The Main Difficulties in Multilayer PCB Manufacturing Process

The Multilayer PCB is More Expensive 

The multilayer PCBs higher than 10 layers are generally defined as high-level boards, which are mainly used in communication equipment, high-end servers, medical electronics, industrial control, military, and other fields. In recent years, the demand for multilayer PCBs in the fields of application communication, base station, aviation, military and so on is still strong, and with the rapid development of the 5G application, the multilayer PCBs market prospects are promising.

The multilayer high-level boards are more difficult to process than traditional circuit boards and have high quality and reliability requirements.

The production of multilayer PCBs not only requires high technology and equipment investment but also requires the accumulation of experience of technicians and production personnel. Therefore, multilayer PCB enters the enterprise with a high threshold and is commonly more expensive than low layer-count PCB.

The Difficulties in Multilayer PCB Manufacturing Process

Compared with the characteristics of conventional low-layered PCBs, multilayer PCB have the characteristics of thicker plates, more layers, denser lines and vias, larger cell sizes, thinner dielectric layers, etc., inner space, interlayer alignment, impedance control, and reliability requirements are more stringent.

1.1 Difficulties in alignment between layers

Due to the high number of layers, the customer design end is more and more strict on the alignment of the PCB layers. Usually, the inter-layer alignment tolerance control within 75μm, considering the multilayer PCB unit size design, the graphics transfer workshop environment. The temperature and humidity, as well as the misalignment and interlayer positioning methods caused by the inconsistency of different core layers, make the interlayer alignment control of the multilayer boards more difficult.

1.2 Difficulties in the production of inner layers

The multilayer PCB adopts special materials such as high TG, high speed, high frequency, thick copper and thin dielectric layer, which puts high requirements on the inner layer circuit production and graphic size control, such as the integrity of impedance signal transmission, which increases the difficulty of inner layer circuit fabrication. The line width is small, the open circuit is increased, the short is increased, the pass rate is low; the signal line of the fine line is more, the probability of the AOI leak detection is increased; the thickness of the inner core plate is thin, the fold is easy to cause poor exposure, etching, it is easy to roll when passing the machine; most of the multilayer PCBs are system boards, the unit size is large, and the cost of scrapping the finished product is relatively high.

1.3 Pressing and manufacturing difficulties

A plurality of inner core sheets and prepregs are stacked, and defects such as sliding sheets, delamination, resin voids, and bubble residues are easily generated during press production. When designing the laminated structure, it is necessary to fully consider the heat resistance, withstand voltage, the amount of glue and the thickness of the material and set a reasonable multilayer PCB pressing program. The number of layers is large, the amount of expansion and contraction control and the compensation of the size factor cannot be consistent, the interlayer insulation layer is thin, which easily leads to the failure of the interlayer reliability test. 

Figure 2 is a defect diagram of the blasting delamination after the thermal stress test.

1.4 Difficulties in drilling

The use of high TG, high speed, high frequency, thick copper special substrates increases the difficulty of drilling roughness, drilling burrs, and drilling. The number of layers is large, the total copper thickness and substrate thickness are accumulated, the drill is easy to break the knife. The dense BGA is more, the CAF failure problem caused by the narrow hole wall spacing, the substrate thickness is easy to cause the oblique drilling problem.

Difficulties in The Manufacturing Process Control

2.1 Material selection

With the development of high-performance and multi-functional electronic components, and high-frequency, the high-speed development of signal transmission, electronic circuit materials are required to have low dielectric constant and dielectric loss, low CTE, and low water absorption rate and better high-performance CCL material to meet the processing and reliability requirements of high panel. 

For high-rise thick copper circuit boards, the high-resin prepreg is used. The inter-layer prepreg has enough glue to fill the inner layer. The insulating medium is too thick and the finished plate is too thick. On the contrary, the insulating medium is thin, which is easy to cause. Quality problems such as media stratification and high voltage test failure are extremely important for the selection of dielectric materials.

2.2 Press-fit laminated structure design

The main factors considered in the design of the laminated structure are the heat resistance, withstand voltage, the amount of glue and the thickness of the dielectric layer. The following main principles should be followed.

1. Prepregs must be consistent with core board manufacturers. In order to ensure the reliability of the multilayer PCB, all layers of prepreg avoid the use of a single 1080 or 106 prepreg (except for special requirements). When the customer does not have the thickness requirement of the medium, the thickness of the medium between the layers must be guaranteed by IPC-A-600G.

2. When customers require high TG sheets, the core and prepregs must be made of the corresponding high TG material.

3. For the inner layer of 3OZ or above, use a high resin content prepreg, such as 1080R/C65%, 1080HR/C 68%, 106R/C 73%, 106HR/C76%, but try to avoid all the use of 106 high-elastic prepreg, which is designed to prevent multiple sheets of 106 prepreg from being superposed. Because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large substrate area, which affects dimensional stability and delamination.

4. If there is no special requirement from the customer, the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%. For the impedance multilayer board, the dielectric thickness tolerance is controlled according to the IPC-4101 C/M tolerance, if the impedance influence factor and the substrate thickness. For related, sheet tolerances must also be in accordance with IPC-4101 Class C/M tolerances.

2.3 Inter-layer alignment control

The accuracy of the inner core size compensation and the production size control require the data and historical data experience collected in the production in a certain period of time to accurately compensate the pattern size of each layer of the multilayer PCB to ensure the expansion and contraction of each layer of the core board consistency. Choose high-precision, high-reliability pre-bonding interlayer positioning methods, such as four-slot positioning (Pin LAM), hot melt and rivet combination. Setting the appropriate press-fit process and daily maintenance of the press is the key to ensuring the quality of the press, controlling the flow of glue and cooling, and reducing the problem of interlayer misalignment. Inter-layer alignment control needs to take into account factors such as inner layer compensation value, press-fit positioning mode, press-fit process parameters, material properties, and other factors.

2.4 Inner layer circuit process

Since the resolution capability of the conventional exposure machine is around 50μm, for the production of multilayer PCBs, a laser direct imaging machine (LDI) can be introduced to improve the graphics analysis capability, and the resolution capability is about 20μm. The conventional exposure machine alignment accuracy is 25μm, the inter-layer alignment accuracy is greater than 50μm. Adopting a high-precision alignment exposure machine, the graphic alignment accuracy can be improved to about 15μm, and the interlayer alignment accuracy is controlled within 30μm, which reduces the alignment deviation of the conventional equipment and improves the interlayer alignment accuracy of the high-level board.

In order to improve the circuit etching capability, it is necessary to give appropriate compensation to the width of the circuit and the pad (or solder ring) in engineering design and to make a more detailed design of the compensation amount of special graphics such as return lines and independent lines. 
Confirm the inner line width, line spacing, isolation ring size, independent line, hole to line distance design compensation is reasonable, otherwise change the engineering design. If there are impedance and inductive design requirements, pay attention to the independent line and impedance line design compensation. If the parameters are well controlled during etching, the first piece can be mass-produced after confirmation. In order to reduce the etching side etching, it is necessary to control the composition of each group of the etching liquid in an optimum range.

Conventional etching line equipment has insufficient etching ability, which can be technically modified or introduced into high-precision etching line equipment to improve etching uniformity, reduce etching burrs, and etchlessness.

2.5 Pressing process

At present, the interlayer positioning methods before pressing include: four-slot positioning (Pin LAM), hot-melt, rivet, hot-melt and rivet combination, and different product structures adopt different positioning methods. For the multilayer PCB, the four-slot positioning method (Pin LAM) is used, or the fusion + riveting method is used, and the OPE punching machine punches out the positioning hole, and the punching precision is controlled at 25μm. When fusing, the first board must be X-ray inspection layer, and the layer is qualified to make batches. In batch production, it is necessary to check whether each board is melted into the unit to prevent subsequent delamination, and the press-fit equipment adopts high-performance matching. The press meets the interlayer alignment accuracy and reliability of the multilayer PCB.

According to the multilayer laminate structure and the materials used, study the appropriate pressing procedure, set the optimal heating rate and curve, and appropriately reduce the heating rate of the pressed sheet and prolong the high temperature in the conventional multilayer circuit board pressing procedure. 
The curing time allows the resin to flow and solidify sufficiently while avoiding problems such as slippage and interlayer misalignment during the pressing process. Laminates with different TG values cannot be the same as grate ones, laminates with common parameters cannot be mixed with the ones with special parameters, ensure the reasonableness of the expansion and contraction coefficient, the performance of different laminates and prepregs is different, the corresponding laminates are required and the special materials that have never been used need to verify the process parameters.

2.6 Drilling process

Due to the superposition of the layers, the board and the copper layer are extremely thick, and the bit is seriously worn, and the drill is easily broken, and the number of holes, the falling speed, and the rotation speed is appropriately lowered. Accurately measure the plate's expansion and contraction, providing accurate coefficients. In order to improve the drilling burr problem of the multilayer thick copper boards, the high-density pad is used in batch verification, the number of stacked plates is one piece, and the bit grinding time is controlled within 3 times, which can effectively improve the drilling burr, as shown in the figure 3.

For the multilayer PCBs for high-frequency, high-speed, and massive data transmission, back-drilling technology is an effective way to improve signal integrity. The back drill mainly controls the length of the residual stub, the consistency of the holes in the two holes, and the copper wire in the hole. Not all drilling machine equipment has a back-drilling function, a technical upgrade of the drilling machine equipment (with back-drilling function), or the purchase of a drilling machine with a back-drilling function. 

All of the above was edited by PS electronics and just for your reference. PS is a 26-year multilayer PCB manufacturer, don't hesitate to contact us for any question, you will get solved within a few hours.