PCB Manufacturing Process
- By : David N. Ford
Printed circuit board processing and assembly are done in an extremely clean environment where the air and components can be kept free of contamination. Most electronic manufacturers have their own proprietary processes, but the following steps might typically be used to make a two-sided printed circuit board.
Making the substrate
1 Woven glass fiber is unwound from a roll and fed through a process station where it is impregnated with epoxy resin either by dipping or spraying. The impregnated glass fiber then passes through rollers which roll the material to the desired thick-ness for the finished substrate and also remove any excess resin.
2 The substrate material passes through an oven where it is semicured. After the oven, the material is cut into large panels.
3 The panels are stacked in layers, alternating with layers of adhesive-backed copper foil. The stacks are placed in a press where they are subjected to temperatures of about 340°F (170°C) and pressures of 1500 psi for an hour or more. This fully cures the resin and tightly bonds the copper foil to the surface of the substrate material.
Drilling and plating the holes
4 Several panels of substrate, each large enough to make several printed circuit boards, are stacked on top of each other and pinned together to keep them from moving. The stacked panels are placed in a CNC machine, and the holes are drilled according to the pattern determined when the boards were laid out. The holes are deburred to remove any excess material clinging to the edges of the holes.
5 The inside surfaces of the holes designed to provide a conductive circuit from one side of the board to the other are plated with copper. Non-conducting holes are plugged to keep them from being plated
Creating the printed circuit pattern on the substrate
The printed circuit pattern may be created by an "additive" process or a "subtractive" process. In the additive process, copper is plated, or added, onto the surface of the substrate in the desired pattern, leaving the rest of the surface unplated. In the subtractive process, the entire surface of the substrate is first plated, and then the areas that are not part of the desired pattern are etched away, or subtracted. We shall describe the additive process.
6 The foil surface of the substrate is degreased. The panels pass through a vacuum chamber where a layer of positive photoresist material is pressed firmly onto the entire surface of the foil. A positive photoresist material is a polymerthat has the property of becoming more soluble when exposed to ultraviolet light. The vacuum ensures that no air bubbles are trapped between the foil and the photoresist. The printed circuit pattern mask is laid on top of the photoresist and the panels are exposed to an intense ultraviolet light. Because the mask is clear in the areas of the printed circuit pattern, the photoresist in those areas is irradiated and becomes very soluble.
7 The mask is removed, and the surface of the panels is sprayed with an alkaline developer that dissolves the irradiated photoresist in the areas of the printed circuit pattern, leaving the copper foil exposed on the surface of the substrate.
8 The panels are then electroplated with copper. The foil on the surface of the substrate acts as the cathode in this process, and the copper is plated in the exposed foil areas to a thickness of about 0.001-0.002 inches (0.025-0.050 mm). The areas still covered with photoresist cannot act as a cathode and are not plated. Tin-lead or another protective coating is plated on top of the copper plating to prevent the copper from oxidizing and as a resist for the next manufacturing step.
9 The photoresist is stripped from the boards with a solvent to expose the substrate's copper foil between the plated printed circuit pattern. The boards are sprayed with an acid solution which eats away the copper foil. The copper plating on the printed circuit pattern is protected by the tin-lead coating and is unaffected by the acid.
Attaching the contact fingers
10 The contact fingers are attached to the edge of the substrate to connect with the printed circuit. The contact fingers are masked off from the rest of the board and then plated. Plating is done with three metals: first tin-lead, next nickel, then gold.
Fusing the tin-lead coating
11 The tin-lead coating on the surface of the copper printed circuit pattern is very porous and is easily oxidized. To protect it, the panels are passed through a "reflow" oven or hot oil bath which causes the tin-lead to melt, or reflow, into a shiny surface.
Sealing, stenciling, and cutting the panels
12 Each panel is sealed with epoxy to protect the circuits from being damaged while components are being attached. Instructions and other markings are stenciled onto the boards.
13 The panels are then cut into individual boards and the edges are smoothed.
Mounting the components
14 Individual boards pass through several machines which place the electronic components in their proper location in the circuit. If surface mount technology is going to be used to mount the components, the boards first pass through an automatic solder paster, which places a dab of solder paste at each component contact point. Very small components may be placed by a "chip shooter" which rapidly places, or shoots, the components onto the board. Larger components may be robotically placed. Some components may be too large or odd-sized for robotic placement and must be manually placed and soldered later.
15 The components are then soldered to the circuits. With surface mount technology, the soldering is done by passing the boards through another reflow process, which causes the solder paste to melt and make the connection.
16 The flux residue from the solder is cleaned with water or solvents depending on the type of solder used.
17 Unless the printed circuit boards are going to be used immediately, they are individually packaged in protective plastic bags for storage or shipping.
This post originally appeared on Made How