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.
Rayming manufacture multilayer PCB up to 56 Layer , accept pure single material or mixed-press materials :Fr4,Rogers,Polymide,metal Core .
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
high assembly density, small size, and lightweight. Due to the high assembly density, the circuits between components (including components) is reduced, thereby improving reliability; more layers can be used, thereby increasing design flexibility; could design impedance control circuit; a high-speed transmission circuit can be formed; a circuit, magnetic circuit shielding layer can be set, and a metal core heat dissipation layer can also be set to meet the needs of special functions such as shielding and heat dissipation; simple installation and high reliability.
High cost; Long manufacturing time; Request high-reliability testing methods.
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.
Multilayer PCB Design
Why are PCB multilayer boards all even-numbered layers?
- 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.
- 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.)
- 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 (without copper) 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:
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
1.6mm 4 Layer PCB Stackup
1.2mm 4 Layer Stack up
0.8mm 4 Layer Stackup
0.6MM 4 Layer PCB Stackup
0.5mm 0.4mm 4 Layer Stackup
Multilayer Printed Circuit Board
2).6 Layer PCB Stack-Up
2.0mm 1.6mm 6Layer PCB Stack-up
1.2mm 1.0mm 6 Layer PCB Stackup
0.8mm 6 layer PCB stack up
3).Typical Stack-Up of 8 Layer PCB
2.0mm 1.6mm 8 Layer PCB Stackup
1.2mm 1.0mm 8 Layer PCB Stackup
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.
1). Positioning of rivet nails: press the layout of the inner layer board and prepreg with pre-drilled positioning holes
- The sequence is set on the template with rivets and then punched with a nail punch
- Rivets to position
2). Solder joint positioning: set the inner layer board and prepreg with pre-drilled positioning holes according to the layout
- The sequence is set on the template equipped with positioning pins, and then through the heating several
- 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.
B= 4.762 ±0.0254MM
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
- 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.
Standard PCB Prepreg Thickness
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
- Instrument: Electronic balance, accuracy: 0.001 g
- 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
- 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:
- lThe gel time (PG) is large,and the resin has strong fluidity;
- lThe fluidity (RF) is large, and the resin has strong fluidity;
- lThe minimum viscosity (MV) is small,and the resin has strong fluidity;
- lLarge 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:
- There is a lot of resin flow and poor board thickness uniformity (easy to be thick in the middle and thin on the edge).
- White edges appear on the edges of the board due to low resin content.
- Skateboarding easily occurs.
- Easy to show texture.
- The resin content of the board is reduced, which affects the dielectric properties and insulation properties. Also,anti-CAF performance is poor.
- 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:
- Dry board, trunk line, dry point.
- Air bubbles.
- The cohesive force between core material layers is weakened, and the board is prone to bursting.
- The peel strength between resin and copper foil is weakened.
PP storage conditions:
- lStorage temperature: 21±2℃ or below 5℃
- lStorage humidity: below 60%
- lStorage time: 90 days and six months
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.
Controlling the height during lay up can ensure the smooth progress of pressing and achieve maximum productivity.
|Machine||Minimum height||Highest height|
-Panel Lay Up Requirements
- l Boards of different sizes cannot lay up together.
- lBoards with a difference of more than 15mil in thickness cannot lay up together.
- lThe 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.
- lSmall 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
- l Lay up the board in the middle of the whole cycle.
- lAdd dummy lay up on the top and bottom of the production board and reach the lowest height.
-Black Oxide Treatment Board Storage Time
- lThe Black Oxide Treatmentboard is stored in the environment for a long time, and it is easy to absorb water, causing delamination after pressing
Lay Stack up design guidelines
1.inner board design requirements
- lThe 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.
- lThe two layers of dummy pads corresponding to the inner layer board mustbe staggered by half the pad distance to balance the pressure during pressing.
- lThe dummy pads of adjacent rows should be staggered to improve flow resin.
- In the PCB design, if the removed area by the rout is large, a dummy pad is required to be added to the rout area to increase the residual copper rate and reduce the filling. The pad diameter is required to be 4.0mm, and the spacing is required to be 1.5mm.
- When designing in array, if the area removed by rout is relatively large, add a dummy pad in the routarea to increase the residual copper rate and reduce the filling. The pad diameter is required to be 1.5mm, and the spacing is 1.0mm.
- For designswith broken edges, dummy pads must be filled with a pad diameter of 1.5mm and a spacing of 1.0mm.
- lThe two layers of dummy pads corresponding to the inner layer board are required to be staggered by half the pad distance to balance the pressure during pressing.
2.PP Design Requirements
- The centrally symmetric structure can avoid the bending phenomenon caused by structural stress.
- High R/C, thin fabric on the outer layer
- lThe same glass cloth combinationand high resin content are placed on the outer layer.
- lDifferent kinds of glass cloth combinations, based on the principle of symmetry, thin fabrics are placed on the outer layer.
- 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.
- Each layer of prepreg has a reasonable thickness
- lThe thickness andglue content is high. The thickness is not suitable for control
- lSmall thickness, low glue content,and low adhesion
- 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.
- Using the upper and lower interlayer copper foils for electric heating, energy-saving,and low operating cost.
- lSmall temperature difference between the inner and outer layers, uniform heating, good product quality.
- lCycle time is short, about 60minutes.
- lFast heating rate (35/min).
- lThe equipment has a complex structure and high cost.
- lSingle machine output is small.
- lThe pressure is a pneumatic working method, which cannot provide high pressure.
Multilayer PCB Pressure curve
Parameter Control and Function of Pressing
It can help to remove gases, air, and small monomer residues generated by solvent volatilization.
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.
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.
Offset the vapor pressure generated by volatiles. Improve resin fluidity. Increase interlayer adhesion. Prevent deformation due to thermal stress during cooling
- lUse a thickness gauge to measure the thickness of the four corners and a midpoint of each plate
- lThe test point is 50 mm from the edge of the board
- lThickness tolerance: general thickness requirements ± 10%
Current thickness control after pressing
- lThickness tolerance requirements: ± 1.5mil
- lStandard thickness: set the standard thickness with the average thickness of 10 plates
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.
Ten Laminated Processes for Multilayer PCB
Multilayer Circuit Board
Autoclave Pressure Cooker
It is a container filled with high-temperature saturated water vapor, and high-pressure can be applied. The laminated substrate (laminates) sample can be placed in it for a period of time to force moisture into the board, and then take out the sample again. Place it on the surface of high-temperature molten tin and measure its "delamination resistance" characteristics. This word is also synonymous with the pressure cooker, which is commonly used by the industry. In addition, in the multilayer board pressing process, there is a "cabin press method" with high temperature and high-pressure carbon dioxide, which is also similar to this type of Autoclave Press.
Cap Lamination Method
It refers to the traditional lamination method of early multilayer PCB boards. At that time, the "outer layer" of MLB was mostly laminated and laminated with a single-sided copper thin substrate. It was not used until the end of 1984 when the output of MLB significantly increased. The current method is the copper-skin type large or mass pressing method (Mss Lam). This early MLB pressing method using a single-sided copper thin substrate is called cap lamination.
The multilayer board pressing often refers to the wrinkles that occur when the copper skin is improperly handled. Such shortcomings are more likely to occur when thin copper skins are below 0.5 oz and laminated in multiple layers.
It refers to the gentle and uniform sag on the copper surface, which may be caused by the partial protrusion of the steel plate used in pressing. If it shows a neat drop of the faulty edge, it is called "dish down." If these shortcomings are left on the line after copper corrosion, the impedance of the high-speed transmission signal will be unstable, and noise will appear. Therefore, such a defect should be avoided as much as possible on the substrate's copper surface.
Caul Plate Partition
When the multilayer board is pressed, in each opening of the press, there are often many "books" of bulk materials (such as 8-10 sets) of the board to be pressed. Each set of "bulk materials" (Opening) Book) must be separated by a flat, smooth, and hardened stainless steel plate. The mirror stainless steel plate used for this separation is called "caul plate" or "separate plate." At present, AISI 430 or AISI 630 are commonly used.
Foil Lamination Method
Refers to the mass-produced multilayer board, the outer layer of copper foil and film are directly pressed with the inner skin, which becomes the mass lam of the multilayer board. This replaces the early traditional single-sided thin substrate Press legal.
When multilayer boards or substrate boards are laminated, kraft paper is often used as a heat transfer buffer. It is placed between the hot plate (Platern) of the laminator and the steel plate to ease the temperature rise curve closest to the bulk material. Between multiple substrates or multilayer boards to be pressed. Try to minimize the temperature difference of each layer of the sheet; the commonly used specifications are 90 to 150 pounds. Because the fiber in the paper has been crushed after high temperature and high pressure, it is no longer tough and difficult to function, so it must be replaced with a new one. This kind of kraft paper is co-cooked with a mixture of pinewood and various strong alkalis. After the volatiles escape and the acid is removed, it is washed and precipitated. After it becomes pulp, it can be pressed again to become rough and cheap paper material.
Kiss Pressure, Low Pressure
When the multilayer board is pressed and the plates are placed and positioned, they will start to heat and be lifted by the hottest layer from the bottom. Afterward, lift with a powerful hydraulic jack (ram) to press each opening (bulk materials in the opening) and are bonded together. At this time, the combined film (prepreg) begins to gradually soften or even flow, so the pressure used for the top extrusion cannot be too large. This is to avoid slippage of the sheet or excessive flow of the glue. This lower pressure (15-50 PSI) initially used is called "kiss pressure." However, when the resin in the bulk materials of each film is heated to soften and gel and is about to harden. It is necessary to increase to the full pressure (300-500 PSI) so that the bulk materials are tightly combined to form a strong multilayer board.
9, Lay Up stacking
Before pressing multilayer circuit boards or substrates, various bulk materials such as inner layer boards, films and copper sheets, steel plates, kraft paper pads, etc., need to be aligned, aligned, or registered up and down to prepare. Then it can be carefully fed into the pressing machine for hot pressing. This kind of preparatory work is called Lay Up. To improve the quality of multilayer boards, not only this kind of "stacking" work must be carried out in a clean room with temperature and humidity control, but also for the speed and quality of mass production. Generally, the large-scale press method (Mass Lam) in construction, even "automated" overlapping methods are needed to reduce human error. To save workshops and shared equipment, most factories combine "stacking" and "folding boards" into a comprehensive processing unit, so automation engineering is quite complicated.
Mass Lamination (Lamination)
This is a new construction method in which the multilayer board pressing process abandons "aligning pin" and adopts multiple rows of boards on the same surface. Since 1986, when the demand for four and six-layer boards has increased, the pressing method of multilayer boards has been dramatically changed. In the early days, there was only one shipping board on a processing board to be pressed. This one-to-one arrangement has been broken by the new method. It can be changed to one-to-two, one-to-four, or even more according to its size. The row boards are pressed together. The second new method is to cancel the registration pins of various bulk materials (such as inner sheet, film, outer single-sided sheet, etc.). Instead, use copper foil for the outer layer, and pre-make "targets" on the inner sheet. This is to "sweep" out the target after pressing and then drill a tool hole from its center. Then it can be set on the drilling machine for drilling.