Ask the Experts
October 16, 2017 - Updated
October 12, 2008 - Originally Posted

Questions About Solder to Flex Circuits

What is the best way to solder fine pitch surface mount connectors to a thin flexible circuit?

S. S.

Expert Panel Responses

Using a Vapor Phase will provide an Oxygen free solder environment for best solder joint results. It will also guarantee 0 Deg delta T and provide a controlled ramp rate. Vapor Phase systems are available as batch systems or in-line and so if this is a relatively low volume application, you could chose to run a batch system. This would save space and power when compared to a typical reflow oven.

Allen W. Duck
ATEK llc
Allen Duck is a 20-year Electronics Industry veteran with Global experience in multiple fields of technology and management. He started A-Tek in 2006 to provide a sales and service channel for international equipment companies wishing to offer value based solutions to USA companies.

Once the flex circuit material is secured, you can align the component by hand, apply liquid flux and using a hoof or mini-hoof soldering tip to hand solder the component in place. The biggest concern you will have is overheating the flex material. I would suggest a Direct Power soldering iron with a 600 series tip cartridge. This combination will provide the speed that you need along with the low temperatures that will keep the assembly safe. The best technique involves the use of a fixed temperature and variable power soldering iron. I would suggest using a hoof style tip to provide the maximum power transfer to the assembly while keeping the temperature as low as possible, which is typically in the 600F or lower range.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

I would suggest that this is completed in a traditional SMT process with solder paste and an oven with blow speed control. The flex circuits are than placed into a "boat " fixture that carries them through the oven and allows for soldering. The boat is also used to keep the flex on a flat surface and at times used to hold the assembly.

John Norton
Eastern Manager
Vitronics Soltec
John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

The best way we've found to do that type of assembly is with fixturing to hold the flex in place, then paste it with solder paste, position the components and reflow in a preset profile oven. The thing you want to do is hold the flex from moving during the paste deposition and component placement process. Once this is done, submit the entire assembly to the reflow process.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

How thin is the flexible section? Can you support the area with FR4 material? This way you can create a pallet and mount via SMT machines. Hand assembly using a stencil/paste will help and don't forget to bake first 2hrs @ 250 degrees.

James Mahoney
Applications Project Manager
Quick Turn Flex Circuits LLC
James Mahoney is a Technical Operations Manager with a 20 year track record in managing new product introduction. He is a skilled leader, motivator and problem solver with a strong background in Product Knowledge and Engineering Management.

Solder will not attach to non-metallic surfaces. Therefore the component body need not be masked. On the other hand solder bridge between component pins is a definite possibility, which can only be alleviated by proper component layout or orientation. Also as a general design rule try to place SMT ICs on the top side of the PCB so that it does not have to be soldered using wave process. If the design does not allow this then specialized fixtures or masking becomes necessary to avoid any bridging of the leads.

Bjorn Dahle
inspīre solutions LLC
Bjorn Dahle is the President of inspīre solutions LLC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

Defining solder attach for flex circuit is quite common. There are a couple of tricky issues that need to be understood when talking specifically about fine pitch components (QFN, uBGA, etc)
  1. Substrate support. It is very important to keep the flex circuit as flat as possible during the attachment of all components. A common method of placing components is using a vacuum jig to hold the flex circuit down.
  2. Substrate support relative to fine pitch areas. When placing fine pitch components, the vacuum structure has a significant influence. If the vacuum is directly under some of the leads of a fine pitch component, there is a chance the vacuum will "pull" the flex into the hole and prevent the component from making contact with the flex circuit lead. This could result in an "open circuit". BGA and like components are more forgiving when it comes to the strategic vacuum design because of the volume of solder.
  3. Solder Printing. Again, the substrate support fixture is a critical element throughout the process of SMT assembly on flex circuits. It is important to make sure the flex does not bend after solder has been applied. If there is any flex within the flex circuit, there is a chance the solder could bridge from one pad to another. This will result in "game over" before a single component is placed on the flex circuit and is even more of an issue with fine pitch components.

Neil O'Brien
Sales Director
Neil O'Brien has worked in the field of electronic manufacturing equipment for over fifteen years and is currently Sales Director for Finetech, a manufacturer of precision rework systems and die bonders.

Soldering to flex circuitry can be accomplished in several ways. Flex circuits are prone to de-lamination, a direct result of excessive or pro-longed heat cycles, so whichever approach you choose for assembly, care must be taken when developing the thermal profile to ensure you avoid such damage. For fine pitch devices, you may consider hot-bar technology which can fuse all leads simultaneously. In this case, heat remains relatively local to the device being applied. You can also successfully reflow flex circuits as long as you observe thermal criteria for the flex assembly. Often when reflow soldering flex circuitry, a fixture or heat-sinking "mask" such as an aluminum template can be utilized to protect the un-processed areas. This assembly method may work very well for your application where the connector area can be exposed, and remaining circuitry remains protected from excessive heat exposure. While not necessarily suggested for bonding connectors, flex circuits can also be assembled using conductive epoxies. These materials are processed under curing profiles which are much cooler than the standard reflow profiles utilized for solder pastes, and as a result are much safer for the flex material. Conductive epoxies/adhesives can have other drawbacks as well, so before proceeding, make sure you check with your material supplier on your specific application.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

The obvious difficulty in soldering a fine pitch surface mount connector to a thin flexible circuit is creating enough support to print paste, place the surface mount connector and reflow to ensure good solder-ability. One of the easiest and most straight forward methods to create proper support is to have a fixture built for your flex circuit that can hold and support the circuit through the entire process. Fixtures vary in size and are often referred to as carriers or pallets. Fixtures can be built for a single circuit, but are often built for multiple circuits to reduce handling. Prices for fixtures can vary depending upon the complexity of the fixture needed; however fixtures in general are considered a low-cost solution to avoid a potentially frustrating and non-productive problem.

Michael Ray
Integrated Ideas & Technologies, Inc.
Mike Ray is founder and president of Integrated Ideas & Technologies, Inc., a manufacturer of SMT stencils. Mike Ray developed the first successful universal frame and holds patents on the AXIS laser system.

There is no one answer for every application. If the flex circuit remains sufficiently planar during processing, including reflow, a standard SMT print-placed-reflow process will work well. If the flex circuit has planarity problems, it may be necessary to hold the part down during reflowwith a small weight to counteract warp in the flex circuit. When properly designed, such re-usable weights can be pick-and-placed like any other SMT component from tape or matrix tray.

John Vivari
Application Engineering Supervisor
Nordson EFD
Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.
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