Spot Checking your Reflow Oven – Part I

Reflow ovens have come a long way, they are much more stable, better controlled, well built and less expensive.   Nevertheless, the need for profiling has not been eliminated, since after all the reflow oven is just a means to an end.  You still need to dial in your oven set points to achieve a desired profile and what oven set point temperatures you choose is not what you see on your PCB.  You may be surprised how many line operators worldwide do not make this connection!

Once you have developed numerous profile recipes, many of you would only like to spot check the reflow oven to see if there has been any change.  The logic goes, if I have developed robust profiles and I can check my oven periodically to confirm there hasn’t been any change, I can therefore infer that my profiles are still valid?  Well, yes and no.   “Infer” is the action word here that can lead to trouble if you are not careful.   Let me give you an example.

For fun I ran some popular oven verification tools you can find on the market to see if they could catch changes that I knew would throw my profiles out-of-spec.   What I did first was to profile an actual PCB, followed by my oven verification fixture.   Running the fixture soon after my profiles gives me a baseline to compare against for future spot checks.  I then took a small clip on desk fan and blew into the entrance of my oven tunnel, knowing that it would change the thermal dynamics inside my process.   I made sure my fan was low enough not to trigger an oven alarm, but just enough airflow that based on my experience would in fact change the process environment inside my oven.

Here is what I found.   My profiled board width was ~12″ wide so a sizable mass.  I used a fixture of the same width, since changing my belt width is not practical.  When I ran the fixture I was given oven statistics to compare against my future spot check.  No problem so far.  When I ran the fixture a second time (fan a blowing), I found a 1 degree delta change.  Now I think you would agree 1 degree doesn’t seem like much at all.   Of course I am not looking at a profile, rather machine data so here is that word, I “inferred” that my process was still fine.   Wouldn’t you?  Now intuitively, I knew this fan blowing up the tunnel of my oven is creating process changes perhaps more than 1 degree.  So how do you know, well you run a profile!   Sure enough when I ran a profile, I did not see any longer just a  1 degree delta.  In fact my profile was out of spec.

So what happen?  I can only assume the fixture which I was using that weighed in at close to 7 lbs, must be absorbing some of the changes inside the oven, masking what is really going on.  Keep in mind the fixture is not designed to profile, rather it is designed to detect  measurable change to my process that would impact my profile.   The mass itself must be factoring into my ability to detect change.  Again this is just my observations and my conclusions are subjective, but I can say that it did not capture what is the whole point of this exercise.   So I guess the mass of your vehicle is important and it is not all just relative.  Any thoughts out there, please post your comments.

Well I guess it is back to profiling, or is it?  Stay tuned for part II as I explore other options.

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