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16 SMT Magazine • March 2017 ture of the circuit. For instance, if a component is inserted or placed with too much pressure, the material can wrinkle and may create a void under the component that could allow foreign material to gather. Another concern during hand assembly is how to keep the circuit in a static condition so that it doesn't move when the solder or iron is placed on the component. It's difficult enough to solder components manually, but that diffi- culty is magnified when the circuit is not stat- ic. Trying to create a compliant solder joint on a moving target is the highest level of frustration. Therefore, it is common to create fixtures to assist in the hand assembly of flexible circuits. The fixture is used to keep the circuit flat and still during the process. The fixture is an invalu- able aid to the assembler. Automated Soldering Process The automated soldering process is typically done on multiple circuits designed in a matrix to form a panel. This panel can then have solder applied, go into a machine that will load all the components to their proper locations/orienta- tions and finally run through an oven to solder the entire panel. This is much easier to describe than to put into practice. The complications are many when flexible circuits are the point of discussion. The screen- ing process and component placement puts a great deal of pressure on the circuit that re- quires tooling to be designed specifically for each circuit. A typical way to fixture a flexible panel is to create a "carrier" that is used to car- ry the circuit through the entire process. It pro- vides a stable surface that assists in consistent assembly and eliminates much of the laborious tedium associated with the assembly of flexible circuits. With the use of carriers, the flexible cir- cuit panel can run through the entire process with little or no issues. Flexible Circuits and Components Every type of component can be soldered to flexible circuits with confidence. Through-hole components, SMT components, wires, switches, BGAs, etc. Some require more skill than others to be attached, but they all can be mounted re- liably to flexible circuits. Some may be soldered automatically like through-hole or SMT compo- nents and others may have to be attached man- ually like wires or cables. The use of a micro- scope is necessary in assembly today. The com- ponents get smaller each year and we are now in an era where a component that measures .020" by .010" is common. That's not much big- ger than a flake of black pepper. Most manual assembly and inspection, therefore, is done un- der a microscope or Automated Optical Inspec- Figure 3: Carrier panel. Figure 4: Surface mount components and through-hole components. ASSEMBLY OF FLEXIBLE CIRCUITS