It is necessary for PCB engineers to understand the main manufacturing difficulties of multilayer PCBs!

Multilayer circuit boards can generally be defined as 10 layers - 20 layers or more high multilayer circuit boards, than the traditional multilayer circuit board processing technology difficulties, its quality, safety and reliability requirements are high, the main method of application in the teaching communication network equipment, high-end servers, medical enterprise electronics, aviation, industrial control, military and other fields. In recent years, the application of information communication, base station, aviation, military and other fields of high-level board market economic demand issues remain strong, and with our China international telecommunication equipment as well as the rapid and healthy development of the market, the high-level board market research prospects are favored.

At present, domestic PCB manufacturers capable of mass production of high-level PCBs are mainly from foreign-funded enterprises or a few domestic enterprises. The production of high-level PCBs not only requires high technology and equipment investment, but also requires the accumulation of experience of technicians and production staff, therefore, high-level circuit boards have a high threshold for entering the enterprise and a long cycle for realizing industrialized production.

The average number of PCB layers has become an important technical indicator to measure the technical level and product structure of PCB enterprises. This paper briefly describes the main processing difficulties encountered in the production of high-level circuit boards, introduces the key production process control points of high-level circuit boards for your reference.

First, the main production difficulties

Compared with traditional PCB products, high-level PCB has a thick board, more layers, circuit and through-hole density, large unit size, thin dielectric layer, etc., the internal space, interlayer alignment, impedance control and reliability requirements are more stringent.

1.1 Difficulty of interlayer alignment

Due to the number of layers of high-level boards, the customer's design side of the multilayer pcb fabrication alignment requirements for each layer is becoming more and more stringent. Usually the interlayer alignment tolerance is controlled at 75 μ m. Considering the large unit size design of the high-level board, the ambient temperature and humidity of the graphic transfer workshop, the misalignment superposition caused by the inconsistent thermal expansion and contraction of different core boards and the interlayer positioning method, it is more difficult to control the interlayer alignment of the high-level board.

1.2 Inner layer line design and production of a difficult point

High thermal weight, high-speed, high-frequency, thick copper, thin dielectric layer and other special materials are used in high-level board, increasing the difficulty of making the internal circuit. Line width is small, open short-circuit increase, micro-short circuit increase, low pass rate; fine line signal layer is more, the inner layer of Aoi leakage probability increases; the inner core board thickness is thin, easy to fold, poor exposure, machine etching is easy to roll the board, high-level boards are mostly system boards, the unit size is larger, the finished product scrapping cost is higher.

1.3 Difficulties in press-fit production

When multiple inner core boards and prepregs are stacked, it is easy to have defects such as slippery boards, delamination, resin voids, and air bubble residues during press-fit production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, voltage resistance, the amount of glue filling, media thickness and other factors to set a reasonable high-level board pressing program. The number of layers is large, so the control of expansion and contraction and the compensation of dimensional coefficients can not be consistent; the thin interlayer insulation layer is likely to lead to failure of the interlayer reliability test.

1.4 Difficulties in drilling production

The use of high heat weight, high speed, high frequency, thick copper special plate increases the difficulty of drilling roughness, burrs and descaling. Multi-layer, cumulative total copper thickness and board thickness, drilling easy to break the knife; BGA dense, narrow hole wall spacing,PCB factory resulting in CAF failure; due to the thickness of the board is easy to cause oblique drilling.

Key production process control

2.1 Material selection

With the development of high-performance, multi-functional electronic components, bringing high-frequency, high-speed signal transmission, so the requirements of electronic circuit materials with relatively low dielectric constant and dielectric loss, as well as low CTE, low water absorption and better high-performance copper-clad materials to meet the processing and reliability requirements of high-level boards. For high-level thick copper circuit boards, select prepregs with high resin content, and the amount of glue flowing out of the prepreg between layers is sufficient to fill the internal graphics. If the insulating dielectric layer is too thick, the finished board will be too thick, while if the insulating dielectric layer is too thin, it will be easy to cause dielectric delamination, high-voltage test failure and other quality problems. Therefore, the selection of the insulation dielectric material is extremely important.

Table 1

2.2 Pressing laminated structure design

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

(1) The manufacturer of the prepreg and core board must be the same. In order to ensure the reliability of the PCB, all layers avoid using a single 1080 or 106 prepreg (unless the customer has special requirements). When the customer has no requirements for the dielectric thickness, it must be ensured that the interlayer dielectric thickness is ≥ 0.09mm in accordance with IPC-A-600g.

(2) When the customer requires a high heat loss material, the core and semi-cured sheets must be made of the appropriate high heat loss material.

(3) If the inner substrate is 3OZ or more, use prepreg with high resin content,but the structural design of all 106 high bonding prepreg sheets should be avoided as much as possible to prevent overlapping of multiple 106 prepreg sheets. As the glass fiber yarns are too fine, the collapse of the glass fiber yarns over a large substrate area will affect the dimensional stability and delamination.

(4) If the customer has no special design requirements, interlayer dielectric layer thickness for tolerance analysis is generally controlled by +/-10%, for an impedance plate, dielectric material thickness dimensional tolerance according to the IPC-4101 C/M level tolerance management control, if the impedance produces an impact on the social factors and the thickness of the substrate as well as related to the manufacturing tolerance of the plate must also be according to the IPC-4101 C/M level tolerance.

2.3 Interlayer alignment control

Inner core plate size compensation accuracy and production size control need to be in the production process for a certain period of time to collect data and historical data experience, in order to accurately compensate for the graphic dimensions of each layer of the high-level plate, to ensure that the consistency of each core plate expansion and contraction. Selection of high precision and high reliability of pre-composite positioning, such as four groove positioning (pinland), hot melt and rivet combination. The key to ensure the quality of the press is to set up a reasonable process and routine maintenance, control the glue and cooling effect, and reduce the inter-ply misalignment. Inter-ply alignment control requires comprehensive consideration of internal compensation values, positioning methods, process parameters and material properties.

2.4 Inner layer line process

Since the resolution of the traditional exposure machine is around 50 μm, LDI can be introduced into the production of high-layer boards to improve the resolution of the graphics, and the resolution can reach around 20 μm.... The alignment accuracy of the traditional exposure machine is 25 μm, and the interlayer alignment accuracy is more than 50 μm. With the use of high-precision alignment exposure machine, the graphic alignment accuracy can be increased to about 15 μm, and the interlayer alignment accuracy can be controlled within 30 μm, which reduces the alignment deviation of the traditional equipment and improves the interlayer alignment accuracy of the high-level sheet.

In order to improve the corrosion capacity of the line, the width of the line and pad (or solder ring) need to be properly compensated in the engineering design, such as back to the line, independent line compensation, in order to make more detailed design considerations. Ensure that the inner line width, line spacing, isolation ring size, independent line and hole spacing of the design compensation is reasonable, otherwise change the engineering design. It is important to pay attention to the design compensation of independent lines and impedance lines, control the parameters in the etching process, and mass production after the first piece is qualified. In order to reduce side corrosion, it is necessary to control the composition of each group of solutions in the optimal range. The traditional etching line equipment has insufficient etching capacity, which can be modified or introduced into high-precision etching line equipment to improve etching uniformity and reduce burr problems such as unclean etching.

2.5 Lamination process

At present, the main methods of interlayer positioning before lamination are:pinning LAM, hot melt, rivets and the combination of hot melt and rivets. Different positioning methods are used for different product structures. For high-level boards, pin LAM or fusion + riveting method is used, and the positioning holes are stamped by OPE press, and the stamping precision is controlled within 25 microns.... When the first piece is melted, the layer deviation should be checked by x-ray, and the layer deviation can be qualified before mass production. When mass production, it is necessary to check whether each board is fused into the unit to prevent subsequent delamination. The laminating equipment adopts high-performance support presses to meet the accuracy and reliability of inter-layer alignment of high-rise panels.

According to the enterprise high-level board laminated structure and the use of materials, research to provide a suitable pressing procedure, set an optimal rate of action of heating and curve, in the conventional multi-layer circuit board pressing procedure, appropriate reduction of pressing board material heating rate, extend as well as high-temperature curing work time, so that the resin can make full use of the flow, curing, and at the same time, in order to avoid the pressing process through the slippery boards, the misalignment of the interlayer and other issues. Material TG value is not the same as the plate, not the same furnace plate; ordinary technical parameters of the plate can not be mixed with special environmental parameters of the plate pressure; to ensure that the coefficient of expansion and contraction for the given reasonableness of the different impact of the plate and the performance requirements of the semi-cure sheet is different, need to use some of the corresponding plate semi-cure sheet parameters of the press, never used the development of special education materials need to be verified by us system process design parameters.

2.6 Drilling process

Due to the excessive thickness caused by the superposition of the plate and copper layer, the wear and tear of the drill bit is serious, and the drill bit is easy to break, for the number of holes, speed and descent speed should be appropriate. The number of layers ≥ 14, hole diameter ≤ 0.2 mm or hole distance ≤ 0.175 mm, hole accuracy ≤ 0.025 mm drilling machine production of flat plate, hole diameter φ4.0 mm and above using segmented drilling, thickness to diameter ratio of 12:1, segmented drilling, positive and negative drilling, hole diameter control in 25μm or less. In order to improve the burr problem of high-level thick copper plates, experiments have proved that the use of high-density buffer plates, the number of laminates is 1, and the drill grinding time is controlled to be within 3 times, which can effectively improve the burr problem, as shown in Figures 2 and 3.

Counter-drilling technology is an effective method to improve the signal integrity of high-frequency, high-speed and massive data transmission of high-level boards. Counter drilling mainly controls the length of the stump, the consistency of the two drilled holes, and the copper lines in the holes. Not all drilling machines have backdrilling function, so it is necessary to upgrade drilling machines (with backdrilling function) or purchase drilling machines with backdrilling function. From the industry literature and mature mass production applications of counter-drilling technology are: the traditional depth control counter-drilling method, the signal feedback layer as the inner layer of the counter-drilling, according to the thickness of the plate than the depth of the counter-drilling, will not be repeated here.

Reliability test

High-level boards are generally system boards, and compared with traditional multilayer boards, system boards are thicker, heavier, larger, and correspondingly have a larger heat capacity. Combined with IPC-6012C, IPC-TM-650 standards and industry requirements, large-scale reliability tests on high-level boards, at 217 ° C (melting point of brazing material) conditions, it takes 50 ~ 90 seconds, high-level boards cooled down relatively slowly, extending the time over the reflow test.