The production process and sampling difficulties of multi-layer circuit boards

At present, the production of circuit boards mostly relies on the subtractive method, which involves subtracting excess copper foil from the raw material copper-clad laminate to form conductive patterns.

The production process of PCB multi-layer circuit board

The reduction method mostly uses chemical corrosion, which is economical and efficient. Only chemical corrosion has no differential attack, so it is necessary to protect the required conductive pattern by coating a layer of anti-corrosion agent on the conductive pattern, and then reducing the corrosion of unprotected copper foil. In the early days, anti-corrosion agents were printed in the form of lines using screen printing with anti-corrosion ink, hence the name "printed circuit board". However, with the increasing precision of electronic products, the image resolution of printed circuits cannot meet product requirements, leading to the use of photoresist as an image analysis material. Photoresist is a photosensitive material that is sensitive to a certain wavelength of light source, forming a photo chemical reaction with it to form a polymer. Simply use a graphic film to selectively expose the graphic, and then peel off the unpolymerized photoresist with a developing solution (such as 1% sodium carbonate solution) to form a graphic protective layer.

The interlayer conductivity function is also achieved through metalized holes, so drilling operations are required during the PCB manufacturing process, and metalized electroplating operations are performed on the holes to ultimately achieve interlayer conductivity.

Taking a conventional six layer PCB as an example, the production process is as follows:

1、 First, make two non perforated double-sided panels

Cutting (raw material double-sided copper-clad laminate) - Inner layer pattern production (forming pattern corrosion-resistant layer) - Inner layer etching (subtracting excess copper foil)

2、 Glue and press the two pre made inner core boards with epoxy resin fiberglass semi cured sheets. Rivet the two inner core boards to the semi cured sheets, and then lay a copper foil on both sides of the outer layer. Use a press machine to complete the pressing under high temperature and high pressure to bond them together. The key material is a semi cured sheet with the same composition as the raw material, which is also epoxy resin glass fiber. However, it is in an incomplete cured state and liquefies at temperatures of 7-80 degrees. A curing agent is added to it, which crosslinks with the resin and solidifies at 150 degrees, and is no longer reversible thereafter. Through such a semi-solid liquid solid conversion, adhesive bonding is achieved under high pressure.

Both in terms of design and manufacturing, PCB multilayer boards are more complex than single and double-layer boards, and one may encounter some problems if not careful. So, what difficulties should we avoid in PCB multilayer circuit board sampling?

Difficulties in sampling multi-layer impedance boards

1. Difficulties in interlayer alignment

Due to the large number of layers in multi-layer circuit boards, users have increasingly high requirements for PCB layer calibration. Usually, the alignment tolerance between layers is controlled at 75 microns. Considering the large size of multi-layer circuit board units, high temperature and humidity in the graphics conversion workshop environment, dislocation overlap caused by inconsistency of different core boards, and interlayer positioning methods, the centering control of multi-layer circuit boards is more difficult.

2. Difficulties in Internal Circuit Manufacturing

Multilayer circuit boards use special materials such as high TG, high speed, high frequency, thick copper, and thin dielectric layers, which place high demands on internal circuit fabrication and graphic size control. For example, the integrity of impedance signal transmission increases the difficulty of internal circuit manufacturing. The width and line spacing are small, resulting in an increase in open and short circuits, an increase in short circuits, and a low pass rate; Multiple thin signal layers increase the probability of AOI leakage detection in the inner layer; The inner core board is thin, prone to wrinkling, has poor exposure, and is prone to curling during etching; High level plates are mostly system boards with larger unit sizes and higher product scrap costs.

3. Difficulties in Compression Manufacturing

Many inner core boards and semi cured boards are stacked, which can easily lead to defects such as slipping, delamination, resin voids, and residual bubbles in stamping production. In the design of laminated structures, the heat resistance, pressure resistance, adhesive content, and dielectric thickness of materials should be fully considered, and a reasonable material pressing scheme for multi-layer circuit boards should be formulated. Due to the large number of layers, the consistency of expansion and contraction control and size coefficient compensation cannot be maintained, and the thin interlayer insulation layer is prone to failure in interlayer reliability testing.

4. Difficulties in drilling production

The use of high TG, high-speed, high-frequency, thick copper special plates increases the difficulty of drilling roughness, drilling burrs, and removing drilling stains. Multiple layers, cumulative total copper thickness and plate thickness, easy to break drilling tools; The problem of CAF failure caused by dense BGA and narrow hole wall spacing; Due to the thickness of the board, it is easy to cause drilling problems.