Ask the Experts
February 26, 2018 - Updated
August 10, 2008 - Originally Posted

Problems Hand Soldering 24 Layer Board

We are assembling a back plane board with 24 layers. We are attempting to hand solder solder through-hole components, but are finding that the solder does not flow through to the other side of the board. Is there is a method or technique I need to use to accomplish this? We are using Tin-lead solder.


Expert Panel Responses

Manual soldering of thick boards is a very delicate operation. Solder iron temperature, solder iron heat capacity, preheating of the board, are all issues which need to be considered when trying to fill these types of plated through holes. More than likely, there is not enough heat or thermal capacity in the solder iron to heat the entire hole up to soldering temperatures, thereby the solder solidifies going up through the hole and that's it, it doesn't fill any further. One has to keep in mind the different thermal requirements when soldering such plated through hole especially a hole through such a thick board with internal layers drawing away the heat from the joint. Once the solder solidifies in the hole the thermal requirements change drastically and more heat and energy is required to remelt that solder to completely fill the hole. My recommendations without seeing or knowing too much about the product, would be to preheat the board as the solder joint is being made. Use a solder iron tip and solder iron which will have plenty of energy to continuously supply heat to the solder joint and keep the solder molten, yet not hot enough to damage the laminate material and plated through hole inner layer connections. There are solder iron suppliers out there who can help you with this application.

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.

On the surface, it seems clear that the bottom side (or the side opposite to the solder side) of the board needs to be pre-heated to offset any heat sinking that is probably happening with such a large layer-count board. It wouldn't help the issue if the through hole parts that you are trying to hand solder into place have a large mass such that the parts themselves are sinking heat away from the solder connection which would make a good top and bottom side fillet difficult to achieve. In any event, in lieu of increasing the solder tip temperature too much more (which could possibly scorch or burn the board) or applying an opposite side pre-heat, you might want to also try the following:
  1. Pre-fluxing the plated through hole (top and bottom) and the device legs themselves manually to help facilitate solder flow through the barrel; and/or
  2. Increase the hole sizes on the board fabrication itself for the device in question as it is possible that the ratio of hole size to lead diameter is too small or tight which could prevent solder from properly wicking up the device leads.
I am a big advocate of DFM for any board design as improper or incorrect board fabrication layout can make downstream assembly much more difficult which ultimately adds cost and quality issues to the finished product (in this case, I would ensure that the layout configuration on the board fabrication meets or exceeds manufacturer specifications as found on the datasheet for the part; circuit layout databases can have incorrect hole information or may not be up-to-date for certain devices so I wouldn't necessarily assume that the layout for the part is correct in the first place). It is also worth noting that full barrel fill seems to be your internal requirement (which is certainly fine) but IPC J-STD-001 specifications indicate that a 75% vertical barrel fill is acceptable for class 2 and 3 assemblies (reference IPC J-STD-001D paragraph 6.3.2 table 6.5).

David Bonito
Sales & Marketing Manager
Technical Manufacturing Corp.
David has been active in all areas of the contract electronics manufacturing industry for over fifteen years. He is currently in charge of all Sales and Marketing related activities for Technical Manufacturing Corporation.

Hand soldering a large masses such as your thick board is always a challenge. Wave or reflow soldering systems have edge rails and well controlled IR and/or convective preheater sections that are contained, effectively ovens to evenly preheat the whole assembly.

It's much harder when hand soldering because the thermal energy is being pushing in from a relatively small source--the soldering iron. In addition to the large mass swamping heat from the connection point, the operator is working in an ambient temperature environment, most likely with moving air, that will draw thermal energy off the board.

You need a process that will put a lot of heat into the board before the operator starts to hand solder the components. There are several options or combinations of options to consider but none are really ideal for through hole where you need to place components from one side and then flip to solder on the other side.

!. Soak to about 125 deg C in an oven and remove the board just before soldering. Concerns are that the operator will have to handle very hot boards, solderability can be impacted, and the board will cool off quite quickly, starting at the outside edges.

2. Hot plate, radiant and/or convective preheaters placed under the assembly while it is being soldered. Concerns are that the area to be soldered has to sit over the preheater long enough to pump a goodly amount of thermal energy in the board, these heaters usually heat only a small area and can cause stress due to uneven heating.

It's hard to drive effectively force the heat through the board--it'll still be cooler on the side away from the heater and solder may still freeze part way through the hole. Either of these preheaters can scorch an area if the operator doesn't pay close attention. These units have been available in the past as stand-alone devices that can be positioned wherever needed.

If this will be along term project, you may want to consider usingthe base portion of a rework station that incorporates a larger preheater assembly, board holders and some level of operator protection against touching the preheater assembly.

I've had to rework large assemblies like this in the past. The best method is a combination of the above. Soak in an oven to as hot as the operator can stand to handle it. Remove from the oven and immediately place in a holder over a radiant or convective energy source to slow down the loss of thermal energy. Stuff components and solder as quickly as possible!

Jack Crawford
Director - Certification & Assembly Technology
Mr. Crawford is Director of Certification and Assembly Technology for IPC. He is technical liaison to the IPC committees that maintain critical industry standards and has presented numerous papers internationally.

For a thick board like yours, the only solution is auxiliary heating. There are a number of options for table top auxiliary heat all the way through full blown work stations. Check with your soldering iron manufacturer for the options they have. It really depends on the capital available for your project. For some specifics... ONLY apply solder to one side of the assembly. If you attempt to fill the PTH by applying solder on both sides, you will trap moisture and other contaminants in the PTH. This may lead to corrosion of the PTH or, as the board heats in operation, an explosive out gassing, damaging the connection. Depending on your assembly Class (1, 2 or 3) , the standards state that on a class 2 assembly when the PTH is mounted in conjunction with a large thermal plane that only 50% vertical fill is required. You may not need the full 75% minimum vertical fill to ensure Form, Fit and Function.

Kris Roberson
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

24 layers is a lot of material to heat up for a soldering iron. My first suspicion would be that the surrounding board area is sinking the heat from the soldering iron, resulting in an insufficient amount of heat to create an adequate hole fill. Another common problem is that the recovery rate of the soldering iron is inadequate for the thermal demand required by this application. Lastly, if you are soldering to gold, there is a secondary eutectic which develops during the soldering process. As the molten tin/lead enters the PTH, the gold plating will immediately begin to dissolve and dilute into the molten alloy. Once the gold has reached the proper concentration, a phase change occurs with a secondary melting point of 217deg. C. This higher melting point adds to the difficulty of achieving complete hole fill. The only way to correct this is with a higher / longer concentration of heat. I would suggest they try one or more of the following:
  • Pre-heat the opposite side of the board to at least 100C if possible. (a heat gun will work for this) This will help you achieve and maintain a proper working temperature, and will also promote capillary action to improve hole fill.
  • Increase the tip size of the soldering iron if possible - the more dense the tip, the more heat can be applied to the solder joint.
  • Increase the wattage size of the soldering iron if possible - the higher wattage will provide a quicker thermal recovery, allowing you to heat the joint and surrounding area more quickly and efficiently.
  • The flux type and/or core size of the wire solder you are using may be inadequate for this application. You may try a wire solder with a more active flux, and/or a larger core diameter, which will provide you with more flux vehicle. Keep in mind that simply changing the flux core percentage does not make the product more active, it simply provides more flux activator.
  • If you haven't already, you may try the addition of a small amount of liquid flux to help facilitate wetting.
I would not suggest attempting to solder these devices from both the top and bottom sides of the board. This practice could result in the entrapment of flux inside the PTH, which could lead to further problems. If this is an ongoing process, my recommendation would be to look into a manual or semi-automatic bench top selective solder. The selective solder would be more capable of handling the thermal requirements of this process.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

I am not an "expert" in this area but in my experience this is typically a function of pre-heat. Soldering needs a certain amount of heat at the joint and the large boards and multiple layers sink the heat away quickly when only applied locally. We have seen some of our backplane customers use hotplates and put the entire board on a hot plate and heat the entire assembly up to 130-160 degrees prior to doing any hand soldering or re-work. Then, when you apply the heat locally, you have a better chance of "keeping the heat" at the particular location. In fact, we have even seen some customers put the entire assembly through a reflow oven set at 130-160 degrees and then pop it into a re-work station or do hand soldering quickly thereafter. The oven can be a helpful tool in this respect as many people already have one in house and hot plates can take a long time to heat up a large assembly and the oven can do the job in 3-4 minutes. One other thing: a little extra flux applied at the PTH's never hurts either!

Marc Peo
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

Thick PCBs can be a challenge for soldering. For consistent solder joints many manufacturers use a robotic Iron Tip soldering system. These systems can place the tip onto the joint for a set period of time (to preheat) before automatically forcing the solder into the hole. Since most solder feeders are be precisely controlled for the amount of solder (mm per sec), you can set the solder feeder to meet the desired fill rate outcome. An operator would have a difficult time feeding the exact amount of solder needed even over several seconds on continuous soldering. In tough cases (heavy thermal planes) a pre-heat process may be required prior to soldering. By preheating the back plane, this assists with the reflow of the solder into the joint area and reduces cycle time (for actual soldering). For preheat, some use Hot Air at the joint area whiles others may bake their PCBs prior to soldering.

Gary Goldberg
President and CEO
Mr Goldberg has practical experience in production line layout, process flow and cycle rate analysis. He knows how to avoid bottle necks and most related PCB or pallet handling questions.

The best process to use would be a direct power iron combined with a convection pre-heater under your board. Without this supplemental heat energy, the solder side gets hot and the solder flows down the via and stops once it hits the colder side of the board, with the pre-heater it will flow properly onto the component side.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

The problem seems to be that there is probably not enough flux that is wetting the entire barrel including the top side. Also, if a flux with increased activity level is used, it could improve the capillary flow and wetting.

Bjorn Dahle
inspīre solutions LLC
Bjorn Dahle is the President of inspīre solutions LLC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

I agree that achieving the right amount of preheat is important, but using a solder preform rather than solder wire would be a better option as far as holefill goes. The preform is solid solder and can be made into any shape or size depending on the hole dimensions. These could be flux-coated as well. So the preform could sit on the pad or be placed inside the hole; and a heat gun blowing hot air can be used to melt the solder and fill the hole.

Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

Everybody agrees that this is a heat transfer related issue. There are just too many variables here to come up with an immediate solution. There are multiple ways to look at this - try an alternative soldering solution - selective soldering, soldering robot. Getting the board temperature high enough is the key but as important is to keep it that way throughout the soldering process. For more questions and/or complete root cause analysis you can contact me. See below.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

The important thing is to have a top qualitytemperature controlled soldering iron that is able to maintain its tiptemperature. Then the tip needs to be selected in order to maximizethe thermal bridge. Depending on how fine the pin/barrel is and themagnitude of the ground plains, an IR type preheater would definitely help.

Gerald Gardener
Sales Manager
JBC Tools Inc
With over 10 years experience in design and product development, I offer technical support to our US Sales force and liase with our Europe based R&D and manufacturing departments to continually offer the end user the best handsoldering solution possible.

Reader Comment
I am surprised no one suggested using a solder fountain, such as an AIR VAC where you can select a well size for the solder job, mask off the areas needed and solder with this mini wave. It works extremely well with heavy copper planes. Proper preheat is necessary and the job is done, full barrel fill! No need or hotplates, irons, etc.
Paul C. Brown, L-3 Communications TE, USA

Reader Comment
Lots of answers to this one! What we've found that works quickly and very effectively for thick boards, or boards with heavy thermal planes, is to use the largest flat, wedge shaped soldering tip possible, flux both sides of the board, apply solder and soldering iron to the secondary side of the board and apply a second soldering iron to the primary side of the board. The heat from the second iron will draw the solder through to the primary side. This requires a board holder, the board held vertically, and 2 operators, but it works every time. Assuming you have access to the pad on the primary side.
Vince Burns, MC Assembly, USA

Reader Comment
Many soldering irons tout claims of quick recovery but a 24 layer board requires thermal mass to solder efficiently. Hexacon makes a product called the Magnum Therm-O-Trac soldering stations which is designed to solder multilayer circuit boards. We just know it delivers a huge amount of heat and can do the job.
Kathi Johnson, Hexacon Electric Company, USA

Reader Comment
For future production Continuous Improvement, you might want to check the PCB design to verify that there are thermals around the holes on any large plane layers connected to these component leads. Otherwise, the Cu on those planes is going to soak a lot of heat needed for soldering.
Julie Ellis, Consultant, USA

Reader Comment
While applying solder to both sides of the board can cause problems and violates the IPC guidelines, applying heat to both sides does not. Many solder techs are skilled enough to use two soldering irons at once. Assuming you can get access to both sides of the board with a soldering iron, try installing the part, pre-apply flux and apply heat to both sides of the board at once with two irons, while flowing in the solder from the bottom side. It may sound complex, but don't be surprised if your soldering tech says they can do it.
Craig Parker, L-3 Communications, USA

Reader Comment
There are many great suggestions in the comments above. However, I don't think enough was made of DFM. Without any other information about your board, I assume the following:
  1. At 24 layers your backplane boards are more than likely over .100" thick.
  2. There are possibly multiple ground planes internally tied to some or all of these vias.
  3. Since you are using tin/lead solder the boards are probably tin/lead finish also.
You also do not mention whether your soldering problems are with all of the through-hole components or just some. This is important to know as you may find that there is not a one size fits all approach to soldering this assembly.

Pre-heat and a properly sized iron tip attached to a power supply with sufficient output to recover quickly are a must. Selective solder is definitely something to be considered as much more heat can be applied to an area quicker than with an iron.

Beyond that, you should look at several items with respect to DFM. Are the vias sized to manufacturer's recommendations to allow the solder to flow around the component lead? Have adjustments to those recommendations been made to account for the specifics of your assembly? Have the internal ground planes been given thermal relief where possible to allow for hand soldering?

One last important item I would look at is the size of the pads around the vias. These help to transfer heat from the iron tip into the via. If you are using a larger tip and the only contact for heat transfer is the component lead that may be a reason you are not getting enough heat to the barrel. Good luck.
Chris Tribble, Salem Technologies, Inc., USA

Reader Comment
Many great suggestions that have proven to work well. One I see missing is TIN THE LEADS. We have overcome this vertical fill/barrel wetting issue like this by freshly tinning the leads to assure the most solderability possible, as well as providing a heat conduit to the destination side via the fresh coating on the lead.
John De Leeuw, TE Subcom

This one sounds like a bear! Often when dealing with similar applications we will utilize a combination of the following to achieve solder fill:
  1. Convection Bake out oven to prebake the board prior to rework or assembly.
  2. A radiant and or convection heating system (Hot Gas Rework System) to drive additional heat to the target location.
  3. Conduction heating with a soldering iron.
We will normally use a wire solder with additional flux paste or liquid flux compatible to the solder. The key is to drive enough heat into the area to get the complete solder fill you are looking to achieve.

Andy Price
Sales Engineer
Circuit Technology Center
Mr. Price has been a key member of the team at Circuit Technology Center since 1985. He has vast expertise, experience and understanding of complex circuit board rework, repair and modification operations. He is one of the most knowledgeable experts in this area across the globe.
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