Posts Tagged Reflow Soldering
Heat Flow Happens
Posted by Paul Austen in M.O.L.E. MAP, OvenCHECKER, OvenRIDER, Profiling, Reflow Oven Verification, Reflow Profiling, Thermal Musings, Thermal Profiling on May 24th, 2011
An often misunderstood concept is heat flow and how it can influence the temperature of the product being heated so here is Wikipedia’s definition of heat flow, followed by a discussion of our own on the subject.
Energy needs a conductor in order to flow
1) An energy difference between two objects.
and
2) There is a conductor to act as a bridge enabling the energy to flow.
Energy always flows through a conductor from an object of high energy to an object of low energy. In this illustration, the high-energy object is a moving hammer, the low energy object is the table and the conductor is a block sitting on the table.
When you hit the block with the hammer, the energy contained in the moving hammer is transferred to the block when it hits. Some is also conducted through the block and transferred to the table it is sitting on. However, because the block is not a perfect conductor, which is true for most things, some of the energy stays in block. That energy bounces between the molecules of the block like balls on a pool table.
Because the molecules rub up against each other, and there is friction between them, some of the moving energy of the hammer is converted to heat energy, which causes a rise in the block’s temperature. It all comes down to molecular motion in an imperfect conductor creating friction that raises its temperature. Therefore, temperature increase is a way of observing energy flow, and energy flow that causes a temperature rise is called heat flow. Read the rest of this entry »
The Cooling Zone
Posted by Paul Austen in Profiling, Reflow Profiling, Thermal Profiling, Wave Solder Profiling on November 18th, 2010
A sometimes forgotten fact about reflow and wave soldering is that anywhere from 25% to 50% of the time a solder joint spends above its melting temperature, aka: time above liquidus (TAL), takes place in the “cooling zone”. Much time is spent getting the heating portion of the oven recipe finely tuned to produce a robust thermal profile, only to toss the product, covered with liquefied solder into a cooling zone where the solder joints must return to a solid state. The rate at which this occurs (cooling slope) is even more critical using lead- free solders. Giving the cooling zone some well deserved attention when defining the requirements of the thermal profile is essential to a good Thermal Quality Management program for your soldering process.
The cooling zone is where the quality of the solder joint is defined, with the cooling slope influencing the joint strength, and overall longevity. These two qualities are often at odds with each other because strength often comes from slower cooling rates, while longevity results from faster rates. Different cooling slopes have been tested to try to find what rate produces the best combination of strength and long life when subjected to accelerated thermal-cycling. These studies have concluded that slow cooling rates (1 to 2 °C/sec) allow too much time for intermetallic alloy growth, a strong but often brittle alloy prone to cracking when stressed. Faster cooling (5 to 7 ºC/sec) can form a softer solder joint with less overall strength, not to mention possible component damage. Cooling slopes between 3 and 4 °C/sec were found to be the best at producing a solder joint with both good strength and overall longevity.
So… don’t forget the cooling zone when developing the best thermal profile for your solder process.
References:
“Cooling Rates in Lead-free and Tin/lead Reflow”
SMT Magazine
by Denis Barbini, PhD.; Ursula Marquez
“Accelerated Thermal Fatigue of Lead-Free Solder Joints as a Function of Reflow Cooling Rate” Journal of Electronic Materials
by Qi Y; Zbrzezny A R; Agia M; Lam R; Et al
“Proceedings of 2005 International Conference on…” Asian Green Electronics
by Qiang Hu; Zhong-suo Lee; Zhi-li Zhao; Da-le Lee
On Reflow Soldering
Posted by Paul Austen in Reflow Profiling, Thermal Profiling on May 3rd, 2010
You know, at ECD we have been in the thermal profiling business for over 25 years. Most of what we profile is the reflow soldering process. There are many others like wave soldering, baking, drying, curing, and a host of other industrial temperature process. Still, reflow soldering is the most popular use for a thermal profiler like the MOLE. And yet, most of what can be found on the subject of reflow soldering, at least on the web, focuses on specific portions of the reflow process and not on the entire process as a whole.
For example, component manufacturers would have you avoid certain limits in temperature or temperature change rate (slope) to avoid damaging their parts. And that’s ok, but that only tells you what to avoid, temperature wise. Most solder paste manufacturers would have you believe that their paste can take most any reflow thermal process so as not to be excluded from purchase. This too is understandable and in reality, most solder pastes are good and will solder your components to your circuit board. Many standards (like IPC standards) on the subject suggest what your product MUST withstand to be considered robust and not necessarily an ideal reflow process thermal profile. This makes sense, because there is no one reflow thermal profile that will solder every possible circuit board assembly, and standards must be general in their application. Then there are the public websites that are often peppered with bias toward a specific brand of profiler in their description of what’s important or how to view it.
Each year new talent enters the work force and training in the art of reflow soldering is limited or costly. Worse yet, some learning about the reflow process only occurs from failures caused by incorrect reflow process settings. And perhaps worst of all, many reflow solder machine are still running the same setting set generations ago because no one currently available has the skill to make them better. Just because many industry veterans understand the issues around reflow soldering and thermal profiling does not mean the new talent can hit the road running. And, since most every electronic assembly will pass through either reflow soldering, wave soldering, or sometimes both, I thought it important to take a look at the reflow solder process, dissect it and consider what’s important to measure and control. Click here for the more in-depth look.
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