|Ask the Experts|
August 31, 2018
Problems With Skewed D-Pak Components
We are having trouble with surface mount D-Pak components skewing during reflow. We have checked our profile and have tried a diagonal hatch aperture on our stencils, but are still having problems with skewing D-packs. Any thoughts?
|Expert Panel Responses|
Keeping any component from skewing during reflow is all about symmetric wetting. If the wetting of the solder is not symmetric, components can easily shift during the reflow process. The two culprits here are thestencil aperturedesign and the design of the board. I don't know the shape and size of the stencil aperture, but my guess is it has been substantially reduced and shifted towards the D-Pak leads. The solder willwet/spread towards the rear of the ground pad. As it wets, the surface tension will pull the D-Pak with it. Also, the surface finish appears to be OSP and there is very little wetting of the ground pad on CR315. The stencil aperture for the CR315 ground pad looks to have too large of a reduction. Regarding the design of the board, both D-Paks are skewing towards the side/corner of the ground pad with the large GND traces on them. Molten solder does not like a temperature differential across the same SMT pad. If there is one, more of the molten solder will move to the side at ahotter temperature. Once the board hits the peak temperature in the oven, it will start to cool. The solder is still above liquidus temperature, but the side/corner of the ground pad with the GND traces will have a higher thermal mass and will cool at a slower rate. It will be at a higher temperature, prior to falling below liquidus, and more of the solder is pooling at this side/corner. This is pulling both D-Paks towards the large GNDtraces. It would bebetter if the tracesentered the ground pad from therear rather than a side or corner. This skewing problem can be overcome with the proper stencil design. If you would like me to make a recommendation, I would need to review the current stencil design and the board files. My email address is firstname.lastname@example.org.
V.P. Technology & Engineering
Fine Line Stencil, Inc.
I have seen this before and in a least one instance the problem was too much solder, it may be worth reducing the solder by 20-30%, just splitting the deposit in half with a large gap up the middle. On a different occasion it was possible eliminate the problem by modification of the profile. As is often the case the cause may be one of several causes or even a combination of them.
Senior Applications Chemist
This is likely the transition of solid to liquid not occurring across the parts at the exact same time. You can potentially prove this by turning the boards through 90 or 180 deg and see of the rotation remains and in the same direction. The solution is to modify your process and or profile.
This is a relatively common defect. The main cause is due to too much solder under the heat sink (big pad) this causes the device to float during reflow and leaves the component slightly askew. It can be fixed by reducing the volume of solder on this pad, either reduce the pad size by about 30% or cut the big pad into 4. It can also be caused by poor wetting to the component, but this is very rare. Alternatively dispense a couple of dots of chipbonder on either side of the component and this will hold it in place during reflow.
The component is moving due to the surface tension of the fluid flow and density of the molten solder. My reasoning being as the solder becomes molten it lifts the component or allows the component to float on the molten metal and allows it to move around the pad and this movement is based upon the surface tension of the solder and the solderability of the component. The amount of solder beneath the ground pad has to be changed to prevent the movement or reduced to prevent the component from floating on the molten solder. Therefore, if the pad is there for thermal relief, change the pad configuration, either on the stencil or on the artwork, to a horseshoe type of design as this would localize the solder and reduce the ability of the component to move around. Whereas, if the pad is there for electrical continuity, then just apply small amounts of solders around the ground pad which when molten would not lift the component up and allow it to move around. This can be done by changing the apertures sizes in the stencil.
Vice President, Technical Director
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address