|Ask the Experts|
May 22, 2020
EMI Shield Reflow Soldering Techniques
When reflow soldering a RF shield frame to a PCB, what techniques can be used to keep the frame perfectly aligned on its footprint?
Through hole pins can cause signal leakage, could a tooling pallet be used without significantly affecting the reflow profile?
After reflow, what effective techniques can be used to ensure consistent solder coverage around the entire perimeter of the RF shield frame?
|Expert Panel Responses|
Reflow fixtures can be successfully implemented with careful attention paid to thermal mass and airflow (less mass and more air flow). In fact the use of fixtures can ultimately improve reflow performance by allowing increased airflow around the board, as we have experienced using our board carriers.
Of coarse this also requires careful measurement of the reflow profile to ensure you are getting the assembly hot enough, but not too hot. In regards to post solder testing, I think x-ray would be the common solution to offer the highest confidence.
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
Fixturing the shield during reflow is the best way to hold it in place. It must be as light weight as possible. A light metal fixture may be best, since the metal will conduct the heat provided by the oven verses a plastic or composite fixture, which is an insulator.
It must not block the convection air flow, as much as possible, which is what carries the heat to the shield. Best to increase the convection flow and lower the zone set points, which will cause faster heating, even on the heaver shield, but to a lower temperature, which is safer for the other components.
You will need to thermally profile the board, specifically in and around the soldered portion of the shield, to make sure it meets the solder paste's thermal requirements, and to make sure the oven settings (recipe) are not too aggressive as to damage the other components on the board.
As for the assurance of consistent solder coverage around the entire perimeter, a good visual inspection for properly wetted solder should be sufficient, unless your requirement for full coverage is an air tight seal, which may demand an X-Ray view.
Senior Project Engineer
Electronic Controls Design Inc
In most cases, the RF Shields can be machine-placed very accurately and the surface tension of the solder should hold the component in place during reflow.
If your design requires a 100% solder seal of the base perimeter, you should be able to achieve that by screening ample solder paste and by minimizing the gaps between your stencil apertures. I would not expect that fixturing would be necessary.
Principal Product Engineer
Benchmark Electronics, Inc.
Tooling fixtures can be used for this application. The reflow profile will have to be modified to compensate for the heat taken up by the fixture. It is best to measure and set up a new profile by running the PCB with the fixture in place.
Consistent solder coverage can be achieved through creative stencil design. Some stencil suppliers offer design services to help optimize solder coverage in situations like this. I suggest that you talk with your stencil supplier about it.
You could use a pallet with spring loaded supports for the shield frames but it will increase your production costs. One economical way is to use short guide pins to avoid any signal leakage and over-print around the perimeter of the frame. I have used this method before with RF and I got excellent results.
Senior Manufacturing Engineer
If the self-centering nature of solder post-reflow is not adequate for the required alignment of the RF shield, a fixture can be used during reflow.
In this case, the fixture thermal mass and its effect as a heat absorber vs the heat going to the component, solder & PCB - has to be measured with thermocouples when designing the reflow profile. In most cases, there is no significant change to the profile.
To ensure consistent solder coverage around the entire perimeter, ensure optimum print transfer efficiency and stencil design. Choose the right solder paste chemistry that has a lower tendency to form solder balls when overprinting. If the PCB pad and RF shield pose constraints on the amount of solder paste that can be printed, consider solder preforms in combination with solder paste that can be placed in strategic locations along the perimeter of the RF shield pad. The preforms simply add to the volume of solder and can be designed so as to not interfere with placement of the RF shield
Technical Manager - Europe
We need an assembly fixture to align it perfectly. A tooling pallet can be used without significantly affecting the reflow profile. Reflow profile is not a major issue.Lucy Iantosca
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