PCB basic manufacturing process

writer: G February 13, 2019

1. Image (forming / wire making)

The first step in the production is to establish the wiring between the parts. We use a subtractive transfer method to present the working film on a metal conductor. The trick is to lay a thin layer of copper on the entire surface and remove the excess. Additive Pattern Transfer is another way of using less people. This is a way to apply copper wire only where needed.

If you are making a double-panel, the PCB will be covered with copper foil on both sides of the substrate. If you are making a multi-layer board, the next step will stick the boards together.

Positive photoresist is made of sensitizer, which dissolves under illumination (negative photoresist is decomposed if not illuminated). There are many ways to handle photoresist on copper surfaces, but the most common way is to heat it and roll it on the surface containing the photoresist (called a dry film photoresist). It can also be sprayed on the head in a liquid state, but the dry film type provides a higher resolution and can also make a thinner wire.

The hood is just a template for the PCB layer in the manufacturing process. Before the photoresist on the PCB is exposed to UV light, the hood overlying it prevents some areas of the photoresist from being exposed (assuming a positive photoresist). These places covered by photoresist will become wiring.

Other bare copper portions to be etched after photoresist development. The etching process can immerse the board in an etching solvent or spray the solvent onto the board. Commonly used as an etching solvent, ferric chloride, alkaline ammonia (Alkaline Ammonia), sulfuric acid plus hydrogen peroxide (Sulfuric Acid + Hydrogen Peroxide), and copper chloride (Cupric Chloride). The remaining photoresist is removed after the etching is completed. This is called a stripping process.

2. Drilling and plating

If a multilayer PCB board is fabricated and contains buried or blind vias, each layer must be drilled and plated prior to bonding. If you don't go through this step, there is no way to connect to each other.

After drilling the machine equipment according to the drilling requirements, the hole must be plated (Plated-Through-Hole technology, PTH). After metal treatment inside the hole, the internal layers can be connected to each other. Before starting the plating, the debris in the hole must be removed. This is because the resin epoxy will have some chemical changes after heating, and it will cover the inner PCB layer, so it must be removed first. Both the removal and plating operations are done in a chemical process.

3. Multilayer PCB press

Each monolithic layer must be laminated to produce a multilayer board. The pressing action includes adding an insulating layer between the layers, and sticking each other. If there are several layers of vias, then each layer must be processed repeatedly. The wiring on the outer sides of the multilayer board is usually processed after the multilayer board is pressed.

4. Treatment of solder mask, screen printing surface and gold finger part plating

Next, the solder resist is overlaid on the outermost wiring so that the wiring does not touch the plating part. The screen printing surface is printed on it to indicate the position of each part. It cannot cover any wiring or gold fingers, otherwise it may reduce the solderability or the stability of the current connection. The gold finger portion is usually plated with gold so that a high quality current connection is ensured when inserted into the expansion slot.

5. Testing

Test the PCB for shorts or open circuits and test it optically or electronically. Optical scanning is used to find defects in each layer, and electronic testing is usually done with a Flying-Probe to check all connections. Electronic tests are more accurate in finding short circuits or open circuits, but optical tests can more easily detect problems with incorrect gaps between conductors.

6. Parts installation and welding

The last step is to install and solder the parts. Both THT and SMT parts are machined to be placed on the PCB.

THT parts are usually soldered in a way called Wave Soldering. This allows all parts to be soldered to the PCB at a time. First cut the pin close to the board and bend it slightly to allow the part to be fixed. The PCB is then moved to the water flux of the co-solvent and the bottom is contacted with a co-solvent so that the oxide on the bottom metal can be removed. After heating the PCB, this time it is moved to the molten solder and the soldering is completed after contact with the bottom.

The way to automatically weld SMT parts is called Over Reflow Soldering. The paste solder containing the solvent and the solder is processed once after the parts are mounted on the PCB, and then processed after being heated by the PCB. After the PCB is cooled, the soldering is completed, and the next step is to prepare the final test of the PCB.


Method of saving manufacturing costs

In order to make the cost of the PCB as low as possible, there are many factors that must be considered:

1. The size of the board is naturally a key point. The smaller the board, the lower the cost. Part of the PCB size has become the standard, as long as the cost is reduced according to the size.

2. Using SMT will save money than THT, because the parts on the PCB will be more dense (and will be smaller).

On the other hand, if the parts on the board are dense, the wiring must be thinner and the equipment used should be relatively higher. At the same time, the materials used should be more advanced, and the wiring design must also be more careful, so as to avoid the problem that power consumption and the like may affect the circuit. The cost of these problems can be saved more than the reduction in PCB size.

3. The higher the number of layers, the higher the cost, but the PCB with a small number of layers usually causes an increase in size.

4. Drilling takes time, so the fewer the guide holes, the better.

5. Buried holes are more expensive than guide holes that run through all layers. Because the buried hole must be drilled before the joint.

6. The size of the hole in the board is determined by the diameter of the part pin. If there are different types of pin parts on the board, then because the machine can't use the same bit to drill all the holes, the relative time is spent, which also represents a relatively high manufacturing cost.

7. Electronic testing using flying probe detection is usually more expensive than optical. Generally optical testing is enough to ensure that there are no errors on the PCB.