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Use the Coffin-Manson Equation to Calculate Number of Thermal Cycles

Category:
  • Solder Reliability
  • Folks,

    Let's look in on Patty and friends....

    Patty, Rob, and Pete were headed to their regular monthly meeting where they, along with the Professor, discussed a book they were all reading.  This month’s book was about General Leslie R Groves.

    “This was one of the most interesting books we have read,” Pete said starting the meeting.  “I think most people are aware of the technical genius of the scientists involved in the Manhattan Project, such as J. Robert Oppenhiemer and Richard Feynman, but few appreciate the contributions of General Groves,” Pete continued.

    “I agree,” Rob said.  “Without Grove’s orchestrating of the overwhelming number of small and large details of the program, it would have taken three times as long,” he went on.

    “Right!" the Professor chimed in. "He set up a $20 billion enterprise to produce the components of the bomb in less than 3 years.  Who else could have done that?”

    “One of the things that I found almost comical was that he was so good at the secrecy of the project that his family had no idea he was working on the bomb until it came out in the newspapers,” Patty exclaimed.

    The four book club members chatted about the book for about 20 more minutes.  Patty felt her cellphone vibrate.  It was a text from Mike Madigan.

    “Rob, Pete, it looks like we may have another assignment from Mike. He wants us to call, so let’s go to my office,” Patty suggested.

    Even though the three of them were all at the engineering school at Ivy U, Mike Madigan, the CEO of ACME, established a blank contract with them to do part-time consulting.  Part-time consulting is quite a common thing in the academic world as it helps the profs and technical staff keep current and also earn a little money.

    Patty called Mike's number and activated the speakerphone.

    “We have a customer who we assemble TVs for.  Each TV goes through 10,000 on/off cycles in its field life.  The temperature change from these on/off cycles is from 20°C to 50°C.  We are performing thermal cycle testing of the PCBs from 0°C to 100°C.  How many thermal cycles will we need to perform to equal the 10,000 field cycles?” Madigan asked.

    Patty chuckled to herself as she had just solved a problem like this for a reliability workshop that she was developing. So, the technique was fresh in her mind.

    “You need to use the Coffin-Manson equation,” Patty explained.

    “Whoa!” Mike chuckled, “Is the problem so serious that we need to worry about coffins?”

    “Coffin-Manson is used to relate strain to temperature changes. It will help us to calculate the right number of cycles,” Rob chimed in.

    Rob, Patty, and Pete all got calculators out to see who could get the answer first.  Pete won the contest.

    “I get an acceleration factor (AF) of 25,” Pete announced victoriously.

    “Agreed,” Patty and Rob sighed in unison.

    “The equation is quite simple," Patty shared.  See the figure below.

    "The Coffin-Manson acceleration factor for lead-free solder, m, is about 2.7," Patty finished.

    “So, you need to perform about 400 (10,000/25) cycles in the test chamber,” Pete said.

    “Wow! I’m really relieved,” Mike said, “I thought it might take 2,500 thermal cycles or more.”

    “There is no way we had enough time for that number of cycles, but 400 is easily doable,” Mike concluded as he sighed a breath of relief.

    The four of them chatted for a while more and then went their ways after having mastered another electronics assembly problem..

    Cheers,

    Dr. Ron