|Ask the Experts|
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?
|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.
Director of Reclaim Business
Alpha Assembly Solutions
I believe the drag soldering technique is not the correct method considering the contacts are only 2mm apart from each other.
Senior Manufacturing Engineer
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.
Director of Marketing & Training
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.
Regional Sales Manager
OK International Inc.
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!
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.
Pillarhouse International Ltd
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.
Deck Street Consultants
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.
Manufacturing Process Engineer
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.
Engineering and Operations Management
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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