The Basics of Quick Turn PCB Assembly Process
The assembly of printed circuit boards is a multi-step process. It begins with the manufacturing of the substrate, followed by drilling and plating holes on the substrate surface, and then printing the circuit pattern on the substrate. After these first 3 basic steps, the contact fingers are attached then fused using a tin-lead coating. The panels are then sealed, stenciled and cut before the components are mounted on the PCB.
PCB assembly- Preparing the Substrate
The substrate refers to the baseboard through which the rest of the PCB will be made. Substrates are often made from woven glass fiber. The glass fiber is impregnated with epoxy resin. The substrate is either sprayed or dipped in a vat of resin.
After impregnation, the glass fiber is passed through rollers to flatten it into the desired thickness. Excess resin is also removed as the substrate passes through the rollers.
The substrate is placed in an oven to semi-cure. After which, the substrate is cut into large panels. The panels are then stacked in layers. In between each glass fiber layer is a layer of copper foil with adhesive backings. The stacked panels are placed in presses under high temperature (340˚F/170˚C) and pressure (1500 psi) for at least about an hour. The resin is fully cured in this process and the copper adhesives are tightly bound to the substrate.
PCB assembly- Drilling, plating, printing
Several substrate panels are piled and pinned together for stability. The panel is then laid out to sketch a pattern. The panel stacks are placed inside a CNC machine where designated holes are to be drilled. After drilling, the holes are de-burred in order to remove any excess from the edges of the holes. Copper plating is placed on the inside surface of the holes, which will serve as conductive circuits. The non-conductive surfaces are plugged to prevent them from getting plated.
The circuit pattern is printed on the substrate either through a substractive or through additive process. In the subtractive process, the entire substrate surface is plated with copper. The pattern is then etched on the copper plating, removing copper on areas that are not part of the circuit design.
In the additive process, the surface is covered in foil, revealing only the areas where the copper plating is supposed to go in. The copper plating is only placed in the desired pattern and the rest of the substrate surface is left unplated.
PCB assembly- contact fingers
Contact fingers are responsible for the conduction in between the layers of glass fiber substrates. These are fastened to the substrate edge and connected to the printed circuit. These are then masked off for plating, using three kinds of metals, namely, tin-lead, nickel, and then gold.
The entire PCB surface with the circuit pattern printed with copper is coated with tin-lead. This coating material is very porous and easily oxidized. To protect the PCB from oxidation (rusting), the entire circuit board is passed through a hot oil bath or a “reflow” oven. The tin-lead coating is melted and allowed to reflow on the shiny surface of the PCB.
PCB assembly- Seal, Stencil and Cut
The panels are then sealed with epoxy. This will protect the printed circuit patterns from damage when the PCB components are being attached. Then, other instructions and markings are stenciled on the circuit boards.
After which, the panels are cut into the desired size of individual boards. After cutting, the edges are smoothed out.
PCB assembly- Mounting
Electronic components of the PCB are then mounted as the boards are passed through several machines. The panels are passed through an automatic solder paster if surface mount technology is used to mount the components.
After all the components are placed, the boards are packaged for storage and shipping.