PCB News - Why is it so difficult to make multi-layer PCB boards?

Why is it so difficult to make multi-layer PCB boards?

With the development of electronic information technology, multi-layer PCB boards are used in more and more fields. In the traditional sense, we define PCB boards with more than 4 layers as "multi-layer PCB boards", and those with more than 10 layers are called "high multi-layer PCB boards". Whether a high-layer PCB can be produced is an important indicator of the strength of a PCB manufacturer. It can be regarded as a PCB company with top-notch technical strength to produce high-multilayer boards with more than 20 layers. It is said that the production of multi-layer PCB boards is expensive because it is difficult to make, but many customers have not understood the problem of "Why is it so difficult to produce multi-layer PCB boards", so they think that the manufacturers are looking for reasons to deliberately charge indiscriminately. Today, let experienced PCB engineers explain for you: Why is it so difficult to make multilayer PCBs?

1. Main production difficulties

Compared with conventional circuit boards, high-level circuit boards are thicker, have more layers, denser lines and vias, larger cell sizes, thinner dielectric layers, etc., inner layer space, interlayer alignment, impedance control, and reliability Sexual requirements are more stringent.

1. Difficulties in alignment between layers

Due to the large number of high-level boards, the customer design side has more and more stringent requirements for the alignment of each layer of the PCB. Usually, the alignment tolerance between layers is controlled by ±75μm. Considering the large-scale design of the high-level board unit and the ambient temperature and humidity of the graphics transfer workshop, As well as factors such as misalignment and superposition caused by inconsistency of expansion and contraction of different core layers, interlayer positioning methods, etc., it is more difficult to control the degree of alignment between layers of high-rise boards.

2. Difficulties in the production of inner circuits

The high-level board adopts special materials such as high TG, high speed, high frequency, thick copper, thin dielectric layer, etc., which puts forward high requirements for the production of the inner layer circuit and the control of the pattern size. Line width and line spacing are small, open and short circuits increase, short circuit increases, and pass rate is low; there are more fine circuit signal layers, and the probability of missing AOI detection in the inner layer increases; the inner core board is thinner, which is easy to wrinkle and cause poor exposure and etching It is easy to roll the board when it passes the machine; the cost of scrapping the finished product is relatively high.

3. Difficulties in pressing production

When multiple inner core boards and prepregs are superimposed, defects such as sliding plates, delamination, resin cavities and air bubbles are likely to occur during production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, the withstand voltage, the amount of glue and the thickness of the medium, and set a reasonable high-level board pressing program.

4. Difficulties in drilling drilling

The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of drilling roughness, drilling burrs and de-drilling. There are many layers, the cumulative total copper thickness and the plate thickness, the drilling is easy to break the knife; the dense BGA is many, the CAF failure problem caused by the narrow hole wall spacing; the plate thickness is easy to cause the inclined drilling problem.

2. Control of key production processes

1. Material selection

The dielectric constant and dielectric loss of electronic circuit materials are required to be relatively low, as well as low CTE, low water absorption and better high-performance copper clad laminate materials to meet the processing and reliability requirements of high-level boards.

2. Laminated laminated structure design

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

(1) The prepreg and core board manufacturers must be consistent. In order to ensure PCB reliability, avoid using a single 1080 or 106 prepreg for all layers of prepreg (except for special requirements of customers). When the customer has no media thickness requirements, the thickness of the interlayer media must be guaranteed ≥0.09mm in accordance with IPC-A-600G.

(2) When customers require high TG sheets, the core board and prepreg must use corresponding high TG materials.

(3) For the inner substrate 3OZ or above, use prepregs with high resin content, but try to avoid the structural design of 106 high-adhesive prepregs.

(4) If the customer has no special requirements, the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%. For the impedance board, the dielectric thickness tolerance is controlled by IPC-4101 C/M tolerance. If the impedance affects the factor and the thickness of the substrate If relevant, the sheet tolerance must also be in accordance with IPC-4101 C/M tolerance.

3. Interlayer alignment control

The accuracy of the inner core board size compensation and production size control requires a certain period of time to collect data and historical data experience in the production to accurately compensate the size of each layer of the high-layer board to ensure that the core board of each layer expands and shrinks. consistency.

4. Inner circuit technology

Since the resolution capability of the traditional exposure machine is about 50μm, for the production of high-level boards, a laser direct imaging machine (LDI) can be introduced to improve the graphics resolution capability, and the resolution can reach about 20μm. The alignment accuracy of the traditional exposure machine is ±25μm, and the interlayer alignment accuracy is greater than 50μm; with the high-precision alignment exposure machine, the graphic alignment accuracy can be increased to about 15μm, and the interlayer alignment accuracy can be controlled within 30μm.

5. Pressing process

At present, the positioning methods between layers before pressing mainly include: four-slot positioning (Pin LAM), hot melt, rivet, hot melt and rivet combination, and different product structures adopt different positioning methods. For the high-level board, the four-slot positioning method, or the fusion + riveting method is used, the positioning hole is punched out by the OPE punching machine, and the punching accuracy is controlled at ±25μm.

According to the laminated structure of the high-rise board and the materials used, study the appropriate pressing procedure, set the best heating rate and curve, appropriately reduce the heating rate of the laminated sheet, extend the high temperature curing time, make the resin fully flow and cure, and avoid pressure Problems such as sliding plate and interlayer dislocation during the closing process.

6. Drilling technology

Due to the superposition of each layer, the plate and copper layer are too thick, which will cause serious wear to the drill bit and easily break the drill bit. The number of holes, falling speed and rotation speed are appropriately reduced. Accurately measure the expansion and contraction of the board to provide accurate coefficients; the number of layers is ≥14, the hole diameter is ≤0.2mm, or the hole-to-line distance is ≤0.175mm, and the hole position accuracy is ≤0.025mm. The hole diameter is larger than φ4.0mm. Step drilling, with a thickness-to-diameter ratio of 12:1, adopts step-drilling and positive and negative drilling methods; to control the drilling front and hole thickness, high-rise boards should be drilled with a new drill or a one-grind drill as far as possible, and the hole thickness should be controlled within 25um.

Three, reliability test

The high-level board is thicker, heavier, and larger in unit size than the conventional multi-layer board, and the corresponding heat capacity is also larger. When welding, it needs more heat and the welding high temperature time is longer. It takes 50 to 90 seconds at 217°C (melting point of tin-silver-copper solder), and the cooling speed of the high-layer board is relatively slow, so the time for the reflow soldering test is prolonged.

The above is the answer of "Why is it so difficult to make multilayer PCB board" explained by experienced PCB engineers. Through the above sharing, I believe you must have a deeper understanding of the production of multilayer PCB board. At the same time, you I also understand why the production price of multi-layer PCB boards is so expensive! Indeed, the PCB board production process is complicated, and the production of multi-layer PCB boards is even more difficult. "You get what you pay for" is the truth. I hope the above sharing can help you.