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February, 2017
How to Properly Calculate Reflow Temperature Gradients
By Vitor Barros, General Manager, KIC Europe T
he theory is simple: Process electronic assemblies in spec every time and the result is a quality product. In practice, however,
this is much more difficult for numerous reasons, including normal and abnormal process variations, human error, and many other factors. First, let’s be clear about what the process
specifications actually are. Critical components such as LEDs, crystals, bottom-terminated compo- nents (BTC), including micro-BGAs and oth- ers, have very specific process limits that can be challenging to achieve and can be devastat- ing when ignored. Electronics manufacturers must fully understand the nomenclature and process window definitions before setting up their assembly machines. For example, max rising and falling slope limits during PCB reflow are a common source of misunder- standing and process failure.
The Devil’s in the Details Take an LED component for example,
and note that the terms slope and gradient may be used interchangeably. A typical reflow process window or process limits for such a component may appear relatively straightfor- ward, but the downside of getting it wrong is sig- nificant. First, a definition of a temperature slope: the rate of temperature change with distance or time. In a thermal profile, this will be measured in degrees Celsius or Kelvin (°C or K) per second. A common mistake is to calculate max rising
and falling temperature gradients as a linear measurement from the start of the profile to the peak temperature, and from the peak temperature
Example of thermal process specifications for an LED component.
need one more specification, the distance or time over which the slope will be measured. Reading the fine print in the LED component spec limits we might find something like the following: Max ris- ing slope to be measured over 10-second intervals. To calculate the max rising slope for the LED
component we need to measure each 10-second slope along the profile from the beginning to peak temperature. To do that we select the profile tem-
to the end of the profile. These calculations are misleading and result in slope profile inaccuracies because they average together all of the various slope calculations along the entire profile. This brings to mind an old joke about a statistician, with his head in the oven and feet in the refrigera- tor, who stated that the average temperature was comfortable. To find the correct measurement of a slope we
perature at 10 seconds, subtract the temperature at 0 seconds, and divide by 10 seconds. Next, we calculate the profile temperature at 11 seconds, subtracting the temperature at 1 second and divide by 10 seconds and so forth. The calculations will continue in one-second increments until the peak.
Finally, the highest number of all these calcu-
lations represents max rising slope. You will find that the 10-second max gradient measure- ment is significantly higher than the aver- age gradient. Similar calculations will be made on the falling slope, but the component supplier will likely specify a different accept- able limit along with new calculation guide- lines. The cooling section of the profile has a shorter duration and may be susceptible to more volatile temperature variations. The component specifications may call for max falling slope measurements over a 5-second interval instead.
Approaches to Slope Calculation The use of average instead of max slope
calculations will be misleading, and it risks component damage. What makes this partic-
ularly worrisome is that stressing LEDs or other optoelectronic and electronic components may introduce latent defects that enable the PCB to pass the factory’s quality inspection, but go on to fail prematurely when in use. This may seem very complicated, but with
today’s profiling software it is straightforward. The calculations are made in just a fraction a sec-
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