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Reflow Profiling in SMT & BBQ: Getting the Right Temperature

During the summer months I'm a big fan of grilling.  Up until very recently I've grilled everything from hot dogs to steaks using nothing but my propane grill and my natural instincts of how hot and how long to cook my dinner.  The results have been pretty good with the occasional dry piece of chicken or too rare hamburger, but overall I've been pretty satisfied.  That was until the other day when I made a purchase that changed my grilling habits forever:  I was at the store and saw a sale on a digital thermometer with a long thermocouple attached to it.  It caught my attention and I impulsively purchased it along with my other necessities. 

Later that night when I got home and opened the package I realized that not only was it a general thermometer, it also had pre-programmed settings for various types of meat (medium rare beef, chicken, pork, etc.).  My wife put together some seasoned hamburger patties and I took them out to the grill.  I set the thermometer for the "well beef" setting, as she likes, and put her patty on the grill.  When I saw that her meat had reached an internal temperature of approximately 130oC, I put mine on the grill, as I like it medium rare.  I waited patiently until the thermometer began beeping that her burger was done, changed the setting to "medium rare beef", and moved the thermocouple over to my burger.  My instinct was good in this case and my burger's internal temperature had the thermometer beeping about a minute later.  Those were the best burgers I'd made to date due to knowing the exact internal temperature of the meat at all times.

So, why am I talking about grilling meat in a solder blog?  Well, many of the same concepts apply to reflow profiling. You need to accurately establish and set your reflow oven zone temperatures and belt speed to effect the optimal temperatures on the circuit board so that you properly reflow the solder.  Just as my meat thermometer has pre-set conditions built in, and a long thermocouple that can be attached to my meat, a solder reflow profiling device does, too. 

I recently visited a customer site at which an engineer had adjusted his oven by instinct.  The customer was getting adequate reflow of their tin-lead solder paste, but was also melting the internal solder on components that were assembled using SAC alloy, damaging the components.  It was obvious that the reflow temperature was too hot, but, without profiling, we had no idea how hot it was or where to make adjustments.  Using a KIC Profiler, we were able to establish conditions such as desired ramp rate, time above liquidus, and peak temperature, and then were able to follow up by actually measuring the temperatures on the board as it passed through the oven to determine how well it matched up to these settings.  As suspected, the initial settings were much too hot, heating the solder joints to almost 30oC higher than needed.  Using the KIC software's prediction abilities, we were able to theoretically adjust the oven zone settings and belt speed until everything appeared normal.  Changing the actual oven settings to the newly predicted settings, we ran the board through again and saw that we were now well within the process window that we were seeking.

In summary, it's very important to actually know what temperature your circuit board (or meat!) is seeing inside the oven.  Just because a setting says 250oC, the board may only be reaching a temperature of 230oC or lower, depending on the size of your components, the board thickness, the amount of copper within the board, pallets being used to hold the board, etc.  Without actually profiling with thermocouples directly attached to the board, it's impossible to be certain what temperatures your solder is experiencing. 

This is just a basic guideline to get a general reflow profile set up.  Once this is achieved, certain defects can also be overcome through careful and specific manipulation of the reflow profile.  My colleague, Ed Briggs, has written some blog posts on how to overcome defects through proper profiling. He covers defects such as the elimination of graping, here.

Have a great summer!