What is the PCB made of？

We all know PCB is printed circuit boards, which is an essential part of electronic products. But do you now what is PCB made of? Basically, PCB is formed by four essential parts: base substrate, cooper foil, solder resist, surface treatment and silkscreen.

1
Substrate board

Substrate boards are the base of PCB. Substrate boards are typically made of glass fibre or non-conductive materials like CEM1, CEM3, FR4, polyimide material. The substrate is used for single-layer or multi-layer.

2
Copper foil

Copper foil is used for current transportation in PCB, so copper is the electric wire for PCB to transmit current.

3
Solder resist

Solder resist is used in the PCB surface. It covers the PCB board surface that does not need to be welded, protects the board surface so that it is subject to small thermal shock during welding and is not easy to blister. At the same time, it also plays a role in preventing bridging, pinching, short circuit, faulty welding and other situations.

4
Surface treatment

We know there are pads free from solder resist on PCBs, which are used to solder electronics parts on it. But these pads may be get oxidiated without proper treatment. So we surface treatment(also called surface finsihes) is needed. Teh most commonly used ones are: HAL, immersion gold(ENIG), immersion silver, tin, OPS, ENEPIG, and also gold plating.

5
Silkscreen

Silkscreens are used for marking electronic components. We know numerous electronic components exist, so how to distinguish which electronic components are soldered to the PCB pad? It is to judge the electronic components soldered at each pad through the silkscreen on the PCB.

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    Preparation before PCB manufacturing

    When the PCB factory receives the data, can you start the manufacturing directly? Of course not. The PCB engineering designer needs to optimize customers’ data with CAD, Genesis2000 and other software, then make them into production files that fit our company. Primary engineering drawing data is shown below, and production fixtures need to be prepared:

    • Cutting drawing: According to the customer's requirements, make the single pcs PCB to the panel, add side strips and space, and then make a cutting dimension drawing. Cutting dimensions must meet the plate utilization rate and include copper thickness/board thickness and other information to make production and material opening convenient.
    • Films for inner layer / outer layer: According to the circuit drawing provided by the customer, like a pad, independent circuit, BGA, IC pin etc., to make optimization compensation, then make made into a film and provided to the production graphic transfer exposure process to complete the circuit.
    • Stack-up drawing: Multi-layer PCB needs fabrication and design of the laminated structure, like the copper thickness of inner/outer layer and PP information, stack-up thickness, impedance dielectric layer thickness, etc., to provide to lamination process to manufacturing according to it.
    • Drilling data: According to the aperture size of customer data, design corresponding drill belt data means the X-Y-Z coordinate, convenient for drilling machine identification and production.
    • Electroplate area: Use software to measure the plated area of the hole, large copper surface and circuit, and provide to plating process to calculate the input current to reach the thickness requirements of copper.
    • AOI optical inspection: Make AOI optical inspection scan data, transform and transmit it to the AOI scan machine, which means data compared with the physical board, ensuring the physical board meets the customer's requirements.
    • Solder mask film: According to the data provided by the customer, like a pad, independent circuit, BGA, IC pin etc., to make optimization compensation, then make made into a film and provided to the solder mask process to make solder mask.
    • Silkscreen film: Extract character layers in customer data for optimization compensation, can add information like corresponding UL number, LOGO, date code etc., and make a character screen and provide it for character process printing silkscreen.
    • Outline routing data: According to the outline dimension of customer data, make routing CNC forming gongs belt data, which means the X-Y-Z coordinate, convenient for routing machine identification and production.
    • E-Test data: For samples, make Flying Probe test data and lead in the Flying Probe test machine, to test the PCB according to the requirement; For batch production, we need to make test fixtures to do the e-test. For Viasion, the PCB area over three ㎡ needs to make test fixtures to save test time and cost.

    PCB manufacturing process

    Someone is wrong if he thinks the PCB manufacturing process is not complex. Just take a look at the following technologies involved in it.

    • Mechanical Processing
    • Ordinary Chemical Reaction
    • Photochemistry
    • Electrochemistry
    • Thermochemistry
    • Computer Aided Design(CAD)
    • Computer Aided Manufacturing(CAM)
    From the above, we can realize how complicated the process is. PCB manufacturing process is discontinuous form. Any steps link error will result in production suspension or a large number of scrapping. If the PCB is scrapped, it cannot be recycled.

    The process flow of boards with different structures is also different, but the PCB manufacturing process can be roughly divided into the following steps:

    1. Rigid PCB manufacturing process

    1. PCB manufacturing Process flow of the double-sided board

    Copper clad plate (CCL) cutting → drilling holes → through hole copper plating (PTH) → whole panel plating → pattern transfer ink or dry film → pattern plating → etching → semi-inspection IQC → screen printing solder resist ink and character ink (SS) or paste solder resist dry film → hot air levelling or tin spraying (HAL) or immersion gold ( ENIG ) → outline routing (Pounching) → finished product inspection (FQC) → electrical test E-TEST → packaging.

    2. Multi-layer board manufacturing process flow

    Copper foil cutting of inner layer copper clad plate (CCL) → inner-layer pattern making → inner-layer Etch → inner-layer black oxide → lamination or pressing process → drilling → copper deposition (PTH) → panel plating → outer-layer Etch → semi-inspection IQC → silk screen solder resist ink and character ink (SS) Or paste dry film of resistance welding → hot air levelling or tin spraying (HAL) or immersion gold ( ENIG ) → appearance (Pounching) → finished product inspection (FQC) → electrical test E-TEST → packaging

    2. Flex PCB manufacturing process

    1. Common double-sided FPC process

    Material cutting → drilling → PTH → electroplating → pretreatment → dry film pasting → alignment → exposure → development → graphic electroplating → coating cover layer → pressing → curing → surface treatment → silkcreen printing → cutting → electrical testing → punching → final inspection → packaging → shipment  

    2. Simple single-sided FPC process

    Material cutting → pretreatment → dry film pasting → alignment → exposure → development → graphic electroplating → coating cover layer → pressing → curing → surface treatment → silkcreen printing → cutting → electrical testing → punching → final inspection → packaging → shipment

    3. Multilayer FPC process

    Material cutting (double sided flex CCL) →pattern making → Etch → flex CCL bonding → lamination or pressing process →drilling → PTH → electroplating → pretreatment → dry film pasting → alignment → exposure → development → graphic electroplating → coating cover layer → pressing → curing → surface treatment → silkcreen printing → cutting → electrical testing → punching → final inspection → packaging → shipment
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    Definition and explanation of each process

    People from our industry may not familiar with all the glossary in PCB manufacturing process. Here we would like to explain more about these terms.

    1
    Cutting

    Cut the PCB substrate to production size from the whole sheet of copper clad plate with corresponding thickness and corresponding copper foil thickness, and reduce the weight by about 10-15%.

    2
    Drilling

    Drill out the different dimension conductive holes or plug-in holes on the board according to drill belt data in the drilling machine. The through-holes between the boards can be used for the subsequent processing of the plug-in and can also help the heat dissipation of the boards; The plate weight is reduced by about 5%.

    3
    Copper deposit

    Deposit a thin chemical copper in the drilled hole, with a thickness of about 0. 3-2um, with less weight increase, aimed at depositing a layer of copper on the non-conductive FR4 substrate (or other substrates) by chemical method, make the circuit of each layer of the board conductive.

    4
    Whole plate Copper plating

    Plating copper to a place where the holes and circuit are not covered with dry film, at the same time, further increases the conductivity and copper thickness and also protects the thin chemical copper from avoiding oxidation.

    5
    Graphic production (graphic transfer)

    Including the production of inner graphics, pasting dry film on the board or screen printing with graphic anti-electroplating ink, and making circuit graphics after exposure and development process;

    6
    Graphic plating

    Thickened copper plating is carried out on the board's surface where the graphic circuit is done, which can make the copper thickness in the hole, and the circuit can reach the copper thickness the customer requests. After that, the holes and circuit can load a particular current. The weight increases by about 15%.

    7
    Etching

    The etching process involves using a chemical solution to remove unwanted copper from the PCB board and create the desired circuit pattern. This process is necessary for ensuring that components are properly insulated from each other to prevent short circuits.

    8
    Lamination

    Laminate the inner layer with the semi-solidified sheet and copper foil and press them into a high-temperature multi-layer plate. The 4-layer plate needs one inner layer and two copper foils; The 6-ply board requires two inner layers and two copper foils. The weight increase of the 4-ply board is about 15-25%, and that of the 6-ply board is approximately 30-40%.

    9
    Printing solder mask by Silk screen or paste solder mask dry film to make solder mask

    Print a layer of solder mask ink on the board surface according to the solder mask film, about 35um thick, and increase the weight by about 2-4%; Or paste a layer of dry solder mask, make solder mask after exposure and development.

    10
    Tin spraying

    A layer of lead and tin shall be sprayed on the place to solder components on the board to facilitate soldering components, and at the same time, it can avoid the oxidation of the copper surface area, and the weight increase is about 1-2%.

    11
    Silkscreen

    Print some symbolic characters like tag numbers or other characters on the board, less weight increase. It is mainly convenient for PCB assembly in further production.

    12
    Outline

    The outline of the plate is routed according to the customer's requirements, and the weight is reduced by 5-10%;

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