|Ask the Experts|
August 25, 2020
Challenges Placing RF Shields During SMT Assembly
When placing an RF shield during SMT assembly, what are some ways to keep the shield flat during reflow? I am seeing the shields lift on one side causing them to exhibit a bow/twist post-reflow.
|Expert Panel Responses|
Reflow solder pallets. There are many creative pallet fixture manufactures out there that can develop pallets to hold your RF shield square and straight through the reflow process.
One could also explore the question, why does it lift?
This could be because of uneven heating, which is hard to avoid because the shield's leading edge enters each zone first and starts to heat, while the trailing edge has not yet entered. The affect of this is increased as the RF shield gets bigger (or longer), say much over 1" in size. If the shield is rectangular, it may warp less of you can turn the board so the longer edge of the shield enters the oven first, perpendicular to the direction of flow through the oven.
If this does not help, a reflow pallet may be the only way to prevent the warp.
Senior Project Engineer
Electronic Controls Design Inc
Very often when components lift during reflow the root cause is due to various factors including solder paste outgassing caused by too rapid of a preheat profile, or displaced components resulting from oven issues…conveyor vibration being the most common. It is suggested to realign the reflow oven conveyor rails and lubricate them because any vibration during the ramp-up cycle and before wetting occurs will cause components to lift. Also check the fan speed of your convection dominant reflow oven and reduce them slightly. If the issue is due to solder paste outgassing we would recommend reducing the preheat slope.
If only the RF shield is lifting and no other components you should investigate other possible root causes such as internal stresses as a result of punching and bending the shield that are being relieved during reflow. Make sure the shields are passivated to relieve internal stresses prior to reflow. This may require the steel to be passivated prior to punching and bending so it doesn't strain relief during reflow. Another possible cause is the differential of thermal expansion coefficient between the shield material and the PCB in an over-constrained design. Calculate differential expansion and accommodate variation in the design.
As an alternative, you could investigate the use of vapor phase reflow soldering since vapor phase has extreme uniformity in heating or depending on the cause fixturing or latching the shield to slots on the PCB may be required.
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