PCB Manufacturing

Guide To Six-Layer PCB Stack-up and Manufacturing Process

6-layer-PCB-Stack-up

Typical Six-Layer PCB Stack-up

Six-layer PCB is a printed circuit board with a total of six copper layers. In a typical six layer PCB stack-up, there are top and bottom layers, plane and power layers, and two signal layers. Six-layer PCBs are widely used in a lot of electronic products since they combine both complexity and cost-effectiveness. The following elements are included in a six-layer PCB stack-up:

Copper Layers: We refer to Copper Layers as a layer of a PCB that has copper on it after etching, usually tracks and pads or power and ground layers

Cores: The core of a PCB is a rigid base material laminated with copper on one or two sides. A CORE is used for manufacturing single-sided and double-sided boards but is also used in the production of Multi-layer PCBs.

Prepreg: As simple as it is, a prepreg is nothing but an insulation layer. Prepreg is a dielectric material that is sandwiched between two cores or between a core and a copper foil in a PCB, to provide the required insulation. You can call it a binding material as well. It either binds two cores or a core and a copper foil.

Solder Mask: A solder mask is a thin polymer layer on a PCB that insulates the copper traces. The primary objective of applying a mask is to eliminate undesired electrical connections between the copper features

Silkscreen: The Silkscreen is the topmost layer of a Printed Circuit Board (PCB) which is used as a reference indicator for placing components on a PCB Board. It requires a specifically formulated ink whose standard colour is white, but it can also be Red, Black, Yellow, Blue and a number of other colors.

Surface Finishes: A PCB surface finish is an intermetallic joint between the bare copper of the solderable area of the printed circuit board and the components. Circuit boards have a base copper surface that is susceptible to oxidation if left without a protective coating, hence the requirement of the surface finish.

Layer stack up for a 6-layer PCB

Manufacturing Process of Typical Six-Layer PCB Stack-up

The Manufacturing Process of Printed Circuit boards is huge. Lots of advanced chemical and physical techniques are used in the production. The files used to produce PCBs are Gerber files and drill files. It is a very complex task to transfer a design into an actual printed circuit board. The best practice is to follow sufficient spacing between the nets, traces, polygons, holes, and vias as per the manufacturer’s capability to avoid electrical or physical failures.

Let us look into the manufacturing process of a 6-layer Printed circuit board by correlating the layers built in every stage of the Manufacturing process. 

DFM by Engineering Department

The Role of the Engineering department is primarily to DFM (Design for Manufacturing) checks, such as if all the files received from the client are properly capable of manufacturability by the factory. This department has a standard checklist for various layer stacks. Once the checklist is verified across all the Gerber and drill files the Board continues to the next department, if not then the PCB query is raised to the customer.

Tools and Materials Prepare

Before the Board is sent to actual manufacturing the factory has to be set up with relative tools and materials, sheets by printing the design on a photoresistor film for all the 6 layers of copper that are mentioned in the Gerber file. In addition to it, the Top paste and bottom paste are also printed with negative images. Process card is also set up and sent with the boards to track the production status and update in the case of discrepancy. 

Base Lamanates Cutting

The base CCL (copper-clad laminate) is set to get ready, before that the panel is cut into the required size to manufacture multiple PCBs at the same time in the same panel. Once the copper layer is cut to processing size by an automatic cutting machine, a copper-clad laminate is placed over the FR4 material and heat-pressed. The main parameters that are tracked in this process are the Size of panel boards, PCB thickness, Copper thickness, sheet material types & space between the edge of the board and components. The edge of the board is not kept sharp to keep the board away from the damage that happens when passed through a furnace or other heat press in the upcoming manufacturing process. The PCB is baked to strengthen the material and remove foreign particles.  

Inner layer Production

  • Inspection & Pretreatment Process

In the Inspection and pretreatment process, the copper oxide particles are removed and the surface is kept rough to maintain adhesion between the film and the copper. Once after the dry or wet film is coated on top of the inner copper layer, the photosensitive film is exposed to UV light thus blocking, plating, and etching happens in the subsequent process. The dry film process is very sensitive so the process completely happens in a brown dust-free room. The Non polymerized film that goes into the UV process is washed off in the alkaline. The copper layer having a coat on top is etched to get the conducting copper layer on the board. The dry film is washed off with sodium hydroxide. The Panel is entirely inspected by AOI- Automated optical inspection for any discrepancies. Copper layer 2, and 3 with core (Dielectric -2) and copper layer 4, 5 with the core (Dielectric -4) are built separately.

  • Lamination -Prepreg

Multilayer or 6-layer boards require a core layer setup and the addition of layers on top of each other. In this step inner layers are kept in a certain order by riveting or fusing and inspection is made to ensure that these do not cause any open or short circuits due to misalignments. Dielectric layer 3 is sandwiched in between the separately built copper-finished panels. Dielectric 1 and Dielectric 5 will also be sandwiched post drilling and inspection is done as the vias and plated holes need to be electrically conductive.

Hot Press

The inner layers and the prepreg are held together and fed into a vacuum heat press machine for lamination. Under heat and pressure, the prepreg flows and drills the gap between inner layers also resulting in all the layers together.

Drilling & X-Ray Inspection

The Boards are taken to the next step for drilling. The vital process is ensuring the drill alignments by x-ray and verifying the alignments. Hole milling is also the most important part. The best practice in PCB designing is to provide a sufficient gap between a trace-to-hole trace-to-solderer mask, and manhole-to-hole to avoid short circuits.  The drill data is taken from the drill files, Drill holes can be majorly classified into two types, plated and non-plated holes. For single- and double-sided boards, the drilling process happens after the board cutting while the 6-layer board takes up drilling, AOI, and X-ray inspection are drilled immediately after lamination. Mechanical drilling is usually done using a high-speed drilling machine where the drill bit automatically changes through a high-speed spindle.

Copper Plating

The holes after drilling ate either no conductive epoxy resin or fibreglass board. To connect the traces in different layers the walls of the holes need to be metalized and the process is done through autocatalytic redox reaction resulting in copper plating. Before plating the copper, the Panel is ensured clean by deburring, alkaline degreasing, and micro etching. The board is ensured clean and copper plating is even throughout the surface.

Outer layer Production

The imaging process of the outer layer is similar to the inner layer. The lamination is done mostly in the positive film by using polyethylene film and the etching process happens. As shown in the image the film is removed with alkali and the process continues the same for the outer layers and processed to solder mask after inspection. Outer layers Copper layer 1 and copper layer 6 are plated on Dielectric 1 and Dielectric 5. 

Outer layer Production

  • Pattern plating

Pattern plating is adding secondary copper plating to increase the copper thickness where the electrical pads of the circuits are placed and this ensures sufficient electrical conductivity of copper to satisfy customer requirements. Degreasing, Micro etching, and Acid picking are the few steps that make the PCB ready for the Pattern plating.

Add copper

  • Tin Plating and Etching

After copper plating, a layer of tin is added to prevent the copper from oxidizing and etching. This layer is seen directly on top of the board, the board is sent for Automated Optical inspection and quality test for etching.

Solder Mask

The solder mask is nothing but the protective non-conductive layer on top where the Tin pads are only exposed on the top and bottom of the PCB. It not only protects but also the electrical resistance to solder and heat on top of the PCB. The Solder mask is printed using two methods, screen printing or electrostatic spaying. The board is cured and the solder mask is on top of the solder pad or plated through a hole. The exposure of these solder max to UV light and hardened. The ink which is not exposed is removed by an alkaline solution. The board is baked at high temperature.

Silkscreen

The silkscreen contains the legend information of the PCB like Designators, Switch settings, test points, Part number, Version number, Company name, and logos. Two widely used methods are screen printing and Direct legend printing. The PCB is again baked at a high temperature to make sure the ink is attached to the PCB.

Surface Finish

To prevent the oxidation process, the surface finish is done. The finish is majorly HASL Lead, HASL Lead-free, Immersion Gold (ENIG), OSP, Hard Gold, Immersion Silver, Immersion Tin, ENEPIG, and Plain Copper. This takes several steps like Pretreatment, Flux coating, Tin spraying, Hot air leveling, cooling, and post-cleaning. The boards go through Quality control.

Profiling

The Process of Profile is to cut the entire production panel to the PCB ordered by the customer. Profiling for the panel by the customer is done in two ways, Tab routing or V scoring. Tab routing is a method where the frame is set around multiple PCBs and the boards are in contact with the frame in very few places. Whereas in V-scoring the board is cut near the edges and the depth is 1/3rd of the PCB thickness. This method is cost-effective and used widely.

E-Testing

After all Manufacturing Processes, the electrical parameters are checked for short or open circuits. It is done by two methods

  1. Flying probe test
  2. Bed of nail test

Both tests have their application. It also depends on the board’s complexity, functionality, and available test points or access points.

Packaging

Packaging is the final step where the customer can also request the type of packaging that they require for their standards.

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