Ask the Experts
January 22, 2017 - Updated
October 3, 2017 - Originally Posted

Insufficient Barrel Fill on Through-hole Components

We are having issues with barrel fill. We are using a lead free, no-clean flux process. The issues is with through hole connectors with lead retention mechanisms. Any thoughts?


Expert Panel Responses

Assuming you are hand soldering, you might try a higher wattage soldering iron. Increasing the temperature on a low wattage iron doesn't work well, you will need higher wattage.

A larger tip will also help. If you have tried this, then a hot plate would be the next step. If selective soldering, then a "jet" tip, increased dwell time, and longer preheat should help. We selective solder as much as we can and have good success with it.

Tom Schardt
Industrial Engineer
Red Lion Controls, Inc.
Tom Schardt has been an Industrial Engineer with Red Lion Controls since 1988 with a degree in Electronics Engineering. He has held the positions of wave solder operator, electronics technician, production assembler, Lead Person and Supervisor. In his current position he is responsible for plant floor design and compliance, implementing new machines and technology, and the selective soldering operation.

The surface finish can have an effect if the assemblies were subjected to one or more prior SMT reflows. Immersion tin can react with copper to create a difficult to wet PTH barrel.

Insufficient flux loading an inadequate pre-heat in the wave process can also inhibit barrel fill. Older boards, subjected to oxidation during storage could also contribute to poor barrel fill.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions
I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

There are several types of lead retention mechanisms. These are design to provide extra support between the lead and the PTH. The ones I've worked with are usually for press-fit connectors which do not require soldering.

The retaining design might block the normal flow of solder in order to meet IPC specifications applied to a non-retained lead. In this case, a pull test might be required to complement the soldered connection. Always contact the component manufacturer or your local rep for additional recommendations.

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.

My experience has identified "Insufficient Barrel Fill" on Through-hole Components can be attributed to the following :

First, have you cross sectioned any PCB samples as received from the vendor/s to verify the thickness of the PCH through hole walls or even if there is any copper in the through hole ? Also, to check for wetting either on the lead or the barrel?

Second, I would revisit your reflow belt speed - compare fast verses slow for time in profile/s. If wave, the check solder bath time and temperature (LF @ 275C pot temp. with 2-3 sec. immersion)and ensure you use a flow accelerator. Typically, immersion times above this will impact most fluxes adversely. More preheat and/or more time on the wave is another area to revisit.

Third, is poor wetting due to oxidation on the leads, oxidation in the barrel, not enough flux, etc. your problem? If you are not using and inert gas atmosphere this may result in oxidation issues and decrease in the barrel fill. The inert gas may increase the wetting of the solder into the barrel and in turn improve your current flux chemistry performance. You can also use a more aggressive flux which will require intensive cleaning post wave.

Fourth, Lead free alloys wet more slowly than do tin led alloys. How thick is your PCB(.093) ? A thicker board exacerbates the problem. There is usually a significant temperature gradient between the bottom of the board and the top side for thicker boards.

Since solder likes heat, the solder is reluctant to flow up the barrel to a cold region. What is your pad finish especially if you are using a tin lead OSP, ENIG, AgImm, etc. Again, you may have to increase the preheat temperature and the time on the wave (again the extent you can do this will depend on the flux).

Mark Northrup
VP of Advanced Technical Operations
IEC Electronics
Mark has over 25 years' experience in electronics fabrication, quality and reliability while working for IEC Electronics, GE, Motorola, ORS, etc. He has most recently established IEC Electronics Analysis and Testing Laboratories (IATL), LLC in Albuquerque, NM for electronics and material analysis testing in the military, medical, and industrial industries. His area of expertise includes PCB, PCBA, components, analytical and electrical analysis techniques.

I'll assume that this is a wave soldering process, and not a selective soldering process (though the concerns are similar). Following are some of the things that can contribute to lack of complete barrel fill:

Incomplete fluxing - Lack of sufficient heat transfer because of
  • Insufficient heating of leads due to short lead protrusion on the solder side
  • Wicking of heat from the barrel due to too many or insufficiently relieved plane connections
  • Solderability issues with the leads or barrels
The fact that your fill problems seem to be focused on connector leads with retention features tells me that it's most likely that you're not getting sufficient fluxing due to the retention feature blocking the hole. Other factors may be contributing, but if you don't get adequate fluxing up the barrel, wetting can't occur.

In order to test this theory, select half of the leads with the retention feature on some boards, and carefully apply a *small* droplet of liquid flux (same flux you use in the wave process) to the top side if accessible (bottom side if not). Let the flux wick down or up the hole. Now pass the boards through the normal process. If the added flux eliminates the issue, then blockage of fluxing is the problem.

If this is the problem, it is not an easy one to solve, unless you have selective control of flux application. Just increasing the application generally is not usually a good idea without further study.

Fritz Byle
Process Engineer
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.

Item 88217: poor barrel fill. My assumption is you are wave soldering this board. Here is a list of items to evaluate your process in order to achieve better barrel fill.
  • Flux deposition do you have full flux coverage on your board?
  • Preheat setup. With a thick board you want to monitor the top of the board for the proper topside temperature and heat it from the bottom. If you heat the board from both the top and bottom the middle of the board still may be cold.
  • Be careful not to over the board prior to soldering.
  • Contact time. With lead free you will need to adjust the conveyor to achieve the proper amount of contact time. This should be 4 - 8 seconds. On a conventional wave solder machine, the contact distance is about 2 inches. Meaning your speed needs to be 2.5 feet / minute or slower.
Please provide more detail about your process if it is not a wave process.

Michael Kaminsky
Sr Field Applications Support Engineer
Kester Inc.
Mr. Kaminsky has 30+ years of circuit board soldering assembly experience along with a patent for wave solder VOC flux process.

  1. More top side heat
  2. Higher amount of flux solids to get more flux in the barrel
  3. Alternative processes: paste and a preform or robotic soldering with flux cored wire

Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

There are many variable to achieving barrel fill in through hole technology. Flux activation, preheat time, board thickness ground plane etc.

Without knowing your process such as hand soldering, wave, or selective soldering, I can only suggest you make sure you are using the right amount of flux and proper preheat time to activate the flux and head the board through all layers. You may also need to increase you dwell time if the above does not work for you. Good luck.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

Barrel fill is affected by several things including: the solder alloy and the composition of the alloy, the preheat temperature and time, the solder temperature and contact time, the agitation level in the wave/selective solder, the flux activity and composition, the age of the flux, the penetration level of the flux, the metal finish on the circuit board and component leads, etc. I suggest the following general steps to improve barrel fill.
  1. Measure the amount of flux applied by weight, and ensure that it meets the manufacturers recommendation. Increase the amount of flux if necessary.
  2. Check that the flux penetration is adequate and is reaching the top side of the board. This can be done with paper placed on top of the board and checking for flux spots on the paper over each through hole.
  3. Check the preheat settings to ensure that the flux is being activated properly per manufacturers recommendation. Do not pre-heat too aggressively or the flux may loose activity before soldering takes place.
  4. Check the composition of the solder alloy and adjust to meet the recommended specifications if necessary.
  5. Increase contact time and/or solder temperature to ensure that the top side of the board is reaching the necessary temperature for complete barrel fill. Taking a temperature profile of the top side of the board is the best way to do this.
  6. Increase the flow rate or agitation rate of the wave/selective solder. This may help the solder to wick properly to the top of the holes.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

In my personal opinion it should not make a difference between Leaded vs ROHS solder, however there are the following number of variables one has to look into at the time of setting up the profile:
  1. Tinning of thru hole connector leads as well as the metallic mass of the components in question
  2. Thickness of the PCB
  3. Number of ground planes
  4. Copper thickness of the foil used
  5. Preheat temperatures of the wave machine based on the above 4 points
  6. How well is the flux spray being spread on the board as well as amount of flux getting into the barrels. Maybe in this case a foam flux tray would be better than a sprayer
  7. How good is the barrel finish at the time of fabrication of the PCB in question
  8. Amount of impurities and dross in the solder pot
  9. The hole to the pin aspect ratio
  10. Adjustments on the "Omega" and "Lamda"
Each one of the above variables have an impact on the quality of the overall solderbility of the pin thru hole components.

Mahesh V Draksharapu
Independent Consultant

Over 28 years experience in the area of PCB assembly with 10 years working in various EMS companies in the US (Avex, GSS Array Technology now Benchmark, Everex computer Systems), 10 years working in OEM companies in the US (Trillium Test Systems, Intel & Meru Networks) and with Aristos EMS company in Bangalore since 2008. Engineering Degree from BIT, Mesra and Masters in Egg Mgmt from Wichita State University.

To answer this question there is very little detail you have provided.
  1. Using FAX paper, cover your load your pcb, place your FAX paper on top, cover with a second PCB, Flux your PCB. Inspect to see if you have any flux penetration.
  2. Check your hole to lead ratio of it is correct.
  3. Ensure that you have correct pre heat, and that your flux, is not pre maturely actived before your soldering process.
  4. Ensure that your dewel time is on the higher end of the spec, 4,5 to 6 seconds contact time.
  5. You can also look at different Flux types, main Alpha has a wide range for fluxes for improved Bill Fill, in the automotive range of fluxes.

Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa
Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

You might consider intrusive reflow as opposed to wave soldering. With proper hole to pin ratio and print set up for complete paste hole fill complete barrel fill is possible with lead free solder. Bill Coleman and George Oxx publishes an article on this in SMT several years ago.

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.

Plated Through Hole Barrel Fill with lead-free solder and low solid content flux.

Although the thickness of the board and the number of layers is not mentioned, hole filling is based upon getting the board hot enough so the solder will not solidify in the barrel during the soldering operation regardless of the solder alloy being used.

The basic requirement is 75% hole fill for all class 3 products, and for Class 2 the requirement is a bit different depending upon whether or not internallayers are attached to the plated through holes. If the inner layers are in fact connected to the barrel then the requirement is 50% hole fill.

The lead retention is based upon the strength of all the solder joints on the component. Information on the strength of the solder joint can be found in Howard Manko's book, Solders and Soldering and with this information one can calculate the overall attachment strength of the connector itself.

As far as the process is concern, verify good flux coverage up into the plated through hole and if oxidation is found due to the flux not being able to protect the reoxidation of the surface to be soldered then an inert atmosphere can be used, such as nitrogen. Subsequently, the time in the wave has to be calculated to verify the solder has enough time to rise up into the barrel through capillary action.

If the board is thicker than 0.060" then it can be run through the wave at a deeper depth to utilize the hydrostatic pressure of the molten solder to force the solder up into the plated through holes. Keep in mind however that fixtures may be needed to accomplish this methodology as the top side of the board has to be protected from the solder coming over the thickness of the board and allowing solder to short everything on the top side of the board.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

There are a number of variables that would need to be addressed to provide specific guidance, but here are some general guidelines. We don't know if this a full wave of selective application, but in either case, higher pot temperatures will improve barrel fill. However, higher temperatures will also increase dross, deplete flux activity and increased risk for PCB thermal damage. If a pallet is in use on a full wave, increasing the wave energy can help with barrel fill.

Low and no silver alloys do not exhibit as robust flow characteristics as SAC305 which can cause barrel fill issues. If the board has not been designed for lead-free processes, the lead-hole ration may need to be adjusted to accommodate the reduced flow of many lead-free alloys. These are just a few considerations. Working with your solder chemistry vendor is key to finding the combination of process/materials that will meet your requirements.

Tim O'Neill
Director of Product Management
Timothy O'Neill is the Director of Product Management for AIM Solder. AIM Solder is a leading global manufacturer of assembly materials for the electronics industry. Mr. O’Neill has 25 years of industry experience is a Certified IPC Specialist.

Mr. O’Neill’s responsibilities include developing product and technical information; he is a technical writer and presenter for industry trade publications and events and has been recognized as a Speaker of Distinction by the SMTA.

This could be a result of several things—from poor solderability of the lead/board due to contamination, the lead/board finish, or intermetallics; not enough flux, or wrong flux; heat issues, or other parameter issues; a board design issue (such as non-relieved ground planes, lead to hole ratios), etc. Or it could be related to something like a lead retention mechanism as you mentioned.

I will assume that you are talking about a Selective Soldering process—as the use of Selective Soldering is on the rise, along with questions like this (however, the issues and resolutions for wave soldering are similar). I will also assume that all the basics (such as solderability and the appropriate flux) are in order and focus purely on process concerns.

Typically, a hole fill problem involving Lead Free solder usually boils down to a heat issue—either too much heat, or not enough. Usually, the issue is not enough heat. Too much heat means you are consuming your flux with the heat. Lets’ look at the different possible issues.

First, regarding the lead retention mechanism—if you remove, or don’t use lead retention, does this result in complete, or better hole fill? If yes, then you know this is contributing to the problem and we need to figure out how to deal with it. What kind of lead retention mechanism are you talking about—simply clinched leads? Does the problem only occur where you have the lead retention system in place?

A lead retention mechanism, such as clinched leads, can not only create an obstruction to the solder wetting, but also an obstruction to the flux getting to where we need it. Or, it may be simply increasing the thermal demand for soldering these leads.

Too Much Heat—Too much heat—it may not be just preheat, but heat generally from the entire process could be consuming your flux (flux “burn-off”). Too much heat will be evident by poor hole fill like you have, and possibly with bridging. It’s hard to know without knowing temperatures, but you can simply test the idea by adding more flux to the process and see if the results are better, or worse (not necessarily resolved, but better means you are heading the right direction)

Not Enough Heat—You do not have enough heat if when you added flux (from above) and the hole fill was worse, because the additional flux adds to the thermal demand of the joint. Is your flux still wet when you contact the leads with solder? If it is, you certainly do not have enough heat. If you are using a VOC-free (water-based flux) vs. an alcohol-based flux you are likely significantly shy of enough heat. If other components around this connector solder well (soldered before or after the connector in question) this can be a clear sign that the process lacks heat for this connector

You can add or reduce heat with a number of parameters, such as preheat, contact time/dwell time, traverse speed, solder flow rate and any localized preheat.

Proper wetting and hole fill all depend on time & temperature and you might need to allow more time (contact time) for those pins/holes to achieve the necessary temperature to wet properly. And if we are really talking Selective Soldering, then on those pins you could increase the heat by increasing the contact time by increasing dwell times, slowing the traverse speed, increasing solder flow rates, increasing preheat (temperature or duration), increasing localized preheat, or by increasing the solder temperature (usually the last resort).

For wave soldering, these same principles of too much and not enough heat—and their resolution—still apply.

You just won’t have the same localized control that you do with Selective Soldering.

A couple notes:

Look at Gerbers/drawings of the board to examine internal layers for grounding and heat reliefs

Use a microscope to look down the hole from the top if possible to determine if the wetting problem is with the part lead or the board, i.e. is solder wetting to the lead and not the hole, or vice versa?

Another thing you can try is to solder the plated through-hole without the part to make sure the board is solderable.

The great thing about Selective Soldering is that you have great control over the process so you can customize what is needed in any area, or for any particular component.

Hope this helps. But, please feel free to contact me with any more detailed information and we can clarify the assumptions made here and I can advise more precisely.

Eddie Groves
Director of the Selective Soldering Academy (SSA)
Selective Soldering Academy
Eddie Groves has over 32 years' experience in the electronics manufacturing industry, specifically in the soldering of circuit boards. His first 11 years was with Lucent Technologies (AT&T), then with Kester, Qualitek and then Pillarhouse. He has over two decades of Selective Soldering experience as a user and a provider of Selective Soldering equipment. His particular focus is Process Development, Process Troubleshooting, flux, and component & PCB solderability.
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