Large format/large scale size PCB


Item

Typical Design

Layer count

20

Card Size

7.5”×33”

Total Thickness

0.122”

Min DHS

10mil

A/R

12.2:1

Size accuracy ( fiducial to fiducial ) 

±5mil

Base Material

M6

Registration

Layer-Layer 4mil in total 5mil

Surface treatment

OSP

With the improvement of integrated circuit and the increase of its I/O number, the progress of electronic assembly technology, the development of high-frequency and high-speed digitization of signal transmission, and the upgrade and replacement requirements of high-speed development of electronic equipment, a kind of PCB is gradually moving towards the functions of carrying function subboard, signal transmission and power transmission. The signal processing function is gradually weakening, and this kind of PCB is the backplane. Backplane has always been a specialized product in PCB manufacturing industry. It belongs to the high-end products in PCB.

Backplane, as one of the key components, is widely used in communication, aerospace, supercomputer, medical equipment, military base station and other important occasions.

This kind of board has the characteristics of large size, high layers, large thickness, high aspect ratio of aperture. Backplane (Backplane or Back panel) is a kind of printed circuit board with lines and many rows of jacks, which is mainly used to carry other functional sub-boards and chips for high-speed signal transmission.

Nowadays, there are only a few PCB manufacturers in the industry that can produce large backboard in batches. The production and development of large backplane PCB board is an important reflection of the technical ability of PCB manufacturing enterprises. Large size backplane PCB production is both a challenge and an opportunity for PCB manufacturing enterprises.

 

Following is the key technology research of rocket PCB in the backplane field.

 

Material selection


Generally choose good heat resistance, good reliability of high TG material, TG ≥ 170C

 

Tin-plated+segmented back drill technology

 

The key role of back hole drilling is to reduce the reflection interference of excess copper of hole to signal in the process of signal transmission, so as to ensure the integrity of signal transmission. At present, back drilling is a low-cost method to meet the performance of high-frequency and high-speed circuit boards.

However, in the actual implementation process, due to the characteristics of the back drill itself and its structural requirements and other technical difficulties, it is easy to have quality problems such as copper wire in the hole, blockage, drill breakage and so on.

Place the back drill after the pattern electroplating, the back drill is used to improve the positioning accuracy and heat dissipation of the back drill because of the low density of tin compared with copper. The copper wire and the burr in the hole are removed by etching potion, and the washing section such as high pressure water washing is used, clean the drilling dirt in the hole to prevent plugging.
To adjust the speed of the drill needle, the "1:2:3:4" subsection drilling technology, which requires 10% of depth for the first drilling, 20% for the second drilling, 30% for the third drilling and 40% for the fourth drilling, is adopted.


Large-size backplane etching uniformity enhancement technology

 

The backplane surface is susceptible to the "puddling effect". The accumulation of etchant on the backplane is not easy to drain, which prevents the replenishment of fresh etchant and reduces the jet force borne by the backplane.
The method of "vacuum etching" is adopted to add "Absorbing potion Pipeline" between the spray pipes on the upper board, which is staggered with the spray head pipeline. The absorbing pipeline can timely absorb the accumulated liquid medicine on the backplane surface, providing a new way for the next spray etching.
Fresh etching solution prevents the formation of "puddling effect" and greatly improves the etching effect. A fan-shaped nozzle is used to increase the spraying area and balance the spraying pressure.


Improvement Technology of High Thickness-Diameter Ratio Deep Hole plating Capability

 

The traditional chemical drilling method (KMnO4+NaOH) is easy to cause problems such as inadequate removal of drilling dirt. Conventional copper precipitation process is easy to produce bubbles in the hole due to the large thickness-diameter ratio of the product.In the follow-up, the problems of copper-free hole and thin copper are easy to occur.
Vertical electroplating (gantry line) is used to make the plating. The uniformity of electroplating is more than 90%, and the drilling contamination is removed by plasma.

Full plate plating is made by low current and long time method. The board is dumped up and down 180 degrees in the middle. Reasonable increase of vibration frequency and amplitude facilitates the exchange of medicine and water in the hole.

Packet controlled layer bias technology of Inner Photo Expansion&Shrinkage and OPE Punching

Inter-layer alignment accuracy is one of the main difficulties in the production of circuit boards. Increasing Tool Diagram of Concentric Circle and Expanding and Contracting PAD to Check Interlayer Alignment.

Expansion and shrinkage PAD: A solid PAD is designed for each layer, and the positions of PAD corresponding to all layers are made into copper-free zones, so as to provide light transmission for subsequent X-RAY measurements of expansion and shrinkage.

Due to the difference of expansion and contraction between high frequency board and FR-4 board, the deviation of each core plate is relatively large. In order to better control the deviation between laminates, the stacking matching production is carried out according to the OPE punching deviation of 0.05 mm. The grouping matching production method takes into account the expansion and shrinkage of each layer more practically, some problems such as poor lamination were improved from the self-expanding and shrinking properties of the material.


Improvement of solder mask quality by double exposure and segmented baking

 

Difficulty description
Ordinary exposure, due to the fineness and thickness of the soldermask bridge, the under Cut of the soldermask after development is larger, and the bridge is easy to fall off; the larger the aperture, the more oil in the hole, the more oil spilled at a single point, and the larger the contaminated area; the higher the proportion of oil explosion, the more problems occur such as hollow in the plug hole and the oil explosed on pad.

Improvement solution
After analysis and research, the ink passes through two curing processes- soldermask pre-baking, no explosive oil problem, after development, re-exposure once.
By using double exposure, poor exposure is avoided caused by excessive exposure time or excessive energy leakage

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