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March 4, 2020

Point to Point Soldering vs Drag Soldering

Our process engineering department has recently made a change to the soldering method used to solder displays to circuit cards. The display has 18 pins on each side and is ~2mm from pin to pin.

The old method is to solder each pin individually. The new method uses a hand drag soldering technique with a slant chisel soldering iron tip to solder all the pins in one pass while adding solder along the way.

We are experiencing solder shorts between some pins. Is this drag soldering technique on through hole components an acceptable method?

T.E.

Expert Panel Responses

A wave soldering process may solve this issue. If you do not have a wave soldering machine, there should be a contract electronics manufacturer (CEM) near you who could provide this service.

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Mitch Holtzer
Global Director of Customer Technical Support
Alpha Assembly Solutions
As the Global Director of Customer Technical Service (CTS) for Alpha, Mitch sets direction and provides coordination for the Alpha CTS group in a global capacity. A major focus of this position is to provide strategic support to OEM, CEM and Automotive customers and target accounts. Mitch joined Alpha in 1998 and has progressed through positions of increasing responsibilities in Marketing, Product Management and R&D. He is a graduate of Purdue University with a degree in Chemistry and holds an MBA from Temple University.

I believe the drag soldering technique is not the correct method considering the contacts are only 2mm apart from each other.

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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.

Drag soldering, sometimes called the Miniwave technique or multi-lead soldering, is a popular method used to install QFP's and other fine-pitch surface mounted components. MiniWave/Drag soldering tips usually feature a concave, spoon-shaped end that acts as a solder reservoir, allowing you to drag a bead of solder across multiple leads at once.

MiniWave tips act as a miniature wave-soldering machine (hence the name), allowing you to swiftly solder an entire row of pins at one time. It is an acceptable method and documented several times in IPC 7711 (see Procedure #’s 5.5.1, 5.5.2 and 5.6.4) as an approved technique.

While it is normally not used to solder through-hole components, I have met some technicians who have successfully mastered this, but I think it is much harder to accomplish and less reliable. I would recommend restricting this technique to surface mount rather than through-hole applications.

It is not uncommon to generate solder bridges during drag soldering, especially if you are using a pb-free alloy. Applying a generous amount of flux is important, as is perfectly aligning the leads to the pads. If you are misaligned by even 1/5th of a pad, you will likely develop bridges.

If you generate a solder bridge, apply a generous amount of flux to the affected joints, wipe the MiniWave tip clean, and gently touch the cleaned tip to the bridged joints. The excess solder should wick onto the clean tip, and the bridge will be gone. A vacuum Solder Extractor or wicking braid can also be used.

For a detailed video, full procedure and instruction on how to properly perform Drag/Miniwave soldering, see PACE's "How to Install a Fine-Pitch QFP using a Miniwave/Drag Soldering Tip" at https://paceworldwide.com/node/442.

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Aaron Caplan
Director of Marketing & Training
PACE Worldwide
Mr. Caplan has been involved in electronics manufacturing technology since the 80's, is an acknowledged expert in Miniature/Microminiature Electronics Repair, and provides technical expertise in teaching of non-destructive PCB rework/repair techniques, educating industry on how to properly utilize these techniques.

Drag soldering for Leaded SMT components has been around since the evolution of the hoof or cup style soldering tip. This method was developed as a solution for fine pitch SMT hand soldering. The problems you describe with your PTH application are caused by the operator simultaneously heating multiple leads while applying solder.

This technique will lead to insufficient heating, poor filet formation and is causing your bridging. I have never seen this method used successfully in a PTH manufacturing environment, nor would I recommend it, as it only leads to poor quality and additional rework.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

I think you have answered your own question! When you were soldering each pin individually you were not experiencing shorts. You changed your process to dragging solder from pin to pin and now have shorts.
This procedure is causing bridging. Go back to your old process & you have solved your problem!

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Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.

From your description it sounds like you are hand-soldering rather than using a form of selective soldering method. One thing you need to consider is that you are not soldering components to a circuit board, but are soldering pins to the circuit board. The contact time for a pin would be different when connected to a ground plane compared to a signal track due to the heat requirements.

Normally hand-soldering operators vary their soldering speed as they move down a component to compensate for this. The other aspect is the speed you move down the connector. You need to get this speed just right to ensure that the solder 'breaks off' the previous pin without shorting. This is often tricky to do when hand-soldering.

I would recommend either going back to a pin-by-pin approach or look at an automated selective soldering solution that would provide a consistent result.

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Simon Smith
Marketing Manager
Pillarhouse International Ltd
Mr. Smith has been in the electronics industry for over 30 years, the last 21 in the Selective Soldering field. He has progressed from electronics design, through after sales and now marketing.

Drag soldering for though hole is difficult. Board design plays into the equation. If the board has high density ground planes, many internal circuit connections or any other attribute that will draw heat away from the PTH it may not work.

Assuming you do not have these issues, the other question is how reproducible is the speed or contact of the hand drawing in the process. I would imagine it is operator dependent.

The wire diameter and flux content also play into this scenario. The defect you are describing is one of insufficient flux or flux that is not stable to long contact times with the iron tip.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

In my experience, drag soldering with a slant chisel or "hoof" tip is strictly a surface mount technique. I would not expect the heat transfer from source to destination side of the PTH to be fast enough to keep up with the pace of the iron across the row of pins, resulting in unacceptable hole fill and destination side circumferential wetting. For this reason, I wouldn't expect this to be an acceptable method for through-hole connections at all. That said, one key ingredient that will alleviate bridging in successful drag soldering is ample flux.

You don't say if all the connections are acceptable for other requirements of wetting, fill, etc. Likely they aren't, so I would abandon this technique.

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John De Leeuw
Manufacturing Process Engineer
TE SubCom
John has more than thirty years of precision metal fabrication, contract manufacturing, fiber optics and electronic manufacturing experience in quality and production management and process engineering capacities. He has been a Certified IPC Trainer for more than ten years. He is currently a Manufacturing Process Engineer for High Reliability Optoelectronic assemblies used in undersea fiber optic cable systems.

This method is acceptable and there is learning curve with it. Speed, angle, heat and solder/flux feeding speed will make the difference between good solder joints and problems.

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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.

Reader Comment
In my experience drag soldering is a fine art to master. It's a mix of tip type, tip heat, solder diameter/type, flux type, drag speed, tip angle. It's something you can't really teach, it's something you learn through practice, feel and touch. There are so many other variables too i.e. component type, component lead pitch/size, pad size, pad plating etc. You tend to build up a knowledge bank over time of what method will work best for each given scenario. Like I say, it's something you learn through experience.
Alan Christmas, Ultra Intelligence & Communications

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