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PCB (Printed Circuit Board), the Chinese name is printed circuit board, also known as printed circuit board and printed circuit board. It is an important electronic component, a support for electronic components, and a provider of electrical connections for electronic components. Because it is made by electronic printing, it is called a "printed" circuit board. There are two types of PCB manufacturing methods: additive method and subtractive method.
Additive method: On a substrate without copper foil, a conductive material is selectively deposited to form a printed board PCB with a conductive pattern. There are silk screen electroplating method, pasting method and so on.
What are the manufacturing methods of PCB
Subtractive method: On the copper-clad board, through photochemical method, screen printing pattern transfer or electroplating pattern resist layer, and then etching off the non-patterned part of the copper foil or mechanically removing unnecessary parts to make a printed circuit board PCB. In the subtractive method, there are mainly two kinds of engraving method and etching method. The engraving method uses mechanical processing to remove unnecessary copper foil, and can quickly produce printed circuit board PCBs under single-piece trial production or amateur conditions; the etching method uses chemical corrosion methods to subtract the unnecessary copper foil, which is currently The main PCB manufacturing method. Below we mainly understand the etching method.
1 Pattern plating etch process
(1) The process takes the double panel as an example. The process is: blanking-drilling-PTH (electroless copper plating)-plate surface copper-photoimaging-graphic plating-alkaline etching-soldering mask+characters-hot air leveling ( HAL)-machining-electrical performance test (ETest)-FQA-finished product.
(2) The main point is to conduct selective plating on conductive patterns only. Board drilling, electroless copper plating, and light imaging to form conductive patterns. At this time, only the lines, holes and pads are patterned with copper electroplating, so that the average copper thickness in the holes is greater than or equal to 20μm, and then tin plating (tin plating is used as etching resistance Etching layer), remove the dry film (or wet film), and perform alkaline etching to obtain the required wire pattern. Remove the tin plating on the surface and in the hole, screen printing solder mask and characters, hot air leveling, machining, electrical performance testing, and get the required PCB.
(3) Features: Many processes, complex, but relatively reliable, and can be used to make fine lines. Most European, American and Chinese companies use this process for production.
2 Panel plaTIng etch process
(1) Process blanking-drilling-PTH (electroless copper plating)-copper plating on board surface-photoimaging-acid etching-solder mask+character-hot air leveling (HAL)-shape processing-electrical test (E-Test) )-FQA-finished product.
(2) Key points
1. After drilling the plate and electroless copper plating (PTH), electroplating the entire board surface and the hole to the required copper thickness, so that the average copper thickness in the hole is greater than or equal to 20μm.
2. Only cover the holes and patterns with dry film, and use dry film as a resist layer.
3. Excess copper is etched in an acid etching solution to obtain the required wire pattern.
(3) Features
1. The process is simpler than the pattern electroplating etching method, but the process control will be more difficult. Many Japanese companies use this process, and a small number of domestic companies use this method for mass production.
2. Difficulty: the uniformity of the copper plating layer thickness on the board surface is difficult to control. There is a phenomenon that the copper layer around the board is thick and the middle is thin, and the etching is difficult to be uniform, and it is difficult to produce thin lines.
3. Dry film cover holes, especially holes with large diameters. If they cannot be covered, the etching solution will enter the holes and the copper in the holes will be etched away, and the board can only be scrapped.
3 SMOBC method (solder mask over bare circuit)
Cover the bare copper circuit with solder resist, and then perform hot air leveling, or sink Ni/Au, or sink Ag, or sink Sn, or OSP (orgamic solderability reserraTIve, organic soldering protection film). The purpose is to not have solder (Pb-Sn, or metal layer Ni/ Au, Ag, Sn, OSP) on the wire, and only coat Pb-Sn (or Ni/ Au, Ag, Sn, OSP) on the holes and pads ).
The role of this process is to prevent the printed board from causing circuit bridging during assembly and welding; save metal costs; and obtain good adhesion for solder mask on the circuit. If lead-tin solder is used on the circuit, the solder mask of the circuit will become brittle during soldering.
SMOBC is actually a pattern plating etching method. This method has been used for 20 to 30 years. In the 1970s and 1980s, the bare copper circuit was coated with solder mask and then leveled with hot air. It is commonly known as tin spraying by the Cantonese. The spray is lead-tin solder, Pb: Sn is 37: 63 or 40: 60, this alloy has the lowest eutectic point, 183·C, and Pb: Sn is 40: 60 when the eutectic point is 190·C.
Hot air leveling (spray tin) process, the Pb-Sn on the pad is not flat enough, which makes it difficult to mount SMT, and the solderability of pure gold is excellent, and the wire patterns, holes, and pads are all plated with sickle/gold (Jin is used as the bottom layer). After etching, all but the holes and pads are coated with solder resist, leaving only the holes and pads as Ni/ Au instead of Pb-Sn. This is what Guangdong and Hong Kong people call Water gold plate craft. The gold layer is 24K pure gold, which can be welded and is very thin, only 0.05 ~ 0.10μm. It needs to be nickel-plated as a primer, 2~5μm thick, and then water-gold-plated. The gold content in the gold plating tank is not much, about Ig/L gold. It should be noted that this solderable thin gold is essentially different from the gold-plated layer on the printed board plug. The plug is gold-plated and plated with hard gold, which is wear-resistant and can be plugged and unplugged hundreds of times. The gold layer is required to have a certain hardness. The gold liquid contains trace amounts of cobalt (nickel, antimony) elements.
In addition, because the lead in the lead-tin alloy is toxic, the use of lead was banned in July 2006 in accordance with EU directives. So the surface coating of SMOBC process has changed to today's chemical immersion silver, immersion tin, immersion Ni/Au, OSP instead of Pb-Sn alloy. Invariably, these processes all belong to the pattern plating etching method SMOBC process in the final analysis.
The above are some methods for PCB factories to make PCB.
