For Pb-free, preheat can make or break the process.

Of the three major components of wave soldering – flux, preheat and solder – the most frequently overlooked and underestimated item is preheat. On the fluxer, we invest our energies measuring flux loading, distribution, and PTH penetration; on the wave pot, we monitor wave contact, immersion depths and flow dynamics. But what do we do with preheat? At a minimum, we ensure the flux is dry to avoid solderballs. Some process engineers measure topside temperature by checking the temperature at the exit of the preheat section, either by applying thermally sensitive stickers or shooting the board with an IR pyrometer. Even fewer engineers actually attach thermocouples to the PWBs to log the process temperature profile. In my opinion, thermocouples are the only way to get an adequate description of the preheat process, and are an absolute necessity on a thermally challenging design.

Hole fill is challenging for Pb-free assembly, particularly on thick or thermally dense PWBs. Our experience has shown that a PTH design that is somewhat challenging to fill with SnPb will be very difficult with Pb-free, and a design that is already difficult to fill with SnPb will be nearly impossible with Pb-free. It seems that as the PWB thickness increases from 0.062" to 0.093" and again to 0.135", hole fill challenges increase exponentially at each step. Designs with poor thermal relief that could barely meet SnPb hole fill requirements are not likely to meet those same workmanship standards with Pb-free.

What’s a process engineer to do when these challenging designs reach the assembly line? Typically, the first response to inadequate hole fill is to increase wave contact, and if that doesn’t work, increase the amount of flux applied. Only when these two approaches fail do we typically consider preheat parameters. Years of SnPb soldering have led us to believe that preheat isn’t usually the root cause in wave soldering problems, so when troubleshooting, it’s naturally the last place we look.

Yet the Pb-free process window is much tighter when wave-soldering thermally dense PWBs, and we must precisely reexamine every aspect of the new assembly process. Preheat needs more attention in the Pb-free world, as it now has the potential to make or break the process.

Preheat serves two main functions: thermally conditioning assemblies to mitigate the risk of shock on contact with the molten alloy, and activating fluxes to prepare board surfaces and components for soldering. The time-temperature window for thermal conditioning is much wider than the window for flux activation. From a thermal conditioning perspective, components must be at a minimum temperature prior to wave contact. It doesn’t matter how long it takes to reach that temperature (there are maximum allowable ramp rates for components, but no minimum ones) or how long they have been maintained at that temperature. As long as they hit the wave at a minimum temperature, it’s all good.

This is not true for flux activation. I’ve learned from the chemists I work with that no-clean fluxes are designed to fully activate and deactivate within specific time-temperature windows. If the process is run too fast or too cool, the flux residues may not be electrically reliable, because the activators or other ionic materials were not fully deactivated. If the process is run too slow or too hot, the activators may be completely spent before the assembly reaches the wave. The latter phenomenon was not often witnessed in SnPb processing, but with Pb-free, we sometimes find ourselves wondering if our soldering defects are a result of a situation we refer to as “flux burnout.”

Flux burnout usually occurs when the time the assembly spends in preheat exceeds the flux chemistry’s activation limits. When seeking to improve hole fill, process engineers usually increase wave contact time by slowing the conveyor. Unfortunately, this also increases the assembly’s residence time in the preheat zones and starts pushing the flux’s limits. Flux burnout is relatively easy to diagnose: If slowing the conveyor helps hole fill, then the flux is still active. If slowing the conveyor hurts hole fill, then the flux is spent, and the engineer has found its practical limits.

There are a few ways to minimize the risk of burnout. First, understand the difference in preheat times that the assembly will experience as the conveyor is slowed. Note that we are referring to preheat time, not preheat temperature. Preheat time was never considered a primary factor in wave soldering. To reinforce the notion of preheat time and separate it from temperature, I now refer to it as “tunnel time,” regardless of whether the preheaters are enclosed. If the tunnel time increases by more than 15 to 20%, preheat temperatures should be reduced, particularly in the first zone. Since the tunnel time is longer, the thermal conditioning can still be achieved, but waiting to activate the fluxes until later in the preheat cycle can help preserve them for where they’re needed most: at wave contact.

Use topside preheaters. About a year ago, I did a series of simple experiments at Electrovert. I worked with some of the most highly respected engineers on these tests. We used my standard wave test vehicle: 0.093" thick; four layers; two ground planes; and four different thermal relief designs to provide multiple levels of thermal challenges. One of the first things we found was that at the same conveyor speed, flux loading and topside temperatures, the effect of topside preheaters jumped right out! There was no denying it; preheating from the topside made a huge impact on hole fill. It was greater than we had expected; but then again, expectations were based on SnPb alloy experience.

Topside preheaters not only provide more uniform heat distribution throughout the PWB thickness, but they also ease thermal demands put on the flux itself. When only bottomside preheaters are used, all the heat required to thermally condition the assembly must flow through the flux. If a portion of the heat is delivered from the opposite side of the PWB, the assembly experiences the same or better heat exposure, but the flux itself experiences less. Limiting the flux’s exposure to heat helps improve its sustainability under extreme thermal demands.

Measure the core temperature. We’ve grown comfortable with measuring topside temperature over the years, and flux data sheets often indicate minimum topside temperatures as part of their recommended processing parameters. Both the information and suggestion to measure topside temperature assume traditional wave solder machine configurations with typical applications: only bottomside preheaters and boards that do not present substantial thermal challenges.

When preheat is applied only from the bottom side of the PWB, the top side is the coolest surface to be soldered, so measuring topside temperature makes sense. It ensures the coolest part of the assembly reaches the minimum recommended processing temperature. The minimum temperature recommendation is usually based on evaporating the liquid carrier of the flux prior to wave contact, typically 95°C for alcohol-based fluxes and 100° to 110°C for water-based ones.

When preheat is applied from both sides of the PWB, the topside is no longer the coolest part of the board. The PWB core is coolest, so it’s the core temperature to consider when profiling assemblies with this machine configuration.

Measuring core temperature takes a little more effort than measuring topside temp, but it is worth the effort. The thicker, more thermally massive PWBs requiring more attention to preheat profile are often the most expensive, and almost always more difficult to rework, so the upfront investment of ensuring the best achievable thermal cycle brings immediate payback in terms of throughput and touchup operations. To measure core temperature, the method of instrumenting the PWBs with thermocouples is similar to the method used when profiling BGAs in SMT: Drill with a small rotary cutting tool like a Dremel, insert a thermocouple, and secure it with epoxy. It’s not much work, especially if the profile board used to dial-in the reflow process is available. If it is available for wave profiling use, then it’s just a matter of relocating some thermocouples. And it comes with the bonus of already having some of them attached to fine-pitch topside components. The longer wave contact times associated with heavy boards raise the risk of topside reflow and necessitate the monitoring of these devices anyway.

When it comes to wave soldering thick and thermally dense PWBs with Pb-free solders, I’ve learned a few lessons during the past couple years:

• Pay more attention to preheat. In the tight process windows, it has the potential to make or break the process.
  
  
• Preheat consideration is no longer limited to final temperature anymore. It’s also about tunnel times and temperature profiles.
  
  
• Consider the sustainability of the flux formulation. Long tunnel times can burn out the flux activity before the board ever reaches the wave.
  
  
• Topside preheaters promote more uniform heating of the assembly and limit opportunities for flux burnout.
  
  
• When using topside preheaters, measure core temperature to get an accurate representation of the process.

Preheat is proving to be a more influential factor in Pb-free wave soldering than it was in SnPb. I wish I could offer a guarantee that adjusting preheat settings or configurations would result in all assemblies meeting workmanship standards, but unfortunately I can’t. I have found it can definitely improve results in most cases, but the board design itself is sometimes the overriding factor in achieving acceptable hole fill. Inadequate thermal relief on PTH barrels tied to ground planes has a far greater impact in Pb-free soldering, and the only proven solution to the problem is proper adherence to DfM guidelines. From a processing perspective, there’s only so much that can be done to compensate for bad design practices. Fine-tuning the preheat process may not be a magic cure-all, but it can certainly help overcome a considerable portion of the soldering challenges that thermally massive assemblies present.

Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics.com); chrysshea@cooksonelectronics.com. Her column appears monthly.

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