Ask the Experts
November 14, 2022 - Updated
December 3, 2014 - Originally Posted

Overlap Solder Joint Failures

Overlap Solder Joint Failures
What is the strongest way to connect two 8mm flexible LED strips together? We now lap solder one strip that has "U" shaped pads onto another strip that has rectangular shaped pads. See the photo showing the two strips prior to soldering.

We need to make many thousands of these connections each month. Currently this process is being done by hand and we are having cold solder joint failures. Can you suggest an automated way to make this solder joint connection?


Expert Panel Responses

Not knowing much about your process and end product; the joint between strips could be flexing thus generating a solder fracture. Consider strip-to-strip connectors. They come in different sizes and they securely hold and connect several LED strips without the need of soldering.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

This is a common issue picked up by multiple manufactures which are into L.E.D lighting. The most reliable way to overcome this issue is to make use of Through-hole technology, and connect the two strips using link wire.

The befits are as follows:
  • Link wire can allow for stress during movement on your assembly process, therefore no in plant failure.
  • Having the though hole plated solder joints increase you products long term reliability, as this can support much more heat and cooling cycles.
Your hardware engineer will have to calculate the thickness of the link wire, as the thickness of the wire will be directly proportional to the total current consumption of your L.E.D arrays.

To answer you second part of your question, Automated soldering does exist, I have used this company before for automatic soldering of a harness to a PCB. They are able to meet a requirement of 3 seconds per joint.


Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa
Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

The description of "cold" solder joints is not conclusive as to root cause, however that should not prevent us from envisioning a semi-automated process for soldering the connections that should hopefully eliminate the condition.

I do have a suspicion as to the root cause of the "cold" joints, which is relative movement of the two flex ends during cooling. Here is what I'd envision for automation:
  1. Assuming that the LEDs are SMT components, we can apply paste to the end terminations of one of the flexes during paste printing for SMT mounting of the LEDs. This will provide a repeatable solder volume on the lands
  2. When the flexes are to be joined, a small amount of no-clean paste flux can be applied to the non-soldered lands on the other flex. A tool can be created to hold each of the flexes in such a position that they are in proper contact:
    a. Pads are aligned, with the fluxed pads in contact with the pre-soldered lands on the other flex
    b. There is a slight deflection of each flex circuit, providing a small force that will hold them in contact
  3. We may now use anon-contact method to heat the flexes to soldering temperature. Given the low mass, I would suggest that a hot air tool mounted to a gantry may be cycled into/out of position on a timed cycle to provide a known heat input to the flexes. The air temperature and flow can be adjusted to give the proper soldering temperature and time.
  4. Once the soldering is complete, a timer can be used to notify the operator that the required cooling period(determined by your process engineer or technician) has passed and the joined flexes can be removed.
A single instance of the above process tool should easily be able to accommodate more than 10,000 parts per month based on a single shift.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

For automated assembly, an investment in fixturing would be needed to hold multiple sets of the 2 strips during heating
  • Position Strip 1
  • Print / Dispense paste on solder strip 1 pads
  • Place strip 2 on top of the paste
  • Hot air solder the assembly
A low-spatter paste would be needed so that you don't end up with solder balls all over the assembly.

A flux-coated preform is another option to solder paste where the preform comes in tape & reel and can be placed. The advantage is that the preform is 100% solder (as opposed to paste that is 50%flux by volume) and would not be prone to spatter.

Comparable in speed to dispensing. Much slower than printing

Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

This is a tough one since there are so many variables we do not know from this statement. We would need much more detail to properly reply.

If J.M. would like to send us some samples and provide us with more details,such as current solder used, flex melting point, pad size, distance between pads, etc: we would be happy to try and find a viable solution for them using our Hot Iron soldering robots with a variety of solders.

Gary Goldberg
President and CEO
Mr Goldberg has practical experience in production line layout, process flow and cycle rate analysis. He knows how to avoid bottle necks and most related PCB or pallet handling questions.

In looking at your application for soldering two 8mm flex circuits, outsourcing this process immediately comes to mind. I would recommend you contact Analog Technologies Corp.

The reason I recommend them is because Analog has developed their own custom-built laser soldering machines designed just for this application and are experts in this area. They know how to do this type of high-volume processing using their laser soldering machines and specialized solder pre-forms with a special type of oxygen evacuation chemical that when activated remains on the pads for the entire solder cycle and produces pristine solder joints with no spattering of solder balls or solder fines (it is not a "flux" in that sense of the word).

The lasers are completely programmable, not only the power and duration are programmed and are variable, but the spot size and ability of the spot to change size, power, AND travel during a pulse are also programmable as well as the angle of the laser, and as such they can develop a perfect laser profile that will precisely pulse from a pre-heat stage and then lase the area to be soldered so you have a perfect intermetallic formation where the solder melts from the inside out, but ONLY that very precise area is heated.

While you could theoretically develop a soldering process yourself using lasers already available on the market or some other technology such as hot bar soldering or directed convection heat, the time and cost involved in this undertaking does not make it a feasible solution.

Customized laser soldering eliminates the delamination and flux entrapment issues so often seen when trying to solder flex circuits with those other methods, but you have to understand the types of solders, fluxes or anti-oxidation compounds, and many other issues (for example, attempting to laser solder with solder paste is definitely not recommended).

I would contact Bill Berg or Joshua Muonio at Analog (952)894-9228. I am sure they can help you bring your product to market much faster with less cost, both up front and thereafter. I would leverage their equipment already in place and qualified, and their expertise, experience, and knowledge of flex soldering and materials including the various types of flex circuit materials. They have performed very well for other clients that I work for.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

I would evaluate using solder preforms and hot bar soldering or IR heating.

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Reader Comment
I can back up Mr. Stadem's comment. ATC (Analog Tech. Corp.) is a really good group to challenge with something like this. They have met (and exceeded) our needs every time.
Tim Griebel, Starkey Hearing Technologies

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