|Ask the Experts|
August 21, 2020
Critical Part Fixture During Reflow
I'm populating a PCA with a component that has critical placement requirements. My pick/place machine does not have epoxy dispensing capabilities.
I'm interested in using a fixture to hold the part in place while it goes through the reflow oven. Is this a wise practice?
Are there standard fixtures for this process?
|Expert Panel Responses|
Fixtures are occasionally used to hold components during reflow. The fixture itself will act as a heat sink which changes the reflow profile.
I suggest setting the profile with the fixture in place to be sure it meets the solder paste suppliers recommendations.
I don't have any experience with the use of fixtures for this, but the obvious concern if using a conventional convection reflow furnace is whether a fixture would interfere with heat transfer to the component in question, and possibly to the surrounding components, because of both its thermal mass and any obstruction it creates to air flow.
This may make getting the reflow profile within specification for the whole board more difficult and I would definitely want one of my reflow logger probes on this component. Depending upon the type of component another concern might be whether the fixture affects the resulting thickness of the solder joints, by interfering with how the surface tension of the solder controls their shape.
This could potentially affect long term reliability, but if the component is suitable for placement onto adhesive then it probably wouldn't be a problem. I have seen attempts to improve the positional accuracy/repeatability of components after reflow by using the solder surface tension forces, in combination with specific pad layouts on component and PCB, to pull them into a specific position.
If you are using a standard component what you can do this way is probably pretty limited, but it might be worth a look at.
Fixtures for holding components in place during reflow, or other soldering processes, are very common and highly recommended. And, if the placement is critical, and you choose to Epoxy the component onto the board first, such a fixture may be required to hold the component in place while the Epoxy cures. So either way, you will need a fixture.
Because most every assembly and component placement situation is unique, most fixtures are custom. Web search "images of SMT solder fixtures" and you will see countless examples. There are many companies that use temperature resistant materials to fabricate fixtures to hold boards and components in place while still allowing the needed heat flow to properly solder the component.
Make sure you run a thermal profile of your assembly in the fixture though the solder process to make sure proper soldering temperatures are achieved.
Senior Project Engineer
Electronic Controls Design Inc
Most SMT parts normally center themselves during reflow pretty well. Between self-centering in reflow and the accuracy of most recent Pick and Place equipment, even 0.4mm pitch QFNs yield very high, especially for just placement. I'd also have concerns about the ability of an Operator to attach a fixture without moving the part of concern or other parts on the assembly.
Besides just having accurate placement initially, it may help to minimize the size of the component pads so that when the part centers after reflow the available variation in possible final location is minimized. This would be more effective with leaded packages than QFNs or BGAs.
If the part is pin-through-hole, there are fixtures with "pushers" that will hold down a part when traveling through the Wave Solder process. This, along with minimizing the hole size may give more accuracy if movement is excessive during soldering.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Since little information is given about the board assembly or component in question, it's difficult to provide a specific suggestion. In most situations the surface tension of molten solder paste during reflow is adequate to center most SMT parts on the pads.
If this is not the case you may want to re-visit the pad geometry and volume of solder paste deposition by your stenciling operation. In most cases the surface tension will also hold most SMT parts in place when they re-reflow in the reflow oven if the board assembly is double-sided, providing the surface tension is greater than the mass of the component.
Adhesives are generally only used to hold SMT parts in place: 1) during second side reflow soldering if the mass of the component is greater than the retention force of the solder paste, or 2) during a follow-on wave or selective soldering operation if the board assembly also has through-hole parts where the continuously flowing volume of molten solder will most definitely re-reflow any SMT parts.
Using fixtures in the reflow process is a common practice. Unfortunately, due to the design of the board, population, reflow profile, etc., these solutions are pretty much customized for each application.
You might want to think also about alternative solutions for the adhesive application.
Engineering and Operations Management
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