Application engineers are a funny breed. They get their kicks from solving real-life manufacturing challenges – and the thornier the better! They like to get up close and personal with an application and help those having trouble to find the right way to weld, mark, cut, bond or machine a part. They also get a great deal of satisfaction from fixing a process that is taking too much time and affecting output (and the bottom line), one that results in unnecessary scrap or one that is out of limits. If the solution doesn’t work out the first time, they stick with it until they develop a one that works.
You heard it here first: there is no single materials processing technology that fits all applications. Manufacturers looking for a robust, production-ready solution must follow a rigorous process to determine the best choice of equipment. There are no short cuts or magic wands – you have to carefully review process feasibility and part design to maximize production reliability. The evaluation must also consider overall system needs.
Most PCB materials like FR2 and FR4 are very resilient to the application of heat during the hot bar reflow soldering (or hot bar bonding) process. But some materials - ceramic substrates in particular - need to be heated in a more controlled fashion to minimize the chance of cracking. Excessive differences in the heat sinking capability of the two parts being joined can also cause solder cracking during cooling. Heat sinking differentials along the solder joint length are the most common design problem to overcome. Small differences may not have much effect on quality, but any large thermal mass change along the joint area will cause inconsistency of temperature and result in a poor solder joint.
A few months ago, a customer came into our Monrovia lab intent on processing application samples using hot bar reflow soldering - the part was specifically developed with that technology in mind. Sr. Lab Technician Vickie Buckley, however, took one look at the part and decided that reflow soldering wasn't the optimum process: the wires were too small and would flatten out before the solder melted and reflowed creating a proper bond on the terminals below. All was not lost, however, her 20+ years of experience told her that the application could be done using thermocompression bonding; they moved over to that equipment and had immediate success! The customer went home content that he had a process that would work!
Flexible circuits are found everywhere: cell phones, tablets, flat screen tvs, cameras, rockets, satellites – anywhere electrical connections are required in three or more axes; where the assembly is required to flex during operation; where weight is an issue; and, as devices get smaller and smaller - where board thickness or space is a driving factor. They are most often connected using hot bar reflow soldering which utilizes a thermode heating element and allows for a more repeatable and consistent joint quality than traditional hand soldering, and are commonly made of two layers of polyimide encapsulating copper traces. Copper trace thickness generally ranges from 0.0007 – 0.004 inches, and thickness of the flex is between 0.001 – 0.0047 inches.