Circuits Assembly Online Magazine - Switching to Pb-Free: Reflow Soldering

Stalagmites look pretty in caves, but not in convection tunnels.

Ed.: The previous installment of this series misreported that the RoHS directive bans halogen flame retardants. FR-4 materials do not contain PBB or PBDE, which are restricted under RoHS.

Wave soldering systems are critical equipment in the Pb-free transition, as aggressive Pb-free solder alloys directly contact machine parts; e.g., solder pot, pump wheels or channels. As stated in part two of our series, wave soldering systems have to cope not only with changes in the flow pattern of the molten solder, but the contact zones also need to be protected against corrosive new alloys. Various equipment manufacturers offer different solutions and protective coatings.

In this part, we look at reflow, selective soldering and hand soldering. Compared to wave soldering, a changeover is easier for those processes, yet there are still issues to be considered. First, we will distinguish between three of the best-known processes: infrared radiation, convection and condensation heat transfer.

Infrared. There are few movable parts in such equipment. Attention must be paid to the conveyor and, perhaps, improved insulation. IR technology is less able to provide small dTs on the assembly. This inadequacy could become more pronounced as temperatures increase. Further, such systems are often relatively short and have problems processing SnPb, such as maintaining specific profiles over longer periods. It is unlikely that the situation would improve for alternative alloys.

We have to question again whether the dT will play the same important role in Pb-free soldering as it has in traditional solder joining. If not, the argument reduces itself to the large temperature difference between the heater source and heater target.

That IR equipment also will need inert atmosphere coverage is obvious. It is never the soldering method itself that reduces wetting, but rather the material contribution. Because new pastes will be based on somewhat higher rosin content, the tunnel atmosphere should be managed to minimize soiling of the interior.

Convection systems. Although reflow is just a “managed heat transfer,” many pieces of equipment do not meet the requirements of Pb-free processes. When thumbing through the maintenance records of one of our customers running SnPb, we noticed that the ventilation unit in the peak zone of the systems had to be replaced every three months. This had become routine as a preventive maintenance procedure. The motors and bearings were not able to cope with the peak temperature for longer periods.

If machines have problems during SnPb processing, we fear that at 20-40°K higher peaks the system would always be approaching shutdown. Again, it is the moving parts (motors, bearings) that pose the problem. Obtaining manufacturer replacement parts free of charge (under warranty) will not solve the situation: It is not the repair but rather the line downtime that costs money. Global companies calculate downtime of a major mass production line to be around ¤10,000 to ¤15,000. As one of our engineering contacts in such a company once said: “After 10 hours of downtime not only can the company buy a better machine, I can find myself another job.” Special attention, therefore, should be paid to high-quality blower units and motors.

Simultaneously, higher process temperatures require a very effective energy transfer. Heating zones, which are located not only in the upper part of the machine but also in the bottom, combined with an optimized air or gas circulation, ensure effective, component-sensitive heating of assemblies.

Therefore, it is possible to set low oven temperatures, which reduces temperature stress on components, especially for Pb-free soldering. Also, the oxidation rate will be kept as low as possible. Moreover, systems with so-called multi-peaks – double or even triple peaks – ensure flexible temperature profiling.

Nitrogen capability is another requirement. Pb-free reduces wetting, and must be compensated for with nitrogen (and not by even-higher temperatures). Low consumption and a reasonable level of residual oxygen have to be targeted. Consumption can be lowered and resources and money can be saved if reasonable ROL values satisfy the process requirements.

We expect higher rosin content in pastes and hence insist on better flux traps and gas management methods within the equipment. Stalagmites and stalactites look pretty in caves but not in convection tunnels. Not only do they demand serious maintenance, they pose a danger to assembly cleanliness. The soldering system should be equipped with a multi-stage condensate management (at least at the beginning of the heating and cooling zone).

A filterless flux management system provides some advantages as filters may clog gradually and thus disturb the process.

The new temperatures will be close to – if not higher than – the glass transition range of the laminate. Expect more problems with warpage and deformation of the circuit board. Once the PCB has lost some stiffness, it has to be supported. A center support adjustable in width and height can help.

At the exit, cooling defines the crystalline structure of the joints. Information is still lacking on desired cooling rates. But we know that the replacement alloys can create different crystalline structures, depending on the cooling rate employed. Current research tries to identify the “best” crystalline structure and the cooling rate that can achieve it. As results are expected soon, adjustable cooling possibilities will be desired at the exit of the machine.

Condensation systems. In condensation reflow systems the reflow temperature is most often (at least in saturated vapor systems) defined by the boiling point of the liquid in use. For alloys with higher melting points, the liquid has to be changed to match boiling point with melting point. Research at the Technische Universität Dresden¹ seems to indicate that a lower “super heat” (distance: melting point of alloy to boiling point of liquid = process temperature) may be used than in other reflow processes. The reason is the very low level of residual oxygen present in the vapor. As many different liquids are available, with a large range of boiling points, we do not see problems arising from special requirements posed on chemicals.

The thermal profiling of condensation reflow is largely controlled by the conveyor speed. Changing the heating capacity is used only for some preheating technology. The question as to whether such systems are Pb-free compatible thus seems to limit itself to an assessment of the heating capability and whether the increased energy requirements of the higher boiling point liquids can be met.

Although the liquid has a limited ability to dissolve rosin, it would accumulate in the sump if not filtered out regularly. For new pastes, better or more frequent filtering may become an issue. This is a question that is maintenance-related only; the vapor, being in a distilled phase, would be clean even if the sump has been contaminated.

Today’s equipment is well insulated and though the temperature inside will be increased by 10-20°K, the skin should remain within acceptable limits. In most machines the conveyor is “basket-like” and does not require additional support structures, except perhaps for double-sided reflow.

What is true about crystalline structures in convection reflow also holds for condensation reflow. Cooling rates at the exit have to be adjustable, less we be left without the means to produce optimal solder joints.

Selective soldering. From the discussion of the main soldering processes one may glean those aspects that also apply to other operations. During selective soldering either liquid solder (mini-waves) or reflow methods (hot gas or laser) are applied. All aforementioned aspects are less problematic as the processes are used only selectively and do not affect the complete assembly. Especially in case of miniwave soldering, Pb-free processes do not differ much compared to traditional SnPb soldering, provided ideal peel-off angles can be achieved with flexible handling (gripper) systems (e.g., tilting, turning of assemblies, etc.). In those cases, the changing flow pattern of Pb-free solder alloys are not relevant.

From a materials perspective, suitable protective measures for solder pots, pumps and so on must be considered with in the context of the aggressiveness of Pb-free solder.

The situation is different for hand soldering. The life of tips of soldering irons will be reduced substantially. And, if several alloys will be in use, the control of the proper tip temperature (m.p. of the alloy plus 70°K for the heat-sinking experienced during soldering plus approximately 35-50°K “super heat” to ensure proper flow and wetting) will be more difficult than in recent times. The difference in melting point would ideally translate into different tip temperatures for each alloy. Maybe the solder station vendors could consider easing life for supervisors and operators by color-coding the different preselected settings?

During equipment assessment, consider the following: