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

  1. Instrument: Electronic balance, accuracy: 0.001 g
  2. 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
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