Why does it take half a day to cook a Thanksgiving turkey? The answer is simple ― you have 20 lb of bird that simply cannot just be nuked in a microwave like last night’s dinner. If not properly thawed, prepared and monitored, you either have an overcooked, dried-out bird or worse: Salmonella. Strangely enough, as you will see in a moment, PCBs are not that much different.
Let’s say you skip the thawing process and in your haste stick a frozen bird in the oven. What happens? The bird may look properly cooked on the outside, but as soon as you try your skill with the carving knife, you either hit bedrock or the inside is completely raw. OK, I will admit I speak from personal experience on this one (please do not bring this up with my wife). Are PCBs any different? Well, your reflow profile has a preheat phase, with the purpose of bringing your PCB to temperature. In other words, the entire mass of the board with all its components is gradually brought to equilibrium. If you do not do this, you run the risk of thermally shocking your components when they hit reflow and peak. Thawing your bird and preheating your PCB ― you have the same objective in mind.
So, for the vast majority of us, we really have no idea when the turkey is fully cooked until getting an internal reading. A PCB is no different. On the surface, both might look great, but upon closer inspection, you discover some components have defects due to improper reflow or, for that matter, when you cut into a turkey that is still pink it really hits home that you aren’t cooking a TV dinner.
Because of this, as we all know, a 20 lb turkey requires a thermometer. I will concede that some of you use the old “poke the bird and check for pink until done” trick. Let’s assume you are not as skilled, like me, for example. Would you seriously cook a turkey by relying solely on the oven’s temperature reading on your stovetop? Of course not, but why do some of you profile your PCB by relying on your reflow oven’s reported readings? Are either situation that much different? Actually, yes. Your nice self-contained turkey cooking oven is more of a steady state, but there remains a large difference between what is reported by the oven and the internal temperature of your turkey. In contrast, your PCB is exposed to anything but a steady state environment because it rides on a conveyor through different heated zones with blowers, extraction systems and both ends of the oven even open to the elements! For this reason, any oven manufacturer will adamantly tell you to profile and with regularity. Alright, you may have learned how to cook a turkey in your Mama’s kitchen and, in fact, be skilled at not using a thermometer; however, I doubt any serious SMT manufacturer would take a similar approach, checking your PCBs regularly for “doneness” in your reflow process.
What about placing the fate of your Thanksgiving feast on the cheap-o plastic pop-up indicator that likely came with the turkey? Do not laugh. How many of us use the trailing wires that came with the reflow oven? Now to be fair, both work in principal; otherwise, you would have the likes of Purdue Farms with food poisoning lawsuits on their hands, but they only give you ballpark readings in many cases. By design, the turkey is going to be a little overdone and dried out. Your PCB, on the other hand, cannot afford to be a little overdone or it is simply OUT of spec. You can get by with eating the overcooked turkey … the gravy and mashed potatoes are there to make up for less than a perfectly cooked bird. But your PCB will not be as forgiving. Trailing wires, never mind being cumbersome to use, have a tendency to kink and stretch, which compromise their readings. They also are susceptible to 50 or 60 cycle noise from some reflow oven environments, further questioning their accuracy in some cases.
So you want to cook the perfect bird. Who doesn’t? So you pony up for a stainless steel large-dial meat thermometer to accurately read the internal temperature of your 20 lb bird. You also pony up for a KIC Explorer with Navigator because you want to create the perfect deep-in spec reflow profile. It will not only tell you the specific temperature of the joints of your $500 BGAs, but it also will find a balance that does not overcook them or any of your other temperature-sensitive components on the PCB. No pop-up indicator profiler needs to apply since the KIC Explorer with Navigator will go the extra mile and tell you not only if you are in-spec but how DEEP in-spec your profile is, along with what can you do to improve the profile in minutes, if not seconds. Now do you know of any turkey thermometers that can do that?
So when you prepare your Thanksgiving turkey, and as you pause to give thanks, consider applying the same care and consideration that you have given to your family’s feast as you do to your PCBs.
Happy Thanksgiving – Profilingguru