Reflow Oven Profiler

Reflow oven profiling can be a slow, costly and painstaking endeavor. Reflow profiles using thermal couples (TCs) can be as useful as ion-chromotography (IC) and strain gauge (SG) testing. However, the incorrect placement of TCs or SGs, or choosing the wrong location to run an IC test, can provide inaccurate results. No matter where a thermal couple (TC) is placed, if the oven is well-constructed, designed, and maintained, the entire assembly will be subjected to the same oven settings.

The first step in profiling is to establish a baseline temperature for the oven. At any point of travel within the oven—from entrance to exit—a TC should measure the same temperature from the front of the oven to the back. Ideally, the proximity of a TC to the transport rails should not affect the heating uniformity as compared to one located in the middle of the gap between the rails or at any point in-between. If using the mesh belt, the heating uniformity should not vary across the useable width of the belt.

Once the baseline is established, it should not vary between shifts, changes in plant environment, preventative maintenance cycles, etc. If the oven is uniform and stable, placing a thermal couple on the smallest component will ensure every similar component measured anywhere else will yield a similar measurement; the same goes for the largest component. If you have access to the PCB Gerber files, look for a location on the circuit board where the lowest density of internal copper layers, copper pours, and thermal sinks can be found. This location will tend to be the hottest spot on the circuit board where the first TC should be placed. The largest component along with the highest density of layers, copper pours, and thermal sinks will be the coolest spot on the PCBA. Place the second TC at this location.

With these two TC locations identified, essentially everything else in-between is considered moot. Dial in a profile that reduces the temperature difference between these two extremes. The profile should also fall within the process window for the solder paste being used. Since the size of the circuit board and the loading (the gap between successive PCBs flowing through the oven) can affect heating uniformity between PCBAs, it is important to characterize the effect these differences can have by simply running the profile PCBA by itself and with maximum loading. Since loading is variable during production, a well-designed oven should minimize such an effect, but proof is needed. The upfront work necessary to provide evidence of oven uniformity and stability is a prerequisite to using this two-TC approach; otherwise, use as many TCs as the profiler can accept.