Minimum Lead to Lead Spacing for Wave Soldered Through-Hole Components

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Minimum Lead to Lead Spacing for Wave Soldered Through-Hole Components

What is the recommended minimum lead-to-lead spacing for through-hole components run through a wave soldering system?
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
And welcome to Board Talk with Phil Zarrow and Jim Hall, The Assembly Brothers pick and place. We are here to solve your process, applications, things along those lines or certainly give you some food for thought. Jim, today our question is from V.P. It is a design-related question.

What is the recommended minimum lead-to-lead spacing for through-hole components run through a wave soldering system? Would the spacing be different for lead-free solder compared to leaded solder? Gee Jim, what is a design guideline or recommended minimum for lead spacing with a wave soldering machine for through-hole?

Jim
When I entered the industry and started working for Dynapert, which made through-hole insertion equipment, all spacing was 100 mils. Boy, we didn’t have too many problems. I imagine with poor wave soldering you can still get a bridge or two. But of course, with the miniaturization facilitated by surface mount but still being assembly and mixed technology with through-hole that 100 mil spacing went by the wayside pretty quickly.

50 mil spacing, 40 mil spacing, and now I believe I have heard about people talking about connectors with 25 mil spacing. Wouldn’t it be nice if we had a standard that said thou shall not go below 50 mil spacing or 40 mil spacing? But unfortunately, there isn’t. It is what they can get away with and what an assembler can build. Obviously, as the spacing gets smaller bridging is going to be a bigger and bigger problem.

Phil
We don’t know exactly what spacing V.P. is encountering here. He may be just talking theoretically or what should I avoid but the better the spacing you have, the better off you are. The other aspect is wave soldering having its limitations. Again, if you are starting to get down to some very, very close spacing you might want to consider an alternative methodology that is better controlled such as a selective soldering system. I am talking system, not a pallet through wave solder but an actual selective soldering machine. Since that is done on a point-to-point basis or certainly much more controllable than wave solder.

Or possibly, depending on what is going through, intrusive soldering using reflow. Just understand wave solder in itself has limitations as you have indicated V.P. with things like lead spacing and things along those lines. So selective soldering has been able to accommodate tighter lead spacing than we have seen with wave soldering.

The other part of the question is an interesting one, Jim. Would the spacing be different for lead-free versus leaded solder? We know about the slower wetting of lead-free.

Jim
Higher surface tension. It has a bunch of different effects. I have never heard about anybody talking about I went from tin-lead wave soldering to lead-free wave soldering and I got a lot more bridges. I have never heard anybody say that.

Phil
Or vice versa, yeah. Probably not, it is more driven by the methodology and all of the other variables involved. Let’s face it have soldering itself, beyond lead spacing, there are a lot of variables going on in that process. V.P., we hope we answered your question. I am sure we will see some commentary on this as well. I would say however you are going to be wave soldering, whatever lead spacing you have, please don’t solder like my brother.

Jim
And please don’t solder like my brother.

Comments

PTH annular ring spacing requirements are found in the IPC Design Standards and are based on current carrying requirements for the circuit, prevention of stray capacitance, high frequency requirements, etc. To ensure PTH component leads comply with the minimum electrical clearance requirements, the best rule for clinched leads is to trim to the edge of the pad, and for straight (un-clinched) leads trim prior to assembly and soldering to meet the maximum lead protrusion specified on the assembly drawing.

Leaving the leads long enough to hold the part in place and trimming after soldering is permissible, but only use the shock-free standoff-shear lead trimmers such as the Excelta 500 series. And never, ever trim leads with glass seals where the lead enters the part body, not before soldering, and not after soldering either! Those should ALWAYS be purchased with the leads at the required length straight from the component manufacturer, especially radial-leaded relays and similar glass-sealed components. The shock imparted on component leads trimmed with a standard trimming pliers or worse, a right-andgled side cutter will easily break glass seals and very often will crack even a PTH solder joint!

I cringe every time I see an operator using those. And if your design engineer selects glass-sealed components that require trimming, or if the design engineer does not call out minimum electrical clearance or maximum lead protrusion on the board drawing, fire their butt and find someone else who knows what they are doing.

Richard Stadem, General Dynamics MS

Thanks for your time, helping others to understand solder issues. In this place I remember, we had bridges mostly at the two last pins of connectors. We added some extra pads behind there in flow direction, so that the bridges appeared on those unused pads.

Frank Rockenstiehl, KARL STORZ SE & Co. KG