Problems With Wave Solder Lead Bridging

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, also of ITM Consulting, broadcasting to you today from high atop Mount Rialto. Jim, it looks like we have wave soldering question.

Jim
We are wave soldering very close pitch through-hole components and see bridging. What is the finest pitch for through-hole components that can be successfully wave soldered? Is there anything you can recommend so we can reduce bridging on this component?

Phil
Whoa, where to begin? You're talking about a technology that's essentially something like 50, 60 years old.

Jim
And there has never been zero-defects. Even with 100-mil pitch, you still see some bridging. So, what is your acceptable level of bridging? With fine pitch connectors bridging can be 100 percent and can be a real issue. So, what do you do, Phil?

Phil
There's been all sorts of innovations, or duct tape if you will, on the wave soldering process, trying to keep up with this stuff.

Jim
First off, obviously, tuning your wave soldering process, getting your flux down, getting your lead lengths cut, doing everything you can on the front end of the process to optimize it.

Solder pot cleanliness, solder pot temperature. All those can effect bridging in general, so they'll help the situation. But then you have to get onto something else.

Phil
Over the decades, there have been all kinds of things. Hollis Wave Soldering was famous in the 70s. I remember working with peanut oil and one of the supposed advantages was for de-bridging.

Other things that may be more practical; possibly worth a try? One is inerting the solder pot, if you haven't done that already. A number of studies that were done many years ago by both SolTec and Electrovert gave very credible data that shows inerting helps the solder go where it's supposed to go. This is done by reducing the tendency towards oxidation.

Jim
We've talked a little bit about them before, and they're not really popular.

Air knife de-bridging tools may be something you have to look at. They are available from some companies, and this may be the situation where you have to grin and bear the reality of paying for that tool and learning how to use it to achieve a reasonable level of bridging.

Phil
The ultimate answer may to give this old technology a ride and take a look at selective soldering with the various new systems that are out there. As well as exploring whether this product might be feasible for intrusive soldering.

Jim
Pin and paste, reflow of through-hole, intrusive soldering, whatever name you want to call it. Bridging and wave soldering is a way of life. You manage it. You never eliminate bridging 100 percent, regardless of the pitch. Obviously, it gets worse at small pitch, and I agree with Phil. At some point, you're just going to have to say, wave soldering is not the way to handle this component.

Jim
So, thank you. It's a lot to talk about. We could expand on this, but I think we gave a reasonable overview of the general principles involved.

Phil
Beyond that we'll say thank you. And no matter what methodology you're using, don't solder like my brother.

Jim
And don't solder like my brother.

The solder shorts can be reduced by lowering the wave high and lowering the conveyor high. The solder shorts are a result of no flux or a low amount of flux present when the solder joint is exiting the solder wave. The solder joint is formed at the exit of the solder wave.

This can be tested by populating the board with the problem component only and lowering the conveyor high until component leads barely clear the solder nozzle. Adjust the wave high for a lite touch of the solder wave and solder the board with the single component only.

You have set the wave up to minimize the displacement of the flux as the board moves through the solder wave.

In my previous production experience I reviewed PCB design for manufacture, and a part of that was adding debridging surface tension modification pads. Running a connector in-line with the flow tends to leave bridges at the end pins, but this can be reduced by adding extra copper (solder area) at both ends (to accommodate either directions flow). Parallel flow connectors you can lengthen the pads. Similar things can be done with ICs. Usually flow solder QFPs are placed diagonally with extra solder copper in the direction of flow. This was generally in analogue video products which I think went down to 0.6mm pitch, newer digital products are much finer pitch and can be considered to not be flow solderable.

As stated before a simple solution which may reduce bridging would be to rotate the product. Depending on the boards layout a simple 90 degree turn may be your answer. Component shadowing is a common issue when it comes to bridging. Altron Inc. has a detailed write up on board design guidelines which touches on component shadowing.

Some of my best results were achieved using the vibrations to break the surface tension of the solder combined with a hot nitrogen air knife and inert atmosphere on the pot. As with any process - it will take an experiment in your facility with your wave to get the proper results.

Firstly, make sure the wave is set up at a 7 Degrees angle for both leaded and lead free Soldering. Wrong angle gives poor drainage. Ensure the PCB exits the wave in the middle of the solder nozzle with adequate correct flow off the front and back of the nozzle to drag off the solder. Lastly 250C solder temp setting for leaded and 275C for lead free alloys which have been correctly Tin/P treated will help immensely. Hope this helps.

Try to reduce the amount of flux and/or type of flux. You might also try to use a rotary chip wave instead of a standard wave.

A solution which may reduce wave solder bridging would be to rotate the product, either in a pallet or fixture, to cross the wave at an angle. The optimum angle would be one that does not allow 2 neighboring pins within the array to contact the wave simultaneously. The more separation, the better.

This method has led to a 75% - 99% reduction in our wave solder bridging depending on the pitch of the connector.

There are many ways to reduce bridging on thru-hole and with limited details it's hard to pin it down. Some fluxes work better than others, make sure the leads are as short as possible, make sure you use enough preheat, pot temp 250 C or above, are some of the common things to try.

There is a growing trend to reflow solder through hole devices using solder paste and, if additional solder volume is needed, add solder preform to the paste prior to reflow. The component must be capable of the reflow temperature. Bridging is eliminated, and hole fill of 100% can routinely be achieved.