Spectrum Assembly, Inc. is experienced in placing components on wide range of substrate materials, including flex circuit assemblies. At SAI, medical and consumer products tend to be the primary users of flex circuit technology.
While SAI’s existing equipment easily accommodates flex circuit placement, strong focus must be placed on stabilizing the substrate during handling activities. Fixturing is an integral part of building high quality flex circuits. SAI utilizes 3D Solidworks, CAD or Gerber data to design carriers able to provide a rigid surface as the flex circuits go through stencil printing, pick and place and reflow. If an aqueous wash is required, a carrier must be designed for that as well. The goal is to utilize tooling holes in the fixtures to keep the substrate flat and rigid as it moves through the processes.
Understanding the end configuration, is critical to good flex circuit design. When a PCB layout designer lays out a rigid printed circuit board assembly (PCBA) it is fairly easy to understand the areas where design rules need to be followed in terms of clearances. With flex circuits, the designer needs to think three dimensionally in terms of locating points of interconnect, since the flex circuits are folded up and put in tight enclosures. If a point of interconnect is too close to a fold, it can create stress on the solder joint and result in a point of failure. Size of components may also create a design constraint. For example, a product with a long connector may be better designed as a rigid flex assembly than as a 100 percent flex assembly.
Equipment choices also matter. When rigid flex combinations or stiffeners are in use, there are two different heights which makes it impossible to screen print without specialized fixturing. Paste jet printers like the MY500 SAI uses are more adjustable and do not have that constraint because it allows us to tailor the amount of solder on individual components without the use of a stencil.
SAI’s Heller reflow ovens with KIC smart oven technology automatically verify in real-time that each assembly is processed in spec. This is critical with flex circuits because the carriers have a thermal mass impact and concomitantly increase the complexity of the reflow process.
The team at SAI also takes extra care in handling during secondary operations, since any flexing of the material puts stress on solder joints, particularly in the case of ceramic capacitors or other large ICs.