Why do I read that the lead-free thermal process window will shrink significantly? Sure, the temperatures are higher, but is not the width of the window the same?

The short answer is no, the lead-free process window is very narrow. Here is the reason:

While the many lead-free alloys allow for a relatively wide process window, certain types of components lower these process limits. It is true that the component manufacturers are actively developing new lead-free components, many of which are spec'd to a peak temperature of 260 °C. There will, however, be components that cannot handle anywhere near this temperature range. The most temperature sensitive component on a PCB will of course define the entire process window as far as peak temperature is concerned. These components may also have additional limitations concerning both increasing and decreasing slope.

To further complicate the matter, lead free solder paste properties differ considerably from leaded paste. It does for example not wet nearly as well. Time Above Reflow, Slope, and Dwell times may all change.

In conclusion, the lead-free process window is narrow and will make oven set up very critical. As the PCB has to be brought up to a higher temperature and cooled back down, we will also face a throughput requirement. The reflow oven should never be allowed to act as the bottleneck in the production line and hence the oven needs to process the lead-free PC B in spec at a conveyor speed faster than the slowest equipment in the line. Fortunately, there are automatic Oven Recipe Search Engines T on the market that selects the most appropriate oven setups for lead-free applications.

With the narrow lead-free process window, temperature sensitive components and substrate, how do I go about defining the process specifications and profile the assembly to set up my reflow oven?

•  Step one. Identify the limiting factors.

Ask your solder paste supplier for their process window: Upper and lower limits for peak temperature, slope, time above reflow etc. Do the same with the most sensitive components. They tend to be the electrolytic capacitors, crystals, headers etc. Finally, check the specifications for your substrate. The process window is comprised of the "lowest common denominator" of all these specifications. It is, however, possible to define separate process windows for unique points or components on the assembly. As long as the profile does not exceed any of the process limits you are OK.

This may seem complicated, but once your material and component suppliers have provided the specs, it is a relatively simple matter of entering the limiting process specifications into the profiling software.

•  Step two. Attach TCs.

It is important to select the correct areas to measure. These are typically the hottest and coldest points on the assembly. The center of the board and the areas with the largest thermal mass tend to be the coolest, while the corners and areas with little thermal mass the hottest. You also want to select the most sensitive component(s). Some engineers prefer to drill a hole in the component and insert a TC to measure the internal temperature. Finally, you can also attach TCs to the bare board to make sure it does not exceed the limitations specified by its manufacturer.

KIC recommends attaching TCs either with aluminum tape or with high temperature solder. Other methods may be used, but it is important for the same method to be used consistently for consistent results.

•  Step three. Run the profile.

Modern profilers will provide a temperature - time profile for each of the selected points on the assembly, and they will display the actual values for peak temperature etc. Some profiling software will also display whether these values are inside the established process window.

•  By using a trial and error approach it is possible to change the oven recipe (combinations of zone temperatures and conveyor speed) until an in-spec profile is found. There are also profilers on the market that automatically search new oven recipes that will bring an out of spec profile into spec, and even to optimize the profile towards the center of the process window for a more stable thermal process. This will dramatically reduce oven setup time and yield a higher quality process.

Related Material:
Attaching Thermocouples for Solder Reflow Board Profiling Using High Temperature Solder (PDF)

A Comparison of Methods for Attaching Thermocouples to Printed Circuit Boards for Thermal Profiling (PDF)