Multilayer PCB

Multilayer PCB stackup design and its lamination process

A multilayer PCB or multilayer printed circuit board is a circuit board consisting of two or more conductive layers (copper layers). The copper layers are pressed together by the resin layer (prepreg). Due to the complex manufacturing process of multi-layer boards, low output and difficult rework, their prices are higher than those of single-layer and double-layer boards.

Due to the increase in the packaging density of integrated circuits, resulting in a high concentration of interconnect lines, which requires the use of multi-layer PCB. Unforeseen design issues such as noise, stray capacitance, and crosstalk arise in printed circuit layouts. Therefore, the printed circuit board design must minimize the length of the signal lines and avoid parallel routing. Obviously, in a PCB single-sided board, even a double-sided board, due to the limited number of cross circuits that can be realized, these requirements cannot be met. In the case of a large number of interconnections and crossover requirements, the PCB board must be extended to more than two layers to achieve satisfactory performance. So a multi-layer circuit board appeared. Therefore, the original intention of manufacturing multi-layer circuit boards is to provide greater freedom in selecting appropriate routing paths for complex and noise-sensitive electronic circuits.

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

Advantages and disadvantages of multilayer PCB

High assembly density

Multilayer PCBs increase their density by layering. This increased density allows for more functionality, increased capacity and speed despite the smaller PCB size.

Small size

Multilayer PCBs reduce overall size by increasing the number of layers to increase the surface area of the board. This will allow higher volume multilayer PCBs to be used in smaller devices, whereas high volume single layer PCBs must fit into larger products.

Light

A multi-layer PCB can do the same work as multiple single-layer boards, but is smaller, has fewer connected components, and weighs less. This is an important consideration for small electronics where weight is a concern.

High cost

Multi-layer PCB is difficult to manufacture and has a high probability of failure, so it is more prone to errors. In addition, multi-layer boards require more material costs, so it is more expensive =

Long manufacturing time

Multilayer PCBs require multiple complex steps to complete the stackup, so it takes longer, which is not good for electronics manufacturers who need fast turnaround

Many tests are required

Due to the difficulty of manufacturing multi-layer boards, it needs multiple verifications to ensure its quality, such as AOI, XRAY, electrical testing, etc.

Multilayer printed circuit is the product of the development of electronic technology in the direction of high speed, multi-function, large capacity and small volume. With the continuous development of electronic technology, multilayer printed circuits are developing rapidly in the following directions: technologies such as high density, high precision, multilayer, tiny line holes, blind buried holes, and high board thickness-to-aperture ratio meet market demand.

Why are PCB multilayer boards even-numbered layers?

In the PCB manufacturing factory, the four-layer board uses a core wire with one layer of copper foil on one side and a three-layer board with one layer of copper foil on one side. They must be pressed together.
The difference in process cost between the two is that the four-layer board has an extra layer of copper foil and bonding layer. The cost difference is not significant. When the PCB factory quotes, it usually quotes in even numbers. Also, tiers 3-4 are usually quoted as grades. (For example: if you design a 5-layer board, the other party will quote the price of a 6-layer board, which means that the price you design for a 3-layer board is the same as the price you design for a 4-layer board.)
In terms of PCB 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 relatively large. At that time, the warpage will exceed this standard, affecting the reliability of SMT assembly and the whole product. Therefore, designers should not design odd-numbered plies. Even if an odd number of layers is required, it will be designed as a pseudo-even number of layers. That is, design 5 floors into 6 floors, and design 7 floors into 8 floors.

Calculation method for multilayer PCB stackup:

A: Thickness of inner layer E: Thickness of inner copper foil X: Finished board thickness

B: Thickness of PP sheet F: Thickness of outer copper foil Y: Finished PCB tolerance

1. Calculate the upper and lower limits of pressing:

Usually tinplate: upper limit -6MIL, lower limit -4MIL

Gold plating: upper limit -5MIL, lower limit -3MIL

For example tin plate: upper limit=X+Y-6MIL lower limit=XY-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 in-cut material of the above conventional four-layer board is 0.4MM smaller than the finished board, and a single 2116 PP board is used for lamination. For the special inner layer copper thickness and outer layer copper thickness greater than 1OZ, the copper thickness should be considered when selecting the inner layer material.

2. Calculate the pressure tolerance:

Upper limit = thickness of finished product + online tolerance value of finished product - [thickness of copper plating, thickness of green oil characters

(conventional 0.1MM)] - theoretically calculated thickness after lamination

Lower limit = thickness of finished product - tolerance value of finished product off-line - [thickness of electroplated copper, thickness of green oil characters

(conventional 0.1MM)] - theoretically calculated thickness after lamination

3. Common PP sheet types:

Generally do not use two PP sheets with high resin content together. If the inner copper skin is too small, please use PP board with high resin content. 1080 PP sheet has the highest density and low resin content. Try not to press the leaflet. Two sheets of 2116 and 7630 PP sheets can be pressed into thick copper sheets above 2OZ. This layer cannot be pressed by a single sheet of PP. 7628 PP board can be pressed in single sheet, 2 sheets, 3 sheets, and up to 4 sheets.

Instructions for calculating the theoretical thickness of multi-layer PCB boards after lamination

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

4. Typical recommendations for multilayer stackups

1): 4 Layer PCB Stack-Up

2).6 Layer PCB Stack-Up

3).Typical Stack-Up of 8 Layer PCB

Introduction to multilayer PCB lamination process

Lamination is to use high temperature and high pressure to heat and melt the prepreg to make it flow and become a solidified sheet. Then one or more internally etched boards (black oxide treatment) and copper foil are processed into multi-layer boards.
The process also includes stacking before lamination, drilling positioning holes, and contour routing after laminating multilayer boards.

1. Lamination process

Note: For PCB stackups of 6 layers and above, two or more inner layers must be pre-positioned so that the holes and circuits of the different layers are properly aligned.

2. Positioning method

1) Rivet positioning: Press the inner layer and prepreg to form the layout of the pre-drilled positioning holes

The order is set on the template with rivets and then punched with a nailer
Rivet positioning

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

The order is set on the template equipped with positioning pins, and then passed through the heating several times
Fixing point, using prepreg to heat, melt and solidify

We are currently using solder joint location - RBM

Pre-drilling positioning holes on the inner board, the current method we use is: punching 4 slot holes on the four sides of the board, two as a group, respectively positioned in the X/Y direction, one of which is an asymmetrical design. The purpose is to prevent the reaction from starting.

A=7.112±0.0254MM

B= 4.762 ±0.0254MM

RBM - Quality Control After Potential Issues

1) Interlayer offset: Poor RBM positioning or poor condensation at the heating point, resulting in interlayer displacement after pressing, and open circuit or short circuit due to misalignment of each layer after drilling.

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 cores are reversed: the order of the inner cores is wrongly placed during RBM, which affects the quality of the customer's panel.

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

3.Introduction to Prepreg

Prepreg refers to fiberglass or other fibers impregnated with resin. After partial polymerization, the resin molecules are slightly cross-linked and can be softened by heating. However, it cannot be completely melted.

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). Definition of resin content: the weight of resin in the prepreg as a percentage of 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) When the basis weight of glass cloth is constant, TW can be used as a control index

Instrument: electronic balance, precision: 0.001g
Sample: 4"X 4"X 4pcs

Resin Content Description

Prepreg Resin Content (RC)

lRC is mainly related to the thickness of the laminate.
l Low RC, thin board;
l The left, middle and right deviations of RC are large, and the plate thickness uniformity is poor.
After controlling the RC of the prepreg, the required thickness can be obtained after pressing, which can increase the Cpk value of the thickness.

Resin content and PP thickness comparison table

Calculation of thickness after resin filling:

Thickness after PP pressing

Thickness = theoretical thickness of single PP-filling loss
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;
High fluidity (RF), strong resin fluidity;
The minimum viscosity (MV) is small, and the resin has strong fluidity;
Large flow window (FW), strong resin fluidity;

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:

There is a lot of resin outflow, and the thickness uniformity of the plate is poor (it is easy to be thick in the middle and thin at the edge).
Due to the low resin content, white fringing appears on the edge of the board.
Skateboarding happens so easily.
Easy to show texture.
The resin content of the board is reduced, which affects the dielectric properties and insulation properties. In addition, the anti-CAF performance is poor.
The internal stress of the plate increases, and it is easy to distort and deform after being compressed.

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

Dry boards, dry lines, dry points.
bubble.
The bonding force between the core material layers is weakened, and the board is easy to burst.
The peel strength between resin and copper foil is weakened.

PP storage conditions:

Storage temperature: 21±2°C or below 5°C
Storage humidity: below 60%
Storage time: 90 days and six months

Stack control points

-Place the board along the laser beam

We currently have two types of layups. Controlling the consistency of the stack can ensure uniform force during pressing and avoid white edges due to pressure loss. This requires adjusting and fixing the position of the laser beam while preparing the layup. Lay-up of circuit boards with laser beams in build-up production

Height Control

Controlling the height of feeding can ensure the smooth progress of pressing and achieve the maximum production efficiency.

Panel Lamination Requirements

Planks of different sizes cannot be stacked together.
Boards with a thickness difference of more than 15 million cannot be stacked together.
Plates of different thicknesses are stacked together, the thermocouple must be placed in the middle of the thin plates, and the ADARA staff should be informed to increase the curing time by 10 minutes.
Small boards (within 10 pieces) of different copper foil thicknesses can be stacked together by cutting the copper foil, and a PE release film must be placed between the board and the conductive copper foil during production.

separate laying requirements

Place planks in the middle of the entire loop.

Add dummy layers at the top and bottom of the production board and up to the minimum height.

separate laying requirements

The black oxide treated board is stored in the environment for a long time, it is easy to absorb water, resulting in delamination after pressing

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