1. Introduction to PCB subtraction method
The subtractive process is a method of selectively removing part of the copper foil on the surface of the copper-clad laminate to obtain a conductive pattern. The subtractive method is the main method of printed circuit manufacturing today, and its advantages are that the process is mature, stable and reliable.
Printed circuits manufactured by the subtractive process can be divided into the following two categories.
1.1. Non-plating-through-hole Board
Such printed boards are produced by screen printing and then etching the printed boards, and can also be produced by photochemical methods. Non-perforated plated printed boards are mainly single-sided, and there are also a small number of double-sided boards, which are mainly used for televisions and radios. The following is the single-panel production process:
Single-sided copper clad laminate unloading-photochemical method/screen printing image transfer-removal of resist printing material-cleaning and drying-hole processing-shape processing-cleaning and drying-printing solder resist coating-curing-printing mark symbol- Curing-cleaning-drying-pre-coating flux-drying-finished product.
1.2. Plating-through-hole Board
On the copper-clad laminate that has been drilled, electroless plating and electroplating are used to make the holes between two or more layers of conductive patterns become electrical connections from electrical insulation. Such printed boards are called perforated plating. plate. Perforated plated printed boards are mainly used in computers, program-controlled switches, mobile phones, etc. According to the different electroplating methods, it is divided into graphic electroplating and full plate electroplating.
a. Pattern Plating: On the double-sided copper-clad laminate, a conductive pattern is formed by screen printing or photochemical methods, and lead-tin, tin-cerium, tin-nickel or gold are plated on the conductive pattern. Etch the metal, and then remove the resist other than the circuit pattern, which is etched. The pattern plating method is further divided into a pattern plating etching process (Pattern Plating Etching Process) and a bare copper clad solder mask process (Solder Mask On Bare Copper, SMOBC). The process flow of making a double-sided printed board with a bare copper clad solder mask process is as follows.
Double-sided copper clad laminates unloading, punching positioning holes, CNC drilling, inspection, dehairing, chemical thin copper plating, electroplating thin copper, inspection, brushing, filming (or screen printing), exposure, developing (or curing), Inspection and revision—pattern electroplating copper—pattern electroplating tin-lead alloy—film removal (or removal of printing material)—inspection and revision—etching—lead and tin removal—connection and disconnection test—cleaning—solder resist pattern—plug nickel/gold plating—plug sticker Tape-hot air leveling-cleaning-screen printing mark symbol-shape processing-cleaning-drying-inspection-packaging-finished product.
b. Full-board plating (Panel Plating) On the double-sided copper-clad laminate, electroplate copper to a specified thickness, and then perform image transfer by screen printing or photochemical methods to obtain a corrosion-resistant positive-phase circuit image, which is then removed after etching. The resist is made into a printed board.
The full plate electroplating method can be further subdivided into hole blocking method and masking method. The process flow of making a double-sided printed board with a masking method (Tenting) is as follows.
Double-sided copper clad laminate, unloading, drilling, one hole, metallization, plating, thickening, surface treatment, photomasking dry film, making positive phase conductor pattern, etching, removing film, plug plating, shape processing, Inspect a printed solder resist coating, a solder coating, hot air leveling, a printed marking symbol, and a finished product.
The advantages of the above method are that the process is simple and the thickness uniformity of the coating layer is good. The disadvantage is that it wastes energy, and it is difficult to manufacture through-hole printed boards without land.
2. Introduction of PCB addition method
2.1. Disadvantages and advantages of the full additive method
The full addition method refers to that after a circuit is printed on an insulating substrate containing a photosensitive catalyst without copper foil, a copper circuit pattern is plated on the substrate by the method of chemical copper plating to form a circuit with an electroless copper plating layer as the circuit. The printed board is called the additive method because the circuit is added to the printed board later.
The full-additive process is more suitable for making fine circuits, but because it has special requirements for the substrate and chemical immersion copper, and also has strict requirements on the bonding force between the copper plating and the substrate, it is quite different from the traditional PCB manufacturing process. The cost is high and the process is not mature, and the current output is not large.
The full additive method can be used to produce WB or FC flip chip substrates, and its process can reach below 10μm.
2.2. The rise of the semi-additive method to adapt to the needs of the times
The semi-additive method refers to covering the photoresist (D/F) on the substrate pre-plated with thin copper, exposed to ultraviolet light and then developed to expose the required places, and then use electroplating to make the copper thickness of the circuit board. Thicken to the required specification, then remove the photoresist, and then remove the excess copper layer under the photoresist by flash etching, and the remaining copper layer forms the required circuit.
The characteristic of the semi-additive method is that the formation of the circuit mainly depends on electroplating and flash etching. In the flash etching process, since the etched chemical copper layer is very thin, the etching time is very short, and the lateral etching of the line is relatively small. Compared with the subtractive method, the width of the line will not be affected by the thickness of the electroplated copper, which is easier to control and has higher resolution. The line width and line spacing of the fine line are almost the same, which can greatly improve the yield of the fine line. .
The semi-additive method is currently the main method for producing fine circuits. The mass production capacity can reach a minimum line width/line spacing of 14μm/14μm and a minimum aperture of 55μm. It is widely used in CSP, WB and FC flip chip substrates and other fine circuit substrates 's manufacture.
On the surface of the insulating substrate, the method of selectively depositing conductive metal to form a conductive pattern is called the addition method.
1. The advantages of the additive method
The printed board is manufactured by the additive process, and its advantages are as follows:
a. Since the addition method avoids a large amount of etching copper and the resulting large amount of etching solution processing costs, the production cost of the printed board is greatly reduced.
b. Compared with the subtractive method, the process of the additive method is reduced by about 1/3, which simplifies the production process and improves the production efficiency. In particular, the vicious circle of the higher the product grade and the more complicated the process is avoided.
c. The additive process can achieve flush wires and flush surfaces, so that high-precision printed boards such as SMT, etc. can be manufactured.
d. In the additive process, due to the simultaneous chemical copper plating on the hole wall and the wire, the thickness of the copper plating layer on the hole wall and the conductive pattern on the board surface is uniform, which improves the reliability of the metallized hole and can also meet the requirements of high thickness and diameter. Compared with printed boards, copper plating in small holes is required.
2. Classification of additive method
The additive manufacturing process of printed boards can be divided into the following three categories:
a. Full Additive Process (Full Additive Process) is an additive process that only uses electroless copper deposition to form conductive patterns. Take the CC-4 method as an example: drilling, imaging, tackifying treatment (negative phase), electroless copper plating, and resist removal. The process uses catalytic laminates as substrates.
b. Semi-additive Process (Semi-additive Process) On the surface of the insulating substrate, use chemical deposition of metal, combined with electroplating etching or the addition of the three to form a conductive pattern. The technological process is as follows: drilling, catalytic treatment and tackifying treatment, chemical copper plating, imaging (electroplating resist), pattern copper electroplating (negative phase), resist removal, and differential etching. The substrate used in the manufacture is a common laminate.
c. Partial Additive Process (Partial Additive Process) is to use the additive method to manufacture printed boards on the catalytic copper clad laminate. Process flow: Imaging (etching resistance) - Etching copper (positive phase) - Removing the resist layer - Coating the whole board with electroplating resist - Drilling holes - Electroless copper plating in the hole - Removing the electroplating resist.
We will introduce the advantages and disadvantages of PCB addition and subtraction, as well as the products of their application in the next article.
Addition or Subtraction? The comparison of three production processes of PCB and the preliminary exploration of similar carrier board technology are as follows:
There are three main processes for PCB products (including IC substrates): Subtractive, Full Additive Process (FAP) and Modified Semi Additive Process (mSAP).
3. Advantages and disadvantages of the subtractive method
The subtractive method is the earliest and more mature PCB manufacturing process. Generally, it refers to the photochemical method, screen printing pattern transfer or electroplating pattern resist layer on the copper clad plate, and then chemical solution is used to etch away the copper foil of the non-pattern part, or mechanically remove the unnecessary part to make a printed circuit board.
However, in the chemical solution etching process, the etching process is not carried out vertically downward from the surface, but is simultaneously etched to both sides of the channel, that is, there is a phenomenon of side etching, which causes the bottom width of the etching channel to be larger than the top. Due to the existence of side etching, the application of the subtractive method in fine circuit fabrication is greatly limited. When the line width/line spacing requirement is less than 2 mil, the subtractive method cannot be applied due to the low yield.
At present, the subtractive method is mainly used to produce ordinary PCB, FPC, HDI and other printed circuit board products.
4. Market trend: from HDI to quasi-substrate, from the subtractive method to the mSAP semi-additive method
At present, the mainstream advanced HDI boards in mobile phone motherboards are manufactured by the subtractive method. After upgrading to a carrier-like board, since the process requirement reaches 30/30 microns, the subtractive method will no longer be used, and the mSAP semi-additive method needs to be used. The process is similar to the IC carrier board.
From the subtractive method of HDI to the semi-additive method of mSAP like carrier board SLP, more copper plating processes are designed in the process, and the required copper plating capacity is greatly increased, and for exposure equipment (more complex process) and lamination The demand for equipment (increased number of product layers) has also increased.
At present, the main players involved in the production capacity preparation of the quasi-substrate include advanced HDI manufacturers and IC substrate manufacturers.
For HDI manufacturers, since the process is upgraded from the subtractive method to the mSAP semi-additive method, new equipment investment is required, and the learning curve of yield ramping is required.
For IC carrier board manufacturers, since the production of the carrier board itself adopts the mSAP process, there is no obstacle in the production of the carrier board in terms of technology and yield, but the requirements for the fineness of the lines of the carrier board are not as high as that of the IC carrier board. The requirements for equipment are also relatively loose, so IC substrate manufacturers may face the risk of declining profit margins when they switch to the production of similar substrates.
On the whole, in the competitive landscape of similar substrates, HDI manufacturers are temporarily at a disadvantage in terms of technology and yield, but may have advantages in cost, while IC substrate manufacturers have no problems in technology and yield, but in cost control In bad situation.
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