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.

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.

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.

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.

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!

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.

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.

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.

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).

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.