Introduction of the Mass Lamination

Brown oxidation

Purpose:

The purpose of brown oxidation is to produce a passivation layer on the surface of the inner copper foil to improve the bonding force between the PP and inner PCB in order to prevent producing amine attacking the copper surface in the resin and thereby avoid popcorn when performing mass lamination.

Process:

Control elements: Tensile force for brown oxidation, solution concentration

Process Outline

Introduction to the Mass Lamination Process:

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Black oxidation

Process:

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Comparison between brown and black oxidation 

(1)  Since the solution contains Cu2O due to its high alkalinity, long-needle or feather-shaped crystals can easily be formed in the black oxide layer. This acicular cuprous can easily be broken off under high temperatures and thereby significantly affect the adhesive force between the copper and resin, resulting in electrical issues because of resin flow causing black spots spreading over the board. In addition, the presence of moisture may lead to high heat and thereby bring about partial delamination and popcorn. The brown oxide layer has gravel-like nodular crystals and a copper surface. Its dense structure without pores provides an adhesive force with the film greater than that of the black oxide layer, and the force is not affected by high temperature or high pressure. Therefore, brown oxidation has become a process necessary for multilayer polyimide PCBs.

(2)  For black oxidation, longer and thicker crystals provide better coverage compared to brown oxidation. The defects on regular copper surfaces can be covered easily to have uniform color on the surface. As for brown oxidation, the surface is often mottled due to imperfect copper surface pre-treatment. Therefore, most quality control personnel do not approve of the process. However, if the treatment time is longer or the temperature is higher, the color will be more uniform. In fact, such non-uniform surface does not affect its excellent peel strength. Since the thickness of regular products is often limited and a sealer is used to prevent pink ring in order to improve acid resistance, the performance of brown oxidation stands out more.

(3)  Pink ring is usually formed after the PTH process is performed as a result of the thicker black oxide layer. This is caused by an attack on the black oxide layer by micro-etching or the activation or acceleration solution, resulting in a reduction and thereby exposing the original copper color. On the other hand, because the brown oxide layer is very thin, it is less easy to form pink ring. The copper foil matt surface of the inner PCB adheres to the substrate firmly after galvanization. However, the side attack by acid solution on the smooth surface of the black oxide layer easily leads to exposure of the color of copper.

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Comparison of Brown and Black Oxidation

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PP cutting

Function: A roll of PP is cut into small sections 2-3 mm larger than the inner PCB as required to assemble the inner PCB.

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Build-up

A. Function: The cut PP sections are assembled together with the brown oxide inner PCB according to design requirements.

B. Process: Put a piece of PP flat on the build-up table and place the brown oxide OK board handed over by the brown oxide board inspection personnel on the PP, and then put another PP on the brown oxide board. Hold both sides of the board with both hands, arrange the PPs and brown oxide board neatly, and erect them against the backrest of the build-up table to melt the edge.

Hot melt and riveting

PCBs with more than six layers need to be fused and riveted together.

  1. Purpose of hot melt: Align the inner PCB and PPs to achieve initial fixation.
  2. Purpose of riveting: Further fix the relative position of each through hole on the multilayer PCB to avoid layer deviation.
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Lay-up

Process: Place the cut copper sheets on the steel plate (the reverse side of the copper sheet is placed face down on the steel plate) as shown in the manufacturing specifications and then put the assembled work-in-process board (laid up in advance) on the copper foil, and finally cover it with another copper foil. Repeat these steps until the lay-up process is completed.

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Lay-up

There is a top and a bottom thermal compression tray at each opening of the laminator. For the lay-up process, place 18-21 sheets of kraft paper on the steel loading tray serving as the bottom tray, put steel plates, copper foil and inner PCBs between the top and bottom trays, and then pile copper sheets and steel plates up until they reach an appropriate height for the loading tray. Finally, place 18–21 sheets of kraft paper and then close the steel cover. The formation is shown as follows:

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Lay-up

Alignment: Use a projector to align layers by adjusting the infrared light.

Install a projector above the lay-up station and adjust the infrared projection to align all layers. A copper foil cutter and a copper foil clamping machine automatically cut and cover a top layer and a bottom layer of copper foil per layer.

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  1. Operating principle: Put the laid up work-in-process board in the loading tray, close the cover, and load the tray into the laminator for mass lamination. The heat used by the laminator is produced by burning heat transfer oil in a boiler and then transmitted through oil pipes to the thermal compression tray at each opening of the laminator to produce heat for work-in-process boards. The pressure is produced by pushing hydraulic oil at the bottom of the laminator upward.
  2. Thermal compression function: Combine the inner PCB, PP and outer copper foil.
  3. Cold compression function: Cold compression is a process designed to eliminate the concentration of thermal stress on the work-in-process board with high surface temperature (around 170 °C–190 °C) just after the thermal compression process and to cool the working temperature of the work-in-process board.
  4. Key control points: Temperature, pressure setting, and temperature rising rate.

Break-down and steel plate scrubbing

Function: Separate the laminated boards with kraft paper and then clean the steel plate by polishing and scrubbing it.

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Description:

a. Polishing the steel plate requires 4 people grouped into 2 groups with 2 people in each group. One group polishes the upper part of the steel plate, and the other group polishes the lower part of the steel plate.

b.“↑↓” indicates the polish directions. Polish the steel plate with uniform force back and forth without leaving unpolished areas.

X-RAY drilling

Operating principle: Use the X-ray light to locate the target in the target area and drill directly.

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Routing

Purpose: Perform the routing process to unify and shape the outer frame of the laminated board neatly (its size should be the laminated board size required in the manufacturing order) to avoid scratches and facilitate the subsequent processes.

Routing requirement:

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Edging

Purpose: Grind the edges of the board after the routing process to have obtuse angles so as to avoid scratches.

Standard: Smooth and non-scratchy.

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