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How SMT Assembly Techniques Vary For Flexible Circuit Boards

SMT Assembly Techniques Vary For Flexible Circuit Boards

Most electronic devices, from medical monitoring devices to formfitting biometric and tracking patches, are outfitted with a printed circuit board (PCB). A PCB is a laminated structure of conductive insulated layers that provide two functions: hosting electrical components in designated areas on the outermost layer and providing reliable connections between those components through copper traces and planes. While many of the same assembly methods are used on traditional PCBs, there are several considerations that must be taken into account when assembling flexible circuit boards.

Surface-mount technology (SMT) is the most common assembly technique for flex and rigid-flex PCBs. Rather than inserting component leads into plated through holes, SMT utilizes surface mount pcb pads and solder paste to connect the components to the underlying layer. The component lead is contacted to the surface of the pad via solder paste that is screen-printed using metal stencils, usually of a thickness of 100 mm or less. The stencil is then etched to expose the copper and a coating is applied to inhibit oxidation of the pad. A solder paste containing tin, silver, gold, palladium or a combination of these and other elements is then screened through the stencil to fill the pads with solder.

Once the solder has melted, it is then reflow-soldered to the pads in the same manner as other PCB assemblies are reflow-soldered with batch or wave soldering. However, it is important to note that the reflow process on a flex or rigid-flex PCB must be controlled carefully to avoid stress in the material that can cause brittle cracks and failure.

How SMT Assembly Techniques Vary For Flexible Circuit Boards

A key difference between a rigid PCB and a flex or rigid-flex is the use of stiffeners. Stiffeners are added to the substrate to add structural integrity to the flex or rigid-flex PCB and reduce sagging or cracking. Stiffeners are typically made of a material such as polyimide or PET (polyethylene phthalate), but can also be made from FR4, PEN, PTFE or Aramid. The stiffeners are bonded to the substrate using thermally-cured acrylic or pressure-sensitive adhesives.

When designing a flex or rigid-flex PCB, it is important to understand that the layout rules are different for this type of assembly. Traces should be routed in the flex or rigid-flex area perpendicular to the direction of movement, and it is best to keep them away from the edges of the flex or rigid-flex area. In addition, vias should be designed to be larger than the diameter of a pin and should be placed away from areas that will bend. Adding annular rings to the vias will help reduce stress concentration points and can be helpful in reducing peeling of the flex circuit.

Also, it is recommended to use a tin/lead/gold/palladium/nickel/aluminum finish on the copper pads to improve solderability and corrosion resistance. This is especially important for flex and rigid-flex PCBs that will be exposed to harsh environmental conditions. Finally, a careful evaluation of the assembly method for a flex or rigid-flex PCB should be conducted during the design phase to ensure that the selected assembly technique will not cause any issues with reliability and performance.

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