|Ask the Experts|
July 30, 2018
Stencil Issue with PLCC Component
We have an insufficient solder joint heel fillet with a PLCC J lead component. The aperture on the solder paste stencil is 1:1 (30 x 100 mils). Visually the solder fillets appear okay, but testing indicates that we need additional solder at the heel joint. We cannot increase the stencil thickness due to presence of TSSOP & QFP packages. What do you advise?
|Expert Panel Responses|
Printing solder paste beyond the land pad boundary is an acceptable, and common, solution for increasing the solder volume without increasing the stencil thickness. Solder, both leaded and lead-free, coalesces very well and will pull back onto the land pad during reflow. I routinely print up to 0.040" beyond the land pad with no solder ball problems. However, the quality of the print and reflow processes are key to preventing solder balls when printing beyond land pad boundaries. With J lead and gull wing components, the component body is up off of the board. This allows you to print beyond the land pad to the heel and/or the toe side. For pitched devices, I only recommend printing beyond the land pad boundary in the length and not the width. Increasing the width of the paste brick can easily cause bridging problems, especially on fine pitch components. In your case, you want to print beyond the land pad on the heel side. I am assuming the resistance of the solder joint is too high, but I don't have enough information on how high to determine the required increase in solder. I would recommend a 15% increase in solder paste volume, which should give you approximately an 8% increase in solder. A 15% volume increase would be an aperture size of 0.030" x 0.115". Extend the aperture 0.005" to the heel side, but make sure you do not print solder paste onto open vias or any other solderable surfaces. If you do get too close, you can also extend on the toe side.
V.P. Technology & Engineering
Fine Line Stencil, Inc.
This issue can easily be addressed by contacting your stencil supplier. Custom adjustments can be made to the stencil data allowing for extra paste to be printed on the heel only. Determine with your stencil vendor the percentage of paste that would need to be added. Be sure to take into account the type of paste you are using and its particular behavior in your facility. If the paste exhibits poor wetting characteristics, over printing the pad by too much will result in solder balls and bridging.
Integrated Ideas & Technologies, Inc.
Addition of a solder preform (which is basically a brick of solder of the same alloy as paste) to add to the paste volume in this case. Since the alloy is the same, the paste and preform melt at the same temperature and time and more solder volume is achieved. Also the flux in the paste which is 50% by volume will be sufficient to take care of the preform. The preform dimensions could be of any shape as they are going to melt anyways. These preforms can be packaged in tape & reel / bulk pack.
Technical Manager - Europe
Have you tried making the stencil aperture larger in heel area maybe overprinting by 10 to 30 mils?
Vice President Technology
There are two issues happening here. Although you have a stencil aperture of 1:1, it does not mean you are receiving 100% transfer from your solder paste. To get an accurate reading of your transfer, a volume measurement system is necessary. If you discover that you are not receiving a full transfer, your next step is to consider using different stencils. Using the right stencil is essential to the efficacy of your printing. Nano coated stencils will help, providing they are from the right company. If you find that you are receiving the full transfer of paste, there may be another issue. Your next step would be to inspect the leads. If you notice they are much thicker after reflow compared to before, your problem may be that your oven is not under control. If this is the case, you could be heating up the leads faster than the board which will result in "solder wicking." Wicking occurs when the solder paste is deposited correctly, however, due to the lead heating up faster than the board, the solder wicks up the lead and sends it off of the board. The result will resemble an insufficient solder connection at the board interface. If wicking is the culprit, heating the board and the lead at the same time or adding more heat to the board should help minimize your chances of this occurring.
Deck Street Consultants
Check the Aspect ratio is greater than 1.0 and Area Ratio is less than .66 for Laser Cut stencil and .50 for electroformed stencil. I think this solve the issue.M.B.Raju, Vinyas Innovative Technologies Pvt. Ltd.,, India
Unsure why they would initially look good but under testing they require more solder BUT if this is corrct then I would double check that you have correct pressures applied to the printer set up and you are not overpressurising the paste causing it to shear thin applying more flux instead of powder. Rule is 1Kg Pressure every 50mm blade length. If this is correct and you are using T4 Powder (Better compaction = better metal load on Pad) then I would simply cheat to prove the point and apply a small amount of Polyimide beneath the stencil to produce an offset to the PCB which will apply more paste on the pad. If this works then speak to your stencil guy to resolve the problem through stencil design.
Technical Sales Manager
BLT Circuit Services Ltd
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