your particular application. IPC standards are a good reference source. I always specify a finish I require but also by its generic name, the name used by the chemistry supplier. Remember like solder paste in your assembly process not all pastes are the same it’s the same with surface finishes!! Designers and quality engineers take note and think about your spec to your suppliers or to multiple suppliers for quoting!!

PCB surface finish failure after reflow due to the incorrect wash off

procedure for reject boards after paste printing

Many of the failures we see with different finishes are poor or slow wetting. We can also see complete non-wetting of pads. De-wetting is where the solder initially wets onto the surface, then draws back away from the pad which may give us an unacceptable solder joint we should easily spot, manually or with AOI. Issues of poor secondary wetting like with nickel gold for instance, when you reflow, you dissolve the gold into the bulk of the solder and then you’re trying to wet the underlying nickel surface. If for some reason that nickel surface which was being protected by the gold remains poorly wetted with a brittle interface then mechanically, the solder joint will not be reliable  

Example of solder joint failure due to the surface finish quality used on the PCB

If you’re using any solder coating, like solder levelling as your solder finish, then at least you’re guaranteed you’ve got a good solderable surface to start with. It is quite uncommon to get solder level boards with poor solderability because you’ve already soldered it once, so that provides a protection to start off. However, if you have poor control of your PCB suppliers, they’re not in control of the type of alloy they’re applying to the printed circuit board. If they’re not in control of contaminants within the solder bath they’re using all these things need to be considered as well. If you can introduce some form of solderability test to have constant monitoring of your reflow process this is a major benefit to spotting changes in PCB process quality early 

Some of you may be aware I’m a great fan of the wetting indicator, which is the simplest and cheapest way of constantly monitoring solderability on your printed circuit boards in production at zero cost. I implement this on any product or any process that I’ve been involved with over the years. We have the design rules and a procedure for using the wetting indicator strips freely available. It just takes a little effort to introduce with your design team. The wetting indicator has been documented in many scientific reports produced by NPL during their work on finishes, lead-free introduction and benefits of nitrogen 

Wetting indicator test strip after paste application before reflow

 

Close up of wetting indicators above used on boards for first side paste and reflow

Example of the wetting indicator strips on project test panel. Normally one test strip would be included on both sides of a production board or waste area on a production multi process panel  

With wetting results these can then be compared with process yield in PPM

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

BGA popcorning
Dendrite formation
Open solder joints
Solder skips
Coating bubbles
Ultrasonic damage
Missing components
Incomplete past print
Component cracking
Tombstone chips
Solder balls
Solder bead formation
Solder shorts
Sulphur corrosion
Crimp connection failures

The list goes on and on, one month at a time

If you would like to download any of our Process Defect Guides or my three soldering books FREE just drop me an email. That is also the case the Wetting Indicator procedure  bob@bobwillis.co.uk 

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