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BLT in Solder Joints

  • 2017 College Interns
  • 2017 College Interns
  • Yes, BLT can stand for a delicious bacon, lettuce, and tomato sandwich. But, with regard to my work at Indium Corporation, it has a very different meaning.

    The bond line thickness (BLT) of a solder joint refers to the height or thickness of the solder joint, post reflow. Decreasing this distance is the secondary objective of my overarching project. By decreasing the BLT we are using less solder paste, allowing us to complete more joints with the same amount of paste. But, the main reason for changing the BLT is its effect on the joint’s electrical resistivity. So, by adjusting the thickness of the joint, we can change its ability to restrict or allow the flow of an electrical current. Mainly, we seek to decrease the BLT to lower the solder joint’s electrical resistance, thus improving performance.

    The easiest and most effective way of decreasing the BLT, is to decrease the volume of paste dispensed or printed prior to reflow. Ah, if only it were that simple. The problem is the volume of paste used affects basically every single aspect of everything. Outgassing, wetting, voiding, joint strength; volume impacts all of these in some way or another. The trick is to find the perfect medium where the BLT is as small as possible, without compromising other aspects of the solder joint.

    Thanks for reading,