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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?

R.W.

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.

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Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

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.

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David Whalley
Senior Lecturer
Loughborough University
Mr. Whalley has over 30 years of experience as a R&D professional within the electronics manufacturing sector, both in industry and academia, and is the author of more than 200 publications. His main areas of expertise are in joining processes and materials, particularly electrically conductive adhesives and soldering processes and their monitoring and control. He is also experienced in electronics thermal management and additive manufacturing processes for electronic interconnects.

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.

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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

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.

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Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

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.

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Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.

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.

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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
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