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"Fat, drunk and stupid is no way to go through life." - Dean Wormer, Animal House

It amazes me how many people do not know how to profile a reflow oven. Come on, gang: we have been reflow soldering for over 25 years now (time flies when you are having a good time). Yet many people still do not have their act together. It is true: No single profile can do it all. Trying to make do with a single profile is going to bite you in your assets when Pb-free manufacturing comes home to roost.

Profiling is the process of determining the proper time/temperature durations a PCB assembly must sustain throughout the reflow cycle.¹ These are dictated by solder paste characteristics such as solder alloy, solder sphere size, metal content of the paste and chemistry. The setpoints of the emitters and conveyor speed setting of the oven required for reflow are affected by the mass, surface geometry complexity and substrate conductivity of a particular assembly, plus the oven's capability to impart sufficient heat energy. The oven's heat transfer efficiency and the operator's experience will determine just how much of the profiling process is an exercise in trial-and-error.

Reflow soldering is not just "the board goes in the oven, heats up, the solder melts, the board comes out, the solder cools." But, it is not rocket science, either. The time/temperature duration information is supplied by the paste manufacturer for the specific formulation being used and can, hopefully, be found on the datasheet.² Typically, for SnPb37 or SnPb36Ag2 the full liquidus or reflow temperature will be a minimum of 205° to 210°C. For the Pb-free SAC305 alloy, the minimum temperature to be attained at peak is around 235°C.³ Again, look to the datasheet for the temperatures specified by the manufacturer; it is their formulation in terms of metallurgy and chemistry (flux, solvents, etc.).

The maximum temperature the assembly can see is dictated by the component (or material) on the assembly with the lowest thermal threshold of pain. Subtract a "buffer" of 5°C from this temperature to determine the temperature of the most vulnerable component (MVC). The maximum gradient across the assembly is defined by subtracting from the MVC value the temperature at which the liquidicity of the molten alloy is ideal for wetting (full liquidus temperature) which is usually 15° to 20°C above the melting point of the solder alloy (which you conveniently obtained from the solder paste manufacturer's comprehensive datasheet). Do not forget to attain the shortest time above liquidus temperature, too.

Not that complex, is it? That said, it is surprising how many people do not indulge in the practice. I am not quite sure how they determine proper oven settings without profiling (osmosis? laying hands across the oven? psychic telepathy?). Some oven salespersons still tout the "one profile does all" line. (If you buy that, I have a bridge I would like to show you.) Profiling each application is necessary but not all that difficult. Of course there is a downside: reflow profiling is a monotonous procedure. Reflowing in a convection oven is unexciting. With printing and component placement, at least there are the dynamics of mechanical motion. In reflow, the assembly disappears down a dark tunnel and emerges about four minutes later. One oven manufacturer once experimented with cameras to observe the melting of the solder. And some vapor-phase machines have windows in the reflow zone. Now that is entertainment! (Inline cleaners have windows so this exciting process can be observed, too. What's next, a reality television show on soldering?) The point is, you want to optimize the profiling procedure to minimize the time spent on this exhilarating task.

Most important is proper attachment of thermocouple to assembly. Do not use Kapton tape: It will not make consistent contact with the solder joint that you think is being monitored. Instead, what is likely being measured is a combination of air temperature, tape temperature and sometimes (maybe) joint temperature, with the latter highly inaccurate. The proper method is to attach TCs with either a high temperature solder alloy or a conductive epoxy. This may mean sacrificing an assembly to the profile gods, but you will have a tool with which periodic checks may be performed on the accuracy and repeatability of the profile for that board in your oven. For very low quantities and a high mix of boards, a number of nondestructive, reusable contacting probes are available.

Use an assembly that is populated with components as it will be passing through the oven. Except when reflow soldering a bare board, do not profile using an unpopulated board. TCs will be attached at interconnections (lead-to-pad junctions) that represent, at least, the warmest and coolest points on the board. The warmest point will typically be a low-mass component (such as a passive) located near a corner or edge of the board. The coolest point will likely be an interconnect of a high-mass component (QFP, PLCC, BGA) near the center of the board. Place other TCs at heat-sensitive components (i.e., the MVC) and other high-mass components to ensure they get sufficient heat.

When using a previously soldered assembly, remove the solder from interconnects to which TCs will be attached. As the board was likely soldered with SnPb37 and you will be using, perhaps, SnPb12, if you simply solder the thermocouple with the latter, you would wind up with a "mystery" alloy - one that will not sustain multiple thermal excursions required of the test board. Regardless of what is covering it, the underlying SnPb37 will reflow at its usual temperature and undermine the durability of the attachment. It is therefore mandatory that the existing solder be removed using solder-wick. This should also be done if conductive adhesive is used to attach the TC. Otherwise, the SnPb37 will reflow underneath the adhesive.

Profiling is tedious; nothing is worse than losing a TC during a run and having to do it over again. Hence, removing excess solder is very important. I have an assembly that I use for evaluating reflow systems that was instrumented in this fashion. This assembly has seen at least 150 reflow cycles and has never lost a TC attachment. The rest of the board does not look too hot but the TCs are intact. After removing old solder, apply a small amount of flux and then, using a soldering iron, add a small but sufficient amount of the high-temperature solder. While SnPb12 will work when profiling Pb-free alloys, consider the conductive epoxy route for attaching TCs to Pb-free applications.

Use Type K, 30 AWG TC wires, preferably pre-welded. After attachment, route the TC leads toward the rear (in terms of direction of travel) of the assembly. Some people prefer to terminate the TC lead at the trailing edge of the PCB with a connector. The lead from the measuring device can then be quickly connected and disconnected. Use Kapton tape to provide hold-down and strain relief to the TC leads at appropriate locations on the PCB.

Most reflow machines are equipped with on-board profiling software. This permits TC leads to be plugged into ports on the oven and tracked in real-time on the system monitor. Many people prefer a data-recorder device (KIC, M.O.L.E., Datapaq) that travels through the oven with the test assembly and records the temperature from multiple TCs at programmable time intervals. These systems are offered as "run-download-read" or as transmitting units that permit observation of the profile in real-time. Over the past few years, these systems have improved in accuracy and ease of use, with more sophisticated software. Capable, accurate profile prediction capability has taken almost all the guesswork out of profiling for most applications. This feature saves time in a way that is nothing short of miraculous. Another excuse for not profiling every application eradicated.

Today's ovens are powerful in terms of heat transfer efficiency; this translates to greater ease of profiling and less profiles required among similar boards. Just ask some grizzled veteran from the infrared and convection/infrared reflow days (the pre-convection-dominant era) about the tedium of endeavor profiling. A relatively easy assembly might have taken five passes while a moderately complex application could take a day to profile. Today, it is fun and easy - well, it is not really fun. But it is relatively straightforward. Care in the profiling process can save time and improve accuracy. All that time saved can be spent on your RoHS compliance efforts. Are we having fun yet?

Remember, we're all in this together.

End Notes

Phil Zarrow is president and SMT process consultant with ITM Consulting (itmconsulting.org); itmconsulting@aol.com. He still bears the scars, physical and mental, of reflowing convection/IR ovens.