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Ask the Experts

June 19, 2025 - Updated
July 25, 2012 - Originally Posted

Insufficient Barrel Fill on Through-hole Components

We are seeing insufficient barrel fill for one through-hole components during wave soldering. We suspect the problem could due to the pin-hole ratio being too small.

Can you point us to a guideline for the recommended through-hole component/hole diameter ratio? What could we do to improve the barrel fill?

Expert Panel Responses

Ensure sufficient flux wicking.
Ensure sufficient top side preheat BUT not too hot so as to burn off all the flux. So typically 100C Topside temperature just prior to wave.
Ensure PCB is deep enough in the solder.
Ensure Solder bath itself is hot enough to improve flow of the Solder while the heat sinks travels through. Typical Lead Free temperature is actually 275C setting on the wave

Greg York
Technical Sales Manager, BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

We have a Board Talk segment on this topic.
See: https://www.circuitinsight.com/programs/54395.html

Jim Hall
Principal Consultant, ITM Consulting

Jim has a wealth of knowledge in soldering and thermal technology, equipment and process basics and is a pioneer in the science of reflow technology.

The answer is 'it depends.'

The IPC has an inspection standard for finished assemblies and I think it is 610 off the top of my head, this will have different pass / fail standards depending on the class of product you are producing, but generally this will range from 60 -100% depending on what you are producing. You then need to determine how you will measure this and X-ray is the only real way without cutting up the product.

Richard Boyle
Global Product Champion, Henkel Electronics

Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

I am sure you will get plenty of practical advise from wave soldering experts. One thing I would like to introduce is the thought that it may not be necessary to wave solder the component. It may be possible to reflow solder the through hole component at the same time as the SMT components, using printed solder paste.

There are a few things to consider, such as peak temperature capability of the component and component standoff height to provide clearance for the solder paste. Press fit or near press fit connectors don't reflow solder well, since the solder needs a reasonable clearance to flow freely during reflow.

The advantage is that 100% hole fill can be routinely achieved, since issues such as temperature gradients top side verses bottom side, barrel connections to internal planes that act like heat sinks no longerapply, since the entire board achieves the same temperature while in the reflow oven.

If there isn't enough board real estate to print sufficient solder volume for each pin, solder preforms from tape and reel packaging can be considered as a method to augment the solder volume. The elimination of wave soldering is a significant industry trend, and may represent the most significant cost savings opportunity since low silver solder was introduced. It is worth examining, especially if you have a problematic board that does not achieve sufficient hole fill with the wave process.

Paul J. Koep
Global Product Manager, Alpha

Mr. Koep is responsible for product planning and technical marketing for the Preform Products at Alpha. He is the co-author of several patents in the areas of soldering applications focusing on reflow and alternative methods.

Guidance from IPC-2222 Revision A
https://www.circuitnet.com/experts/uploads/1/lead_ratio-2.jpg
For better fill, some possible solutions are:a slower conveyor speed on the wave solder a higher operating temperature more flux.

Mike Green
Design Engineering, Lockheed Martin Space Systems

Mike Green is co-chairman of the IPC Terms and Definitions Committee. He has been working with board design and manufacturing for 33 years.

Circuit community recommendations range from .005" to .020" with.015" being commonly listed as a target.

Given that you need to play the hand you are dealt, I would suggest a couple of strategies for the existing boards:

Preheat the board if possible to eliminate thermal lag
Pre-tin the component to improve wetting speed
Use additional flux to improve capillary action and improve wetting.

Additional flux maybe accomplished by pre dipping the component leads in flux prior to board placement.

Nick Koop
Product Manager, Minco Products Inc

Nick has been in the circuit industry since 1985, in technical and management roles. Nick is the Vice Chair of the IPC Flexible Circuits Committee, and Chair of IPC 6013 Flexible Circuits Perfomance Subcommittee. Nick holds a Bachelors Degree in Mechanical Engineering and a Masters Degree in the Management of Technology from the University of Minnesota.

This is a difficult number to find in the "standards."

I can tell you that board designers have a "rule of thumb" for the finished hole diameter which is: 0.016" (0.4mm) larger that the component lead. This gives 0.008" (0.2mm) clearance around the component lead for solder to wick.

As for the standards,the IPC would be the best place to search for this information.

Paul Austen
Senior Project Engineer, Electronic Controls Design Inc

Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.

If you search for generic recommendations on how much over-size your hole needs to be, you'll find a wide range of sometimes-conflicting results. The reason is that some of the recommendations are focused on auto-insertion needs, while others are based on optimizing the soldering operation,and some incorporate both.

To complicate matters, the solder alloy to be used has a big impact, as does the pin shape (round, square, flat). For a round lead, if your diameter is 0.008" to 0.015" over the pin diameter, you're probably not in big trouble. Where the optimum lies depends on a lot of variables.

Let's talk about what can impact flow-through:

Lack of adequate heat transfer up the lead and barrel. Both the barrel and the lead must reach soldering temperature throughout the PWB thickness, or solder flow up the barrel will be halted. Problems can be caused by lack of lead protrusion, too many ground plane connections to the PTH or insufficient thermal relief on ground connections, too low solder temperature or too-short contact time.
Slow or absent wetting. Both the lead surface and the hole wall must be made wettable by a flux in order for a capillary force to be generated that will cause the solder to flow up the hole. Wetting problems can be caused by poorly-solderable surface finishes, lack of flux penetration, or improper pre-heat (either too-aggressive or insufficient), or contamination of hole walls or leads.

Careful examination of the partly-filled holes can be informative. Are holes on ground pins more problematic? Is there evidence of wetting (low contact angle) to the hole wall,but a high contact angle to the lead, or vice versa? Are particular components more susceptible? Can flux penetration be verified? The answers may guide you toward one or more root causes.

Fritz Byle
Process Engineer, Astronautics

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.

It is very hard to answer this question. Board thickness, pin size, thermal relief and more play a role. One company for small electrolytic caps with a .024 pin size use a 0.049 hole. A large electrolytic with a 0.032 pin may require a 0.070 hole.

There is an open project by one of the consortia evaluating this question currently.

Jack Fisher
Chairman of the IPC Technology Roadmap Committee, Interconnect. Technology Analysis Inc.

Jack Fisher is a retired IBM executive, chairman of the roadmap since 1994, and member of IPC Hall of Fame.

I only have experience with Intrusive reflow pin in hole. In this case too big of a pin to hole ratio leads to voids. 25 mil pin in 48 and 44 hole leas to voids where 25 pin in 40 and 36 hole was OK.

Maybe it is reversed for wave since wave must fill in around pin. For Intrusive reflow the paste is already in the hole.

Bill Coleman
Vice President Technology, Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

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