Q: What happens when defects occur when the thermal load on the oven increases? Do you slow down production? Do you change the oven set points by cranking up the heat to compensate for the increased load?
A: The answer is to establish a NEW profile that is deeper in spec., a profile that is able to better stand up to the daily variations of a reflow process.
Today, profiling software allows you to establish these new deep-in-spec profiles with relative ease. You can precisely define your specifications and run various predictive scenarios. For example, you know that you can’t slow down your conveyor speed, but you can change your oven set points. The profiling software can give you a predictive result that puts you as deep in spec as possible before ever having to run a profile.
In practice, how much work needs to be done to take this out-of-spec process and bring it within spec depends on a lot of factors. How far out of spec are you to start with? What inputs can be changed? How tight are your specs?
If your process is already taking up most of your process window or not far out of spec, then only minor changes will most likely be needed to bring your profile much deeper into spec. In this case, only one additional profile is likely required to bring your profile very deep within spec. In my experience, this profiling process takes about 30 minutes, most of which is waiting for the oven to cool. If your profile is far out-of-spec., you may need up to an hour to bring it within spec.
Each time you re-profile, it is an opportunity to further improve your profile, bringing it further into spec with each effort. Profiling software will tell you a possible scenario for improvement each time, which takes your excellent deep-in-spec profile still deeper within spec. Each one of these changes, on average, takes about 30 minutes.
A word of caution: having a profile in the center of the spec or at 0% PWI, is not always the optimal improvement. While “0%” PWI is statistically desirable, there are other factors to consider. For example, though 30% PWI indicates that you are only utilizing 30% of the allowable process window of your solder paste, in practice, when you find that a PWI of 65% produces a physically better connection, which is better? Specs are just that: specs. They have a range for a reason. In this case, at the upper end of the spec (opposed to the center of the range), a joint may solder better. Advantageous about most profiling software is that you can go back and re-define your specs to see what your new profile will look like without having to rerun the profile. The allowed range can be further narrowed to a particular spec, which results in a better joint. In essence, you are now re-defining your spec, and all future profiles will only consider this new range.
