Problem Meeting Minimum Hole Fill During Wave Soldering

Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers. We are coming to you from ITM headquarters, high atop Mount Rialto.

We are here to talk about electronic assembly, materials, equipment, components, practices procedures, goof-ups, screw-ups and many, many other things. And boy those last two categories, we sure know about those.

So Jim, what is today's question.

Jim
It comes from W.E. We are having problems achieving the minimum hole fill of 75% on electrolytic capacitors using our wave soldering. What do you suggest?

Classic wave soldering problem - hole fill. Well, first we can say that you are obviously attempting to build a class three assembly because that is the only one that requires 75%, so we know that.

This is a basic wave soldering issue. How do I get the solder to fill the holes adequately, in this case 75%. It is the whole process.

This is why wave soldering is a challenge, because you are applying flux and you have to make sure that the flux has penetrate up through the entire hole. If the flux doesn't get up there, the solder won't go there.

Then you have to make sure that the flux is properly activated and all of the solvent has evaporated right up to the top of the hole. That is why when we profile or preheat we put thermocouples on top of the board, as well as the bottom, to make sure we have heated that entire hole.

With thick boards our associate, Bob Klenke wave soldering expert, says put a thermocouple in the middle of the board, in the middle of the hole, to make sure that even though you have top side and bottom side pre-heating, that you are getting that heating and flux activation all of the way to the center of the hole.

If not, even though if may be hot on top if it is cool in the middle the solder will come up and stop when it hits that cool section where the flux is inactivated and the barrel of the hole isn't up to wetting temperature. And then of course, adjusting your wave to the proper immersion depth, proper immersion time, so that again the heat has time to penetrate up into the hole.

Phil
Yeah, make sure your rails are parallel. Take nothing for granted.

And then once you get through all of the wave solder machine parameters, as Jim mentioned there are tons of them, then possibly take a reexamination of your design in terms of that you have the proper gap between the outer diameter lead and the inner diameter of the barrel.

Jim
Or lead to hole ratio, as it is sometimes described.

Phil
It is amazing for technology that has basically been around over sixty years, we're still trying to figure it out. It is definitely not a zero defect process. But getting adequate hole fill, in spite all of these variables, is obviously what you want to do.

That is what it is all about. Watch also your time going over, your immersion depth, your immersion time, your time over the wave. There are just so many factors.

Jim
Phil, we forgot the obvious, as we often do. We are fallible like everybody else.

Solderability. Make sure your leads are adequately solderable and that the barrels of your holes are solderable. Particularly concern some people have if you are using and OSP and it has already been through a couple of reflow cycles.

Are you controlling that and getting the proper coating so that it is still solderable when you come to that wave soldering step.

Phil
And of course cleanliness, as Jim is intoning, of the leads and of the board itself, and the inside of the barrels. Also another thing, it is a little finite there, but that you have adequate plating inside the holes.

Take nothing for granted. It is a massive checklist you can go through here. But that is the wave soldering process.

Jim
Phil, taking a step back and seeing as how W.E. mentions specifically electrolytic capacitors, my focus initially might be on heating. These tend to be big, heavy units and they may not be heating enough.

So maybe your preheat, or time of immersion, or something like that may be the first thing to look at. But although any of these things could contribute.

Phil
Right, make sure you have all your i's dotted and your t's crossed the things that Jim enumerated there. It is just good wave solder practice. Well great, we're always practicing too.

You just spent five minutes or so with us. We thank you very much and whatever you do.

Jim
Don't solder like my brother.

Phil
And don't solder like my brother. And when you are examining those electrolytic don't stand to close to that flux pot either.

I've had these problems where there were no thermal reliefs for the vias connected to the ground planes. A design change solved the problem.

Everything that has been described is correct. If it is a thermodynamic problem, preheating is essential because it’s part of the demand on soldering heat. The higher the heat capacity of the PCB, the longer the preheating time required to achieve an even and homogeneous heat up. Medium-wave IR and convection heaters are advantageous. With high power short-wave IR heaters, there is a risk that the surface of the PCB will become too hot and the flux will already be consumed during preheating. The maximum allowable preheating temperature should be fully used, it depends on the flux type and the components.

We do not recommend top side temperatures higher than 130°C - 135°C. If the Elcap pins are made of alloy 42, they have a poor thermal conductivity. In this case, additional thermal vias in the pads of the thermal reliefs are helpful. They carry additional thermal energy to the top side of the PCB (during soldering) to achieve the required wetting temperature for solder fill. Top side heaters do not help in this case, as large Elcap’s shield the solder joints from the top side heat energy.

Our past experience is that the use of an inert atmosphere, Nitrogen, significantly improves through wetting performance. The nitrogen will allow for the flux to stay active longer and promote improved wetting. N2 inerting retrofit kits are available to use with wave soldering systems.

1. Ensure correct through hole wetting of flux
2. Set solder Bath Temperature to 270-275C for Lead Free to allow for the 5-10C loss of solder temperature as it flows up and over the nozzle. This also ensures the largest heat sink (the PCB) is adequately heated to ensure the solder stays molten through the holes and doesnt chill prematurely
3. If using ENIG finish ensure both the Au and Ni is correct thickness or you will have partial hole fill. Can be seen with both thin Nickel or thin Gold 4. If you exhaust the above then check there isnt too much gold brightener used which may cause dewetting effect

It could be your machine is not up to it. We had similar problem and it went away with a new 5 zone wave with jet fluxer.

We have found in hundreds of experiments that the temperature of the solder in the pot correlates to hole fill. Try raising the temperature in 5 degree C increments.

IPC Rev F mentioned clearly for the 50% solder fill acceptable, class 2 for the leads count more than 14. Earlier it was reflected as GND or PWR plane connection.

The alloy and flux type in use are significant variables that will impact hole fill. Some material sets can make life very difficult with high mass assemblies.

Everything everyone said is true. I had a problem with a mixed technology aerospace board with all the components on the top. 16 heavy copper layers, the boards were 12 x 14 inches (board was originally designed in the 1970's). BIG electrolytics next to SMD parts (board revision was in the double digits). OA flux. We made sure that the components were solderable, The board had been properly cleaned, handled with clean gloves, and prebaked. The flux was sprayed on and the viscosity was in range. The immersion depth was correct, the angle was correct, Speed was correct. Requirement by print was 100% hole fill with a uniform solder fillet on top. DID NOT HAPPEN! The solder that did flow through was dull was grainy. We thought the problem was the automated setup for the heat and board speed. The top was not getting to the needed preheat and what was confusing was the not the electrolytics, but the inner layer ground and traces, away from the electrolytics was starving the preheat from sections of the board.

We tried adding multiple thermocouples on several runs. Cranked the preheaters up, slowed it down, started getting a wide difference in preheat from the thermocouples and we were concerned that we were on the edge of the degrading the boards, yet still no luck.

It was the preheaters, they did not fail, just were degrading and inefficient, and the surface profiles on the heaters themselves showed that the heat varied greatly across them. Once upgraded, they did perform, and once we got a fairly uniform preheat of 525F across the top of the board (verified with thermocouples), the problem disappeared. A fully automated wave solder system is great when it works. Most of the time the adjustments work. But in the final analysis, the boards may just be too complex to fit in the profiles, so you have to go old school and use surface thermocouples. This issue probably cost us more than $350,000 in ruined boards, excessive rework, degraded performance, and a really nasty relationship with our customer, the prime. I hate to say it, but there really is an art (not an app) into producing legacy and mixed technology boards.

Maybe you could check the wetting angle in the hole of capacitors with and without component and an other hole.

I've had those problems with components attached to the ground planes. A lack of a thermal relief has been the cause of most problems or thick boards. If I see that in the first article I then run a slower conveyor speed to get the board heated internally and give the solder a chance to wick up. Make sure your preheat tunnel sufficiently heats the top side of the board too.

As per IPC-A-610 Rev. F, fill requirements for Class 2 & 3 are 75%. For class 1 it is not specified (table7-4). And the exemption of 50% for Class 2 connected to a thermal layer (7.3.5.1)has been removed in Rev. F

For the latest revision of IPC-610 (F), there is information that 50% of vertical fill is allowed only for components with 14 or more leads.

If want to be sure what is the possible root cause of vertical issue:

• put the spacer between the the capacitor leads - to check if the flux does or does not evaporate
• for second board please insert different component - let say lightest - ie.resistor (perfectly if the leads diameter will be equal)

Last not least, the design should have thermal reliefs (if not can not be implemented IPC 610)

Another design factor, beside the lead to hole ration, which you should reexamine is the thermal relief between the ground planes and the PTH holes. It is difficult to achieve the required hole fill if the hole does not have adequate thermal relief at the connection with ground and power planes.

Inert atmospheres with nitrogen in the previous reflow oven process should help to decrease oxidation coming from that step. Correct nitrogen injection in the wave machine would also help to reduce metal parts oxidation and then improve wettability, resulting in better barrel filling.

One item not mentioned in the article is to ensure there is a proper venting path built into the capacitor. I have seen issue before where the capacitor is held tight against the PCB to prevent tipping and if the component does not have a venting path, pressure will build up under the component and force the solder out of the hole once the lead clears the wave.

Another cause to consider is how flat does the Electrolytic Capacitor sit on top of the PCB. We have seen situations where the cap sits flush to the board and causes a back pressure that will prevent the solder from filling completely. If you can, you may consider holding the cap off the board to allow for the pressure release that will help the solder flow.

Ensure the drill inner walls completed platted. Try backing the board ( not OSP ) before wave process. If the board is OSP, try to complete the wave process as much as possible in two hours from prior SMT assembly process.

Fill requirements per IPC are 75% for ALL three Classes. Not just Class 3. There is an exception for Class 1, 2 if connected to thermal or conductive layers acting as heat sinks.

1. Pre-tin the cap leads. Minor cost adder, but best fix.
2. Brush tacky flux around the topside hole, including the cap leads. Tacky flux sticks around for the whole job and will immensely help promote hole fill. Be sure to match the formulation with the flux used in the machine.
3. Make sure the cap lead protrusion is between .030" to .060" if the assembly drawing allows it. Insufficient lead protrusion contributes to insufficient hole fill, solder voids, and lead contour not visible. Your touch-up operators could be removing solder to fix lead contour defects and be inadvertently giving you insufficient hole fill as a result.

Actually in IPC-A-610 Rev F which was released last summer, the only acceptable condition for less than 75% barrel fill, is when it's on a component with 14 leads or more. This is in regards to class 2 only, class 3 still has a minimum of 75% no matter the lead count.