What is Double-sided PCB Manufacturing?
The double-sided PCB (or 2-layer PCB) is the printed circuit board with copper coated on both sides, top and bottom. There is an insulating layer in the middle. To use circuits on both sides, there must be a proper circuit connection between the two sides. The “bridges” between such circuits are call vias. A via is a small hole on the PCB board coated with metal, which can be connected with circuits on both sides.
Figure 1. Double-sided PCB
Planning and Preproduction
Before manufacturing, the manufacturer reviews the CAD data and other information (films, mechanical drawing, and specifications).
- Number of boards per mother panel
- Decide the panel size for the most economical reason.
- Features and information to be added during panelizations. Such as UL symbols, test coupons, layer numbers, and borders are selected at this time
- Layer materials
- Drilled hole sizes
- Tooling holes or target locations
Double-sided PCB manufacturing process
The following section describes the steps involved in producing a double-sided board with solder mask over bare copper (SMOBC), plated through-holes (PTH), and gold-plated contacts and the component legend.
- Preparing Material
Using the information on the traveler- including the numbers and sizes of the panels, as well as any special instructions, the manufacturer prepares the materials necessary to process the order. PCBs start with copper-clad epoxy glass as the raw material. There are a lot of materials used in PCB manufacturing for users and PCB manufacturers to choose from. Different brands and materials have different characteristics, and different materials also provide different benefits, such as FR4, a ceramic substrate, iron substrate, aluminum substrate, etc.
Fr-4, one of the flame retardant materials widely used in PCB base substrates. FR4 board is economical and affordable and can maintain the stability and safety of the PCB board under extreme temperature conditions.
However, FR4 is not suitable for high-frequency and high-speed PCBs. At this time, we need to choose high-frequency materials, such as Rogers’ RO4000 series, RT5000/6000 series, Tacanic’s TLX series, and so on. Aluminum, metal, or copper as the substrate for LED PCB or aluminum PCB are used in the LED lighting industry.
- Cutting of CCL (Copper Clad Laminate)
The next step is to cut the board according to the requirement. The raw PCB board is quite large. There are various sizes available, such as 37 x 49 inches, 41 x 49 inches, and 43 x 49 inches. Therefore, it is cut in the required sizes that can be used in the machines. The board size obtained after cutting is not according to your circuit size; it is much larger. Your PCB size could be small, so multiple circuits on the board can make the process economical.
The circuit board goes to an automatic drilling machine that creates holes in the board quickly. The machine changes the drill bits on its own; everything is automated.
As drilling processes improves, burr-free holes can be produced. But most manufacturers process drilled panels through a deburring machine. The panels pass through brushes or abrasive wheels what mechanically remove any copper burns at he rims of the holes. Deburring also removes any fingerprints and oxides to create a smooth, shiny surface.
- Electroless Copper Deposition (Plating Through Holes, PTH)
Electroless deposition of Cu through the holes as holes are composed of epoxy initially. After Cu deposition, panel is dipped in acid dip and anti-tarnish solution to prevent against oxidation. It is of two types- horizontal and vertical. Horizontal PTH is for carbon deposition and vertical PTH is for Cu deposition. Electroless copper is one of the most important steps in double-sided PCB and multilayer PCB manufacturing processes. Because all PCBs with 2 or more layers use plated through holes to connect the conductors between the layers.
Figure 2. Electroless Copper Deposition (Plating Through Holes, PTH)
- Photo Imaging
In photo imaging, a negative image circuitry pattern is transferred to the PCB panel. First, the panel is covered with a layer of photoresist. The most common photoresist material is dry film plating resist is an ultraviolet (UV) light sensitive photo polymer. It’s supplied on a roll and applied by processing the panel through heated rollers on a hot roll laminator. Once the film is applied, the board is ready to be exposed to UV light for circuit printing.
The whole process is carried out in a room where there are only yellow lights. It is because photo-resistive films are sensitive to other lights. The film that has the circuit design is applied over the board; it is applied on both sides. Then, the board passes through a UV light chamber. When the board is exposed to UV light, the circuit part is hardened, while the excessive part remains the same.
- Pattern Plating
First the panels are clamped in plating racks and immersed in a series of chemical bath that clean the copper pattern that makes up the circuitry. Next, the panels are immersed in a copper plating solution. The solution and panels have opposite electrolytical charges. These opposite polarities cause copper ions to migrate to the un coated copper areas on the panel, depositing the desired thickness of copper on the plates surface and in the holes. After copper plating the panels are moved from bath to bath. The circuitry pattern is covered with extra copper, is further electroplated with tin or tin/lead solder.
- Developing and Etching
The panels are placed in a tank or spray machine to remove the imaging material. Theis step is also called resist stripping. After the resist is stripped off, the panels are placed in the conveyorized spray etcher or batch tank, where a chemical etchant (an ammonia-based compound) removes the uncovered copper but doesn’t’ attack the tin or tin/lead plating, which protects the copper underneath. The tin or tin/lead plating is called the etch resist. Then the tin or tin/lead is chemically stripped from the copper, revealing the copper circuitry pattern.
- Solder Masking
Green, white, blue, and other colors of solder mask on the circuit is a thin layer of polymer that works as an insulator between two conducting lines. It prevents the formation of short circuits. The mask is applied all over the board, and then it is dried. Remove the excess solder mask that is over the circuit. A film that contains circuit patterns is applied over the board. Then the board goes through a UV chamber. The solder other than the circuit is hardened while the solder mask over the circuit remains the same. Finally, the solder mask over the circuit is cleaned.
- Surface Finishing
The copper on the board can undergo oxidation. It cannot last for a long time. Therefore, it is necessary to apply a surface finish over copper to protect it from oxidation. There are many types of surface finishes available, and customers can pick according to their needs. You can choose HASL, OSP, ENIG, ENEG, ENEPIG, Immersion tin, Immersion silver, etc.
- Gold and Nickel Plating
Other plating finishes are used, most commonly gold. However, copper and gold tend to undergo solid state diffusion into each other (with copper doing so at a faster rate); the process is accelerated by increased temperature. Copper on a trace surface oxidizes, resulting in increased contact resistance (copper migrating into the gold can cause the gold to tarnish and corrode). This can be minimized by plating a barrier layer between the copper and gold. Nickel is commonly used as a barrier layer to prevent the gold migrating into the copper on the tracks. (The nickel barrier helps to reduce both the number and the effect of pores compared with plating gold directly over the copper base.) The nickel protective coating provides several benefits. It serves as a backing to the gold for extra hardness as well as providing an effective diffusion barrier layer between gold and copper. The nickel/gold provides a finish that is heat and corrosion resistant, environmentally stable, wire solderable and durable (the nickel underplate enhances the wear characteristics of gold) albeit at a higher cost than simple solder finishes. Traditionally, nickel/gold plating has been applied over copper tracks used for keyboard contacts or edge fingers to provide the conductive, corrosion resistant coating. This approach provides benefits for soldering,
- Applying the Component Legend
The labels on the PCB are called silkscreens. These can be used to mark components and insert the logo. In this step, the PCB board enters into a giant printer that prints the labels on the board. Silkscreens are available in various colors, such as red, blue, yellow, and black, but the standard color is white.
- Separation or Cutting
a cutting machine cuts the circuits and makes them separate pieces.
- Electrical Testing
For this purpose, the Flying Probe Test is used. It is a simple test in which there are multiple probes. The probes are placed over the connections, and the current is passed through them. It checks whether the circuit is working as expected or not. For instance, if there is no connection between two paths, then the current should not pass if the probes are connected to them.
Figure 3. Double-sided PCB Manufacturing Process
Read, also: How to design PCBs?