PCBA Tech - PCB material, composition structure and process flow

One, PCB board material:

1, spray tin plate: because of the low cost, good solderability, good reliability, and the strongest compatibility, but this kind of spray tin plate with good soldering characteristics contains lead, so the lead-free process cannot be used.

2. Tin plate: This kind of substrate is easy to be polluted and scratched, and the process (FLUX) will cause oxidation and discoloration. Most domestic manufacturers do not use this process, and the cost is relatively high.

3. Gold plate: The biggest problem of this type of substrate is the "black pad" problem. Therefore, many large manufacturers do not agree to use the lead-free process, but most domestic manufacturers use this process.

4. Silver plate: Although "silver" itself has strong mobility, which leads to the occurrence of electric leakage, the current "immersion silver" is not pure metal silver in the past, but "organic silver" co-plated with organic materials. "Therefore, it has been able to meet the needs of future lead-free processes, and its solderability life span is longer than that of OSP boards.

5. OSP board: The OSP process has the lowest cost and is easy to operate. However, the popularity of this process is still not good because the assembly plant must modify the equipment and process conditions and the reworkability is poor. Using this type of board, after high temperature heating, The protective film pre-coated on the PAD will inevitably be damaged, which will result in a decrease in solderability, especially when the substrate is subjected to a secondary reflow. The situation is more serious. Therefore, if the process needs to go through a DIP process again, the DIP end will face Welding challenges.

6.Gold-plated plate: The cost of the gold-plated plate process is the highest among all plates, but at present, the most stable of all existing plates, and the most suitable for use in lead-free process plates, especially in some high-unit prices or high-reliability electronics Products are recommended to use this sheet as a substrate.

Second, the PCB board is mainly composed of the following:

1. Circuit and pattern (Pattern): The circuit is used as a tool for conduction between the originals. In the design, a large copper surface will be additionally designed as the ground and power layer. The circuit and the pattern are made at the same time.

2. Dielectric layer (Dielectric): Used to maintain the insulation between the circuit and each layer, commonly known as the substrate.

3. Hole (Throughhole/via): The through hole can make the lines of more than two levels connect to each other, the larger through hole is used as a part plug-in, and there are non-through holes (nPTH) usually used as the surface Placement and positioning, for fixing screws during assembly.

4.Solderresistant/SolderMask: Not all copper surfaces have to eat tin parts, so the non-tin area will be printed with a layer of material that isolates the copper surface from eating tin (usually epoxy resin), avoid There is a short circuit between the lines that do not eat tin. According to different processes, it is divided into green oil, red oil, and blue oil.

5. Silkscreen (Legend/Marking/Silkscreen): This is a non-essential composition. The main function is to mark the name and position frame of each part on the circuit board, which is convenient for maintenance and identification after assembly.

6. Surface Finish: Because the copper surface is easily oxidized in the general environment, it can not be tinned (poor soldering), so it will be protected on the copper surface that needs to be tinned. The method of protection is tin spraying ( HASL), chemical gold (ENIG), chemical silver (ImmersionSilver), chemical tin (ImmersionTin), organic soldering flux (OSP), each method has its advantages and disadvantages, collectively referred to as surface treatment.

Three, PCB board production process

1. Print the circuit board: Print out the drawn circuit board with transfer paper, pay attention to the slippery side facing yourself, generally print two circuit boards, that is, print two circuit boards on one piece of paper. The best printing effect is selected among them Good to make the circuit board.

2. Cut the copper clad laminate, and use the photosensitive board to make the whole process diagram of the circuit board. The copper clad laminate, that is, the circuit board with copper film on both sides, cut the copper clad laminate to the size of the circuit board, not to be too large to save materials.

3. Pretreatment of the copper clad laminate: Use fine sandpaper to polish off the oxide layer on the surface of the copper clad laminate to ensure that the carbon powder on the thermal transfer paper can be firmly printed on the copper clad laminate when the circuit board is transferred, and the polished standard The surface of the board is bright and there is no obvious stain.

4. Transfer circuit board: Cut the printed circuit board into a suitable size, and paste the printed circuit board side on the copper clad laminate. After alignment, put the copper clad laminate into the heat transfer machine. Be sure to put it in. The transfer paper is not misaligned. Generally speaking, after 2-3 transfers, the circuit board can be transferred firmly on the copper clad laminate. The thermal transfer machine has been preheated in advance, and the temperature is set at 160-200 degrees Celsius. Due to the high temperature, pay attention to safety during operation!

5. Corrosion circuit board, reflow soldering machine: first check whether the circuit board transfer is complete, if there are a few places that have not been transferred, you can use a black oil pen to repair. Then it can be corroded, and wait for the exposed copper film on the circuit board When it is completely corroded away, the circuit board is removed from the corrosive solution and cleaned, so that a circuit board is corroded. The composition of the corrosive solution is concentrated hydrochloric acid, concentrated hydrogen peroxide, and water, and the ratio is 1:2:3. When liquid, first drain the water, and then add concentrated hydrochloric acid, concentrated hydrogen peroxide. If concentrated hydrochloric acid, concentrated hydrogen peroxide or corrosive liquid accidentally splashes on the skin or clothing during operation, it should be cleaned with clean water in time. Because a strong corrosive solution is used, be sure to be careful!

6. Circuit board drilling: The circuit board is to insert electronic components, so it is necessary to drill the circuit board. Choose different drill pins according to the thickness of the electronic component pins. When using a drilling machine to drill, the circuit board must be To press firmly, the rig cannot be driven too slowly, please carefully watch the operator's operation.

7. Circuit board pretreatment: After drilling, use fine sandpaper to polish off the toner on the circuit board, and clean the circuit board with water. After the water is dry, apply rosin to the side with the circuit to speed up the rosin For solidification, we use a hot air blower to heat the circuit board, and the rosin can solidify in only 2-3 minutes.

8. Soldering electronic components: After soldering the electronic components on the board, turn on the power.