Multilayer PCB Stack-Up Design & its Lamination Process

Multilayer PCB or multilayer printed circuit boards are circuit boards composed of two or more conductive layers (copper layers). The copper layer is pressed together by the resin layer (prepreg). Due to the complexity of the multilayer PCB manufacturing process , low production volume, and difficulty in rework, their prices are relatively higher than a single layer and double-sided PCB.

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Multilayer PCB Manufacturing

Due to the increase in the packaging density of integrated circuits , a high concentration of interconnection lines has resulted, which necessitates the use of multilayer PCB. Unforeseen design problems such as noise, stray capacitance, and crosstalk have appeared in the printed circuit layout. Therefore, the printed circuit board design must minimize the length of signal lines and avoid parallel routes. Obviously, in the PCB single-sided board, even the double-sided board, due to the limited number of crosses circuit that can be achieved, these requirements cannot be satisfied. In the case of a large number of interconnection and crossover requirements, the PCB circuit board must be expanded to more than two layers to achieve satisfactory performance. Thus a multilayer circuit board has appeared. Therefore, the original intention of manufacturing multilayer circuit boards is to provide more freedom in selecting appropriate wiring paths for complex and noise-sensitive electronic circuits.

Multilayer PCB circuit boards have at least three conductive layers, two of which are on the outer surface, and the remaining layer is integrated into the insulating board. The electrical connection between them is usually achieved through plated through holes on the cross-section of the circuit board. Unless specified, multilayer printed circuit boards are the same as double-sided boards, generally plated through-hole boards

Advantages and disadvantages of multilayer PCB

Advantages:

Multilayer PCB circuit boards have at least three conductive layers, two of which are on the outer surface, and the remaining layer is integrated into the insulating board. The electrical connection between them is usually achieved through plated through holes on the cross-section of the circuit board. Unless specified, multilayer printed circuit boards are the same as double-sided boards, generally plated through-hole boards

Disadvantages:

Multilayer printed circuit is the product of electronic technology development in high speed, multi-function, large capacity, and small volume. With the continuous development of electronic technology, especially the extensive and in-depth application of large-scale and very large-scale integrated circuits, multilayer printed circuits are rapidly developing in the following directions: high density, high precision, and high layers, tiny lines and small holes, blind and buried holes, high plate thickness to aperture ratio and other technologies to meet market needs.

Why are PCB multilayer boards all even-numbered layers?

1、It can be manufactured in a PCB factory. The four-layer board generally uses a core with one copper foil on each sideand a three-layer board with one copper foil on one side. They must be pressed together.
2、The process cost difference between the two is that the four-layer board has one more copper foil and bonding layer. The cost difference is not significant. When the PCB factory makes a quote, they are generally quoted on an even number basis. Also, 3-4 layers are commonly quoted as a grade. (For example:If you design a 5-layer board, the other party will quote at the price of a 6-layer board. That is to say, the price you design for 3 layers is the same as the price you design for 4 layers.)
3、In the PCB process technology, the four-layer PCB board is better controlled than the three-layer board, mainly in terms of symmetry. The warpage of the four-layer board can be controlled below 0.7% (IPC600 standard), but the size of the three-layer board is large. At that time, the warpage will exceed this standard, which will affect the reliability of the SMT assembly and the entire product. Therefore, the designer shouldnot design the odd-numbered layer board. Even if the odd-numbered layer is necessary, it will be designed as a fake even-numbered layer. That is to design 5 layers into 6 layersand 7 layers into 8 layers.

Calculation Method of Multilayer PCB Stack-Up:

A: Thickness of inner layer             B: Thickness of PP sheet
E: Thickness of inner copper foil     F: Thickness of outer copper foil
X: Finished board thickness            Y: Finished PCB tolerance

1. Calculate the upper and lower limit of pressing:

Usually tin plate: upper limit -6MIL, lower limit-4MIL
Gold plate: upper limit -5MIL, lower limit -3MIL
For example, tin plate: upper limit=X+Y-6MIL lower limit=X-Y-4MIL
Calculate the median = (upper limit + lower limit)/2
≈A+the area of the second layer of copper foil%*E+the area of the third layer of copper foil%*E+B*2+F*2

The inner cutting material of the above conventional four-layer board is 0.4MM smaller than the finished board, using a single 2116 PP sheet to press. For special inner layer copper thickness and outer layer copper thickness that more than 1OZ, the copper thickness should be considered when choosing the inner layer material.

2. Calculate the Pressing Tolerance:

Upper limit = Finished board thickness + Finished on-line tolerance value-[Plating copper thickness, green oil character thickness

(Conventional 0.1MM)]-The theoretically calculated thickness after pressing

Lower limit = finished board thickness-finished product off-line tolerance value-[electroplating copper thickness, green oil character thickness

(Regular 0.1MM)]-The theoretically calculated thickness after pressing

3. Commonly types of PP sheets:

PP	KB	KB
1080	0.07MM	0.065MM
2116	0.11MM	0.105MM
2116	0.11MM	0.105MM

Generally, do not use two PP sheets with high resin content together. If the inner layer of copper is too small, please use PP sheets with high resin content. 1080 PP sheets have the highest density and low resin content. Do not press single sheets as much as possible. Only 2 sheets of 2116 and 7630 PP sheets can be pressed into thick copper plates above 2OZ. The layer cannot be pressed by a single sheet of PP. 7628 PP sheet can be pressed by a single sheet, 2 sheets, 3 sheets, or up to 4 sheets.

Explanation of theoretical thickness calculation of multilayer PCB board after pressing

Thickness after PP lamination = 100% residual copper lamination thickness-inner copper thickness*(1-Remaining copper rate%)

4. Typical Recommendation of Multilayer Stack-Up

1): 4 Layer PCB Stack-Up

2).6 Layer PCB Stack-Up

3).Typical Stack-Up of 8 Layer PCB

Introduction to the Multilayer PCB Lamination Process

★ Lamination uses high temperature and high pressure to melt the prepreg by heat, make it flow, and turn it into a cured sheet. It is then processing one or more inner etched boards (Black Oxide Treatment) and copper foil into a multilayer board.

★ This process also includes layer stack-up before lamination, drill positioning holes, and profile routing after laminated multilayer boards.

1. Lamination Process Flow

Multilayer PCB Manufacturing Process Steps

Remarks: For 6 layer PCB stack-up and over, two or more inner layers must be pre-positioned so that the holes and circuits of different layers have the correct alignment.

2.Position Methods

1) Positioning of rivet nails: press the layout of the inner layer board and prepreg with pre-drilled positioning holes

    1、The sequence is set on the template with rivets and then punched with a nail punch 

    2、Rivets to position 

2) Solder joint positioning: set the inner layer board and prepreg with pre-drilled positioning holes according to the layout

    1、The sequence is set on the template equipped with positioning pins, and then through the heating several 

    2、A fixed point, using the prepreg to melt and solidify when heated 

We are currently using solder joint positioning-RBM

Pre-punched positioning hole for the inner board, the current method we use is as follows: Punch 4 slot holes on the four sides of the board, two as a group, respectively locating in the X/Y direction, one of which is asymmetrical design. The purpose is to start to prevent a reaction.

A= 7.112±0.0254MM

B= 4.762 ±0.0254MM

Thickness	<40mil	40mil<T<60mil	>60mil
temperature	300℃	300℃	300℃
Time	0.3-0.5min	0.6-0.8min	0.8-1.0min

Quality control after RBM-potential problems

1) Interlayer offset: poor RBM positioning or poor heating point condensation, causing

Shift between layers after pressing, after drilling due to

Dislocation of the lines on each layer causes open or short.

Possible reason:
    • uInner layer punching deviation
    • uThe expansion and contraction of the inner plate is very different
    • uRBM staff deflection
    • uRBM parameters do not match-the coagulation effect is not acceptable
    • uRBM heating head wear-bad condensation effect
    • u Lay up personnel put the board improperly, causing the heating point to fall off

2) The inner core is reversed: the order of the inner core is incorrectly placed during RBM, which affects the quality
of the customer’s assembled board.

Introduction to Layer Stack-Up Process: The layout process arranges the inner core, prepreg, and copper foil with
aluminum plates according to the structural requirements and reaches the required height for pressing. CEDAL layer
stack-up can be divided into four main layouts according to the right picture

3.Introduction to Prepreg

A prepreg refers to glass fibers or other fibers impregnated with resin. After partial polymerization, the resin molecules are slightly cross-linked, which can be softened by heat. However, it cannot be completely melted.

Prepreg specifications

Prepreg specifications

Main Performance characteristics of prepreg

Resin content (R/C) 

Resin fluidity (R/F) 

Gel time (G/T) 

Volatile content (V/C)

Test - Resin Content

Resin content (RC)

1). Resin content definition: the percentage of the weight of resin in the semi-cured to the weight of the prepreg;
2). Calculation formula: RC=(TW-DW)÷TW ×100%;
RC: Resin content; TW: weight of prepreg; DW: weight of glass cloth after burning.
3) TW can be used as a control indicator when the base weight of the glass cloth is constant
    2、Instrument: Electronic balance, accuracy: 0.001 g
    3、Sample: 4 “X 4” X 4 pieces

Description of Resin Content

Resin content of prepreg (RC)

    • lRC is mainly related to the thickness of the laminate.
    • lThe RC is low,and the thickness of the board is thin;
    • lIf the deviation of the left, middle,and right of the RC is large, the thickness uniformity of the board will be poor.
    • After controlling the RC of the prepreg, the required thickness can be obtained after pressing, and the Cpk value of the thickness can be increased.

Comparison table of resin content and PP thickness

Thickness calculation after resin filling:

Thickness after PP pressing

    1、Thickness = theoretical thickness of single PP-filling loss
    2、Filling loss = (1-A side copper residual copper rate) x copper foil thickness + (1-B side copper residual copper rate) x copper foil thickness + 0.4*(D2)2*H(inner layer thickness)*N(hole Number)/the whole board area

The relationship between PP film characteristic parameters and resin fluidity:

    • The gel time (PG) is large,and the resin has strong fluidity;
    • The fluidity (RF) is large, and the resin has strong fluidity;
    • The minimum viscosity (MV) is small,and the resin has strong fluidity;
    • Large flow window (FW), strong resin fluidity;

The influence of resin fluidity on board quality

When PG is long, RF is high, MV is low, or FW is long, the following situations may occur after pressing:

    1、There is a lot of resin flow and poor board thickness uniformity (easy to be thick in the middle and thin on the edge).
    2、White edges appear on the edges of the board due to low resin content.
    3、Skateboarding easily occurs.
    4、Easy to show texture.
    5、The resin content of the board is reduced, which affects the dielectric properties and insulation properties.
Also,anti-CAF performance is poor.
    6、The internal stress of the plate is increased, and it is easy to twist and deform after pressing.

When PG is short, RF is low, MV is high, or FW is short, the following situations may occur after suppression:

1、Dry board, trunk line, dry point.
2、Air bubbles.
3、The cohesive force between core material layers is weakened, and the board is prone to bursting.
4、The peel strength between resin and copper foil is weakened.

PP storage conditions:

    • Storage temperature: 21±2℃ or below 5℃
    • Storage humidity: below 60%
    • Storage time: 90 days and six month

Key Points of Layer Stack-Up Control

-Placing the Board Along the Laser Beam

Our current lay up is two types. Controlling the consistency of lay up can ensure uniform force during pressing and avoid white edges due to loss of pressure. This requires the position of the laser beam to be adjusted and fixed when preparing for lay up. Laying the board along with the laser beam in the lay up production.

- Height Control

Controlling the height during lay up can ensure the smooth progress of pressing and achieve maximum productivity.

Machine	Minimum height	Highest height
48#	160mm	170mm
73#	220mm	260mm

- Panel Lay Up Requirements

    • Boards of different sizes cannot lay up together.
    • Boards with a difference of more than 15mil in thickness cannot lay up together.
    • The boards of different thicknesses are lay up together, the thermocouple must be placed in the middle of the thin
board, and the ADARA staff shall be notified to increase the curing time by 10 minutes.
    • Small plates (less than 10 pieces) of different copper foil thickness can be lay up together by cutting the copper foil,
and PE release film must be placedbetween the board and the conductive copper foil during production.

- Separate Lay Up Requirements

    • Lay up the board in the middle of the whole cycle.
    • Add dummy lay up on the top and bottom of the production board and reach the lowest height.

- Separate Lay Up Requirements

The Black Oxide Treatmentboard is stored in the environment for a long time, and it is easy to absorb water, causing delamination after pressing

Process	Storage time
B/F	72hours
B/O	24hours

Lay Stack up design guidelines

1. Inner board design requirement

    • The edge of the inner board is filled with dummy pads.The pad diameter is required to be 4.0mm, and the spacing is required to be 1.5mm.
    • The two layers of dummy pads corresponding to the inner layer board mustbe staggered by half the pad distance to balance the pressure during pressing.
    • The dummy pads of adjacent rows should be staggered to improve flow resin.

2. PP Design Requirements

    1、The centrally symmetric structure can avoid the bending phenomenon caused by structural stress.

    2、High R/C, thin fabric on the outer layer

    • The same glass cloth combinationand high resin content are placed on the outer layer.
    • Different kinds of glass cloth combinations, based on the principle of symmetry, thin fabrics are placed on the outer layer.
    

    3、Warp to warp, weft to weft

The glass cloth yarn contains different yarn counts in warp and weft directions, resulting in different glue content and differences in thermal expansion in the two directions.

    4、Each layer of prepreg has a reasonable thickness

    • The thickness andglue content is high. The thickness is not suitable for control
    • Small thickness, low glue content,and low adhesion

    5、Minimum number of layers

    • Many layers, high cost, andnot suitable for process control

Pressing Method – Hydraulic Press

The structure of the hydraulic pressing machine is vacuum type and standard pressure type. The plate between the openings of each layer is clamped between the upper and lower hot plates. The pressure is from bottom to top, and the heat is transferred from both upper and lower hot plates to the plate.

Advantages: simple equipment, low cost, large output.

Disadvantages: large amount of glue flow, poor thickness uniformity.

Pressing method - ADARA SYSTEM Cedal

DARA SYSTEM Cedal

The Cedal is a revolutionary laminating machine. Its operating principle uses continuous coiled copper foil that laminates in a closed vacuum chamber. Current is then applied at both ends. Due to its resistance, the copper foil generates high temperature and heats prepreg, and the pressure is applied by the upper air bag to achieve the compression effect.

Advantages:

    • Using the upper and lower interlayer copper foils for electric heating, energy-saving,and low operating cost.
    • Small temperature difference between the inner and outer layers, uniform heating, good product quality.
    • Cycle time is short, about 60minutes.
    • Fast heating rate (35/min).
    • Disadvantages:
    • The equipment has a complex structure and high cost.
    • Single machine output is small.
    • The pressure is a pneumatic working method, which cannot provide high pressure.

Pressure curve

Parameter Control and Function of Pressing

Vacuum:

It can help to remove gases, air, and small monomer residues generated by solvent volatilization.

Temperature:

The curing agent DICY is very stable at room temperature and can be cured quickly after the temperature rises.
Experiments show that 170°C is the ideal curing temperature. Therefore, it is necessary to control the temperature above 170°C during the pressing process to complete the curing reaction.

Heating Rate:

Maintaining a specific heating rate can appropriately increase the resin’s fluidity, thus improving the resin’s wettability and preventing problems caused by thermal stress.

Pressure:

Offset the vapor pressure generated by volatiles. Improve resin fluidity. Increase interlayer adhesion. Prevent deformation due to thermal stress during cooling

Thickness Control

Thickness test

    • Use a thickness gauge to measure the thickness of the four corners and a midpoint of each plate
    • The test point is 50 mm from the edge of the board
    • Thickness tolerance: general thickness requirements ± 10%

Multilayer PCB is manufactured by stacking two or more circuits on top of each other, and they have reliable pre-set interconnections. Since drilling and plating have been completed before all layers are pressed together, this technique violates the traditional manufacturing process from the beginning. The two innermost layers are composed of traditional double panels, while the outer layers are different. They are composed of single independent panels. Before pressing, the inner substrate will be drilled, through-hole plated, pattern transferred, developed, and etched. The outer layer to be drilled is the signal layer, which is plated through so that a balanced copper ring is formed on the inner edge of the through-hole. The layers are then rolled together to form a multilayer PCB, which can be connected to each other (between components) using wave soldering.

Pressing may be done in a hydraulic press or an overpressure chamber (autoclave). In the hydraulic press, the prepared material (for pressure stacking) is placed under the cold or preheated pressure (high glass transition temperature material is placed at a temperature of 170-180°C). The glass transition temperature is the temperature at which an amorphous polymer (resin) or part of the amorphous region of a crystalline polymer changes from a hard and brittle state to a viscous, rubbery state.

You probably want to widen your knowledge about multilayer PCB. This is one good reason why you have visited this page. Also, you may wish to import multilayer Printed Circuit Boards. Whatever the case may be, our guide will offers the best solution for all your PCB needs.

What does Multilayer PCB Mean?

Multilayer PCB can be referred to as a multi layer circuit board that is composed of three or more layers. This is made up of a substrate layer, which features a conductive metal on its two sides. It also offers enhanced functionality. They are utilized in complex devices that require a very high number of connections.

Multilayer Printed Circuit Boards must feature at least three layers of conductive layers, which are located at the center of the material. This multilayer PCB manufacturing entails the fusing of fiber-glass alternating epoxy-infused conductive core materials and layers.

Both of them are laminated using high hydraulic press pressure and temperature. This process leads to the prepreg melting, which later results to the joining of these layers.

Steps Used in the Manufacturing of Multilayer PCB

The manufacturing process of multilayer circuits involves some steps. Let us consider them.

Developing the Desired Design

Plan the layout PCB design following all the requirements and encode it. By doing this, you are making sure that the different aspects and parts of the designs are error-free. A completed PCB design is then ready for fabrication building.

Printing of the PCB Design

As soon as the check completion has been finalized on the design, then it can be printed. You punch the registration hole to serve as a guide for aligning the films as you continue with the process.

Print the Copper Utilized for the Interior Layer

This step is the first while making the inner layer of the PCB. You print the multilayer PCB design; then copper is re-bonded to the lamine piece that serves as the PCB structure.

Discard unwanted copper

Copper that photoresist does not cover is removed with a strong and effective chemical. As soon as it is removed, it leaves just the needed copper for your PCB.

Lamination of the PCB Layers

Once the layers are free from defects, then you can fuse them. You can achieve this process in two spes, which include the lay-up and the laminating step.

Drilling

Before you drill, the drill spot is located with an x-ray machine. This helps in securing the PCB stack.

PCB Plating
This process helps in fusing the different PCB layers making use of a chemical.

Imaging and Plating of the Outer Layer

By doing this you are guarding the copper found on the outer layer by applying the photoresist.

Final Etching

To protect the copper during the process, a tin guard is utilized. This gets rid of unwanted copper. This also ensures properly established PCB connections.

Applying Solder Mask

After cleaning the PCB panels, you apply an ink epoxy with a solder mask.

Completing Silk-screening and PCBs

PCB plating is done to make sure that the soldering of the components can be achieved. The process of screening points all the important information on the PCB.

Electrical and Testing Reliability

To ensure functionality, the technician carries out tests on several areas of the PCB.
Profiling and Cutting
According to the requirements of the customer, different PCBs are cut from the initial panel. Then the inspection of the board is done, and the errors rectified before it is sent for delivery.

Materials Used in the Manufacturing of Multilayer PCB

The different materials utilized in manufacturing multilayer PCBs are boards, copper foil, resin system, substrate, vias, infused fiberglass sheet. Using an alternating sandwich, you can laminate these materials together.

All the planes of copper are etched and the plating through of all internal vias is done before the layers.

Multilayer PCB: Benefits

Multilayer PCBs come with lots of great benefits. Some of them include:

    • Higher assembly density
    • Provision of high speed and high capacity, as a result of their electrical properties
    • Weight reduction of devices
    • Elimination of connectors needed for multiple separate PCBs, thereby simplifying its construction.

Multilayer PCB: Uses

Multilayer PCBs can be utilizes in many areas. Let’s consider some of them.

    • They are used in manufacturing CAT scan, heart monitors, and modern x-ray equipment.
    • Utilized in the production of high-speed circuits due to their functionality and durability
    • Used for headlight switches and onboard computers due to their high functionality and heat resistant ability
    • The running of machinery and industrial control system utilize them due to their small size and durability.
    • Consumer electronics such as microwaves and smartphones also make use of multilayer PCBs as a result of their small size and functionality.
    • Satellite applications, GPS, and signal information, also make use of multilayer PCBs
    • Used in the production of computer electronics that are utilized in motherboard servers due to its performance and
space-saving attributes.

Identifying a Multilayer PCB

You can identify a multilayer PCB through the following

    • How your electronic equipment operates briskly, as well as the ultimate board’s operational setting