Hello, my name is Bob Willis and welcome to Defect of the Month with WNIE TV. Here is another video to watch and short article with photographic examples to add to your collection of training material. Help your team understand process and product failures.
This month we will discuss soldering and assembly pallets/fixture, the unsung heroes in manufacture that provide so much support. Pallets and fixtures are used in most companies; they are expensive and often not looked after or loved. They are used for wave and selective soldering plus reflow of printed circuit boards. In some cases with thin substrates and flexibles they pass through the complete SMT process.
Examples of different soldering pallets above from the very simple, complex and the best I have ever used in production
One of the first things to consider if you’re going to use pallets on your boards or a customer’s build is to decide on a supplier. You need to select a material which is robust and won’t cause too many issues in terms of warpage and sag during the soldering process. You want the board to be properly supported and flat, if you can’t keep boards flat what is the value in using pallets.
I also believe it should be the responsibility of one engineer to coordinate/review palette design within a company, they gain experience with looking at all the different designs coming through production. Having one engineer responsible per project or customer is not ideal. The reason being that mistakes can occur with multiple engineers dealing with a chosen supplier. One engineer will benefit experience dealing with many different types of customers product and build plus a great relationship with the fixture manufacturer.
Example pallets used for flexible circuits and thin high frequency substrates
We most commonly use pallets for wave and selective soldering, but we can use them for reflow screen printing, placement soldering, and more commonly now robotic soldering. We will illustrate some different pallet types in this short article, show you a couple of examples of each but also what I want to focus on is defects. Now when you’ve decided on your supplier it’s worthwhile investing in one or two pallets to conduct process trials and refine the design of the palette to give you the best process results. It will also highlight some of the process issues that might occur during the design review
I would always suggest that you number the pallets, possibly not in the early stages, but if you invest 10 or 20 pallets. They should be regularly cleaned and regularly checked to make sure they’re still performing effectively hence the numbering. I’ve spent many hours trying to isolate defects which were all associated with soldering pallets. As the pallets were numbered, we could track faults to pallet numbers. Today even barcoding is used on fixtures in some factories
Now what sort of process problems can occur?
First of all solder skips where solder does not make contact with the termination, that’s a lot to do with pallet design. If the board or the pallet flexes during the soldering process it may not come into proper contact with the solder. A fixture will not wet hence it will displace the solder around the edges preventing full contact. Using a high temperature glass plate and video camera is great way to show engineers and operators flux and solder contact and any displacement points during failure analysis. If there isn’t enough contact area with the solder heat penetration into the printed circuit board will not be effective so through hole fill will not be effective leaving limited fill and no visible wetting to the top of the plated through holes.
Close proximity of solder points to the edge of the pallet will cause defects.
You can have a great design but remember in the real world of manufacture the solder wave does change height during the day
We need to eliminate excess flux traps in the palette it can outgas during the soldering operation and actually push the solder away from termination points. This is one of the ways in which skips can occur partially on SOT 23 packages. This has been illustrated in the past with Kapton Tape on high temperature glass plates originally called a LevCheck in my past onsite workshops.
If we have too much flux penetrate underneath the edges of the board and pallet when running double sided reflow and selectively this can create issues. If no clean flux gets trapped between the board and the pallet and doesn’t have enough time to deactivate flux a no clean product may lead to corrosion or cleaning issues at a later stage. These are the sort of issues that you need to bear in mind.
The reason I mentioned about regular checks and maintenance is solder slithers and balls do get trapped around the edge of pallets and can lift the board by a couple of millimetres and actually cause soldering defects. I once watched a fully automated robotic line loading assembled boards into pallets in Europe. Unfortunately no one told the robots to check the pallets before inserting the board in the pallets and locking the hold down clips. The concept was great but the execution not so good.
Now here’s just a couple of examples of some of the worst evidence of poor consideration in terms of wave soldering and not using pallets. This is one of the worst examples I ever experienced in all my years of running training courses, you can see clearly why. That board is not flat.
What you don’t want to see on any manufacturing line
What we want the board to be is flat supported by a palette, possibly by some other techniques as well. But the key thing is good design, good engineering. One other point I’d like to raise is that if you’re interested in more information on soldering pallets, Peter Swanson and I wrote an article on the subject a few years back (The Use of Pallets in Wave & Selective Soldering Process). It’s available if you’d like a copy, just ask.
Thank you for listening to defect of the month. My name is Bob Willis, and I look forward to presenting another defect of the month next month.
We have many Defect of The Month videos and WNIE Online articles which we hope will help you solve your process and product failures. We have listed a small selection from over 100 plus videos below created over many years with NPL, IPC & WNIE TV
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
Our defect list goes on and on, one month at a time