Hello, my name is Bob Willis and welcome to Defect of the Month with WNIE TV. This month I wanted to talk about components floating. During reflow soldering components can float on the surface of the solder paste, they can also float on the solder after reflow. When the solder is in a liquid or semi liquid state parts can wet and float based on the design rules and the process conditions.
Lifting or floating can be caused by the volume of solder paste, it can also be due to poor termination solderability and slow wetting. This is an example of classic reflow soldering using convection, but what you can see happen in our online video is the component actually lifting during the paste reflow.
Reflowing would normally tend to wet the termination and pull the component down to the surface of the pad. But in this case not if the wetting action is slow. Excess movement exaggerates the degree of movement hence all of the different names for this type of defect. Tombstone, drawbridge, billboard, Manhattan skyline, it’s a long list
To rule out solderability there is a very simple Bob Willis test for solderability of surface mount components which if you look at some of our past Defect of the Month videos when we have illustrated this test with glass slides. Looking first at the solderability of the components could eliminate this as a cause. We are happy to tell you how to do it if you would like, with easy-to-follow written instructions. Our test method is also available as a set of training posters FREE from the SMTA website www.smta.org
Incorrect design rules can lead to lifting, you can’t have a generic set of footprints for some components. Yes, if it’s 1206 and 0402 chips the package dimensions are fairly well defined which is fine. But you have to bear in mind that some components are not generic and there may be subtle differences between suppliers, the key thing is making sure that the solder paste is not lifting the body of the component as opposed to the termination, if you have the design rules wrong then that may be an issue with your stencil and paste placement position
However, if you have poor pad dimensions in relationship to the terminations, there’s no reason why you can’t move the solder paste aperture to a slightly different position on the pad. Or even onto the solder resist and let the paste flow back to the joint area in a controlled way during reflow. If its starting underneath the package, it will lift the package and potentially leave you with an open solder joint as illustrated above
One of the things that you can see with components that lift or float this can be exaggerated when you’re using vapour phase soldering because of the speed of wetting and which is much faster than an air environment. With vapour phase, we should allow more time controlling the heat input to the printed circuit assembly before reflow. If you go from let’s, say just below reflow temperature straight up to reflow temperature, there’s a very small window. So what it will tend to do is exaggerate any movement of the component. It’s the same with nitrogen reflow If you’re using a nitrogen in you reflow process and you’re suddenly getting a lot of lifted components, one of the simple old school engineering tricks is just to reduce the nitrogen level within the reflow process. If you’ve never had a problem and then suddenly you have changed your process going possibly from air reflow to nitrogen, so reduce the nitrogen concentration that quite often will reduce the problem of lifting components. However you must go back. Don’t use this as a sticking plaster you must correct the problem, which is the design or the solar paste volume.
Thank you for listening/watching our Defect of the Month videos on WNIE TV. If you have a defect that is really annoying you and can’t find a solution on any forums, drop me an e-mail. I’d be more than happy to create another Defect of the Month video in the not-too-distant future. Thank you very much.
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 of over 100 plus videos created over many years with NPL, IPC & WNIE
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
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