1. Overview:

HDI board refers to High Density Interconnect, which is a relatively new technology developed in the PCB industry at the end of the 20th century.

The traditional PCB board drilling is affected by the drill, when the drilling hole diameter reaches 0.15mm, the cost is already very high, and it is difficult to improve again. The drilling of HDI boards no longer relies on traditional mechanical drilling, but uses laser drilling technology. (So it is sometimes called laser board.) The hole diameter of HDI board is generally 3-5mil (0.076-0.127mm), the line width is generally 3-4mil (0.076-0.10mm), and the size of the pad can be greatly improved. As a result, more circuit distribution can be obtained per unit area, and high-density interconnection comes from this.

The emergence of HDI technology has adapted and promoted the development of the PCB industry. This makes it possible to arrange denser BGA, QFP, etc. in the HDI board. At present, HDI technology has been widely used, among which the first-order HDI has been widely used in PCB production with 0.5PITCH BGA.

The development of HDI technology promotes the development of chip technology, and the development of chip technology in turn promotes the improvement and progress of HDI technology.

At present, 0.5PITCH BGA chips have been gradually adopted by design engineers, and the solder fillets of BGA have also gradually changed from the form of hollowing out the center or the form of center grounding to the form of signal input and output in the center that needs to be routed.

Therefore, the first-order HDI can no longer fully meet the needs of designers, so the second-order HDI has begun to become the common concern of R&D engineers and PCB manufacturers. The first-order HDI technology refers to the hole-forming technology in which the laser blind hole only connects the surface layer and its adjacent sub-layers. The second-order HDI technology is an improvement on the first-order HDI technology, which includes the laser blind hole directly drilled from the surface. There are two forms of drilling to the third layer, and drilling from the surface layer to the second layer and then from the second layer to the third layer. The difficulty is far greater than that of the first-order HDI technology.

two. Material:

1. Material Classification
a. Copper foil: basic material composed of conductive graphics
b. Core board (CORE): The skeleton of the circuit board, the double-sided copper-clad board, can be used for the double-sided board made of the inner layer.
c. Prepreg: an indispensable material for the production of multi-layer boards, the adhesive between the core board and the core board, and at the same time play the role of insulation.
d. Solder mask ink: It can prevent soldering, insulation and corrosion of the board.
e. Character ink: marking function.
f. Surface treatment materials: including lead-tin alloy, nickel-gold alloy, silver, OSP, etc.

2, laminated Insulation material
2.1 List of plates used by SYE< /span>

2.2. HDI insulation layer material

2.2.1 List of SYE HDI insulation materials 

2.3 Introduction of special materials:
The special material RCC used for HDI insulation:
Resin Coated Copper
Resin Coated Copper refers to coating a special resin film on the electroplated copper foil. This film can completely cover the inner circuit to form an insulating layer.
There are two main types: B stage (Mitsui) and B+C stage (Polyclad)
 
Features:
*without glass dielectric layer , Easy to laser and plasma micro-hole forming.
*Thin dielectric layer.
*Extremely high peel strength.
*High toughness, easy to operate.
*Smooth surface, suitable for micro-narrow circuit etching.

Resin Coated Copper: Generally speaking, the laser drilling of HDI boards is to make holes on the coated copper foil. The shape of the aperture is not exactly the same as the shape of a hole drilled by a general machine. The shape of the laser-drilled hole is an inverted trapezoid. In general mechanical drilling, the shape of the hole is columnar. Considering the energy and efficiency of laser drilling, the aperture size of the laser hole cannot be too large. Generally, it is 0.076-0.10 mm.

Other materials required for HDI board are: sheet material; prepreg and copper foil, etc., there are no special requirements. Since the current of the laser board is generally not too large, the copper thickness of the circuit is generally not too thick. The inner layer is generally 1 ounce, and the outer layer is generally half ounce of bottom copper plated to 1 ounce of finished copper thickness. The thickness of the sheet is generally thin. And because RCC only contains resin and no glass fiber, the hardness/strength of HDI board using RCC is generally worse than other PCBs of the same thickness.

2.4 The general structure of the current HDI board:
 
1-HDI


Non stacked 2-HDI


Stacked But Non Copper filled 2-HDI


Stacked & Copper filled 2-HDI

2.5 The effect of different HDI insulation layer materials
These are different types of first-order blind hole slices (A)
 
RCC
 
FR4（1080）
These are the slice images of second-order HDI blind holes
 
RCC


FR4

Three. Process:

Below we will take a 2+4+2 8-layer board as an example to illustrate the HDI production process:
 
1. Cutting (CUT)
Cutting is the process of cutting the original copper-clad board into boards that can be made on the production line.

First let’s understand Several concepts:
1. UNIT: UNIT refers to the unit graphics designed by the customer.

2. SET: SET means In order to improve efficiency and facilitate production, the customer puts multiple UNITs together to form a whole graphic. It includes unit graphics, process edges, etc..

3. PANEL: PANEL refers to a board formed by putting together multiple SETs and adding tool board edges to improve efficiency and facilitate production when PCB manufacturers produce. The large materials we purchased come in the following sizes: 36.5 INCH × 48.5 INCH, 40.5 INCH × 48.5 INCH, 42.5 INCH × 48.5 INCH and so on. As a PCB design engineer, a design engineer, and a PCB production engineer, utilization is a common concern for everyone.

2. Inner layer Dry film: (INNER DRY FILM)

The inner layer dry film is the process of transferring the inner layer circuit pattern to the PCB board.
In PCB production, we will mention the concept of graphics transfer, because the production of conductive graphics is the basis of PCB production. Therefore, the graphics transfer process is of great significance to PCB production.

The inner layer dry film includes multiple processes such as inner layer film, exposure and development, and inner layer etching. The inner layer film is on the copper plate A special photosensitive film is pasted on the surface. This film will be cured when exposed to light, forming a protective film on the board. Exposure and development means that the board with the film will be exposed, and the light-transmitting part will be cured, and the light-transmitting part will be cured. Part of it is still a dry film. Then after developing, the uncured dry film is removed, and the board with the cured protective film is etched. After the film is removed, the inner circuit pattern is transferred to the board.< /span>

For designers, our main consideration is the minimum line width of the wiring, the control of the spacing and the uniformity of the wiring. Because the distance is too small, it will cause the film to be sandwiched, and the film cannot be completely removed and cause a short circuit. If the line width is too small, the adhesion of the film is insufficient, resulting in an open circuit. Therefore, the safety spacing during circuit design (including line and line, line and pad, pad and pad, line and copper surface, etc.) must be considered during production.

3. Blackening And browning: (BLACK OXIDATION)

The purpose of blackening and browning
1. Remove oil, impurities and other pollutants on the surface;

2. Increase the specific surface of the copper foil, thereby increasing the contact area with the resin, which is conducive to the full diffusion of the resin and the formation of greater bonding force;

3. Turn the non-polar copper surface into a surface with polar CuO and Cu 2 O, and increase the polar bond between the copper foil and the resin;

4. The oxidized surface is not affected by moisture at high temperatures, reducing the chance of delamination between the copper foil and the resin. The internal circuit board must be blackened or browned before it can be laminated. It is to oxidize the copper surface of the inner board. Generally, the generated Cu 2 O is red and CuO is black. Therefore, the Cu 2 O-based oxide layer is called browning, and the CuO-based oxide layer is called blackening.

4. Laminating :(PRESSING)

1. Laminating is the process of bonding each layer of circuits into a whole by means of B-stage prepreg. This bonding is achieved through mutual diffusion and penetration between macromolecules at the interface, and then interweaving.

2. Purpose: To press the discrete multi-layer board together with the adhesive sheet into a multi-layer board with the required number of layers and thickness.

Typesetting
Laminate copper foil, bonding sheet (prepreg), inner layer board, stainless steel, isolation board, kraft paper, outer layer steel plate and other materials according to the process requirements. If the board is more than six layers, pre-typesetting is required.

Laminating process< /span>

Send the folded circuit board to the vacuum heat press. The heat energy provided by the machine is used to melt the resin in the resin sheet, thereby bonding the substrate and filling the gap.

For designers, the first consideration for lamination is symmetry. Because the board will be affected by pressure and temperature during the lamination process, there will still be stress in the board after the lamination is completed. Therefore, if the two sides of the laminated board are not uniform, the stress on the two sides will be different, causing the board to bend to one side, which greatly affects the performance of the PCB.

In addition, even in the same plane, if the distribution of copper is uneven, the resin flow speed at each point will be different, so that the thickness of the place with less copper will be slightly thinner, and the thickness of the place with more copper will be thicker. Some. In order to avoid these problems, various factors such as the uniformity of copper distribution, the symmetry of the stack, the design and layout of blind and buried vias, etc. must be carefully considered during the design.

5. Drill blind Buried holes DRILLIN G)

There are many ways to process holes on printed boards, and mechanical drilling is currently the most used. Mechanical drilling is a high-speed cutting method using a drill to form up and down through holes on the board (mother board or daughter board). For perforations with a finished hole diameter of 8MIL and above, we can all process them in the form of mechanical drilling.

At present, the aperture of the mechanical hole must be above 8mil. The form of mechanical drilling determines the non-intersection of blind buried holes. As far as our eight-layer board is concerned, we can process 3-6 layers of buried holes, 1-2 layers of blind holes and 7-8 layers of blind holes, etc. at the same time. But if the design is to have both 3-5 layers of buried holes and 4-6 layers of buried holes, such a design will not be realized in production. In addition, we can understand the necessity of symmetry from the previous lamination. If it is not a buried hole of 3-6 layers but a buried hole of 3-5 or 4-6 layers, the difficulty of production and the scrap rate will be greatly increased. , Its cost will be more than 6 times that of 3-6 layers of buried holes.

6. Shen copper With thickened copper (metallization of holes)

• The substrate of the circuit board is composed of copper foil, glass fiber, and epoxy resin. In the production process, the hole wall section after the base material is drilled is composed of the above three parts of materials. The substrate of the circuit board is composed of copper foil, glass fiber, and epoxy resin. In the production process, the hole wall section after the base material is drilled is composed of the above three parts of materials.

• Hole metallization is to solve the problem of covering a uniform layer of copper with heat shock resistance on the cross section. Hole metallization is to solve the problem of covering a uniform layer of copper with heat shock resistance on the cross section.

• The process is divided into three parts: one de-drilling process, two electroless copper process, and three thick copper process (full board copper electroplating).

The metallization of the hole involves a concept of manufacturing capability, the ratio of thickness to diameter. The thickness-to-diameter ratio refers to the ratio of the plate thickness to the hole diameter.

When the board continues to thicken and the hole diameter continues to decrease, it becomes more and more difficult for the chemical solution to enter the depth of the hole. Although the electroplating equipment uses vibration, pressure and other methods to allow the solution to enter the center of the hole, the center is caused by the difference in concentration. It is still unavoidable that the coating is too thin. At this time, there will be a slight open circuit phenomenon in the drilling layer. When the voltage increases and the board is impacted under various severe conditions, the defects are completely exposed, causing the circuit of the board to be disconnected and unable to complete the specified work.

Therefore, designers need to know the process capability of the board manufacturer in time, otherwise the designed PCB will be difficult to realize in production. It should be noted that the thickness-to-diameter ratio parameter must be considered not only in the design of through holes, but also in the design of blind and buried holes.

7. Second Sub-inner dry film

After 3-6 layers of buried holes are metallized, we plug the holes with resin ink, and then our board will be transferred back to the inner dry film to make the 3rd and 6th layers of the inner circuit. As shown below:
 
After finishing the 3rd and 6th layers, we blacken or brown the board, and then we send it to the second lamination. Since it is the same as the previous steps, it will not be described in detail.

8. Second Sub-lamination (HDI pressing board)

The pressing plate of HDI board: Because the thickness of the insulating layer of HDI is relatively thin. Therefore, it is more difficult to press the plate. Since the strength of the same thickness of LDP is much better than that of RCC, the flow speed is also slower, so it is easier to control.

Lines where there are blind buried holes in the inner layer are more likely to cause open circuits due to recesses. Therefore, if the inner layer has blind buried vias, the circuit design of the outer layer should try to avoid the position of the inner blind buried vias. At least the line should not pass through the middle position of the blind buried hole.

In addition, if there are too many buried holes between the second layer and the penultimate layer when the plate is pressed, the thickness of the dielectric layer on the upper layer will be thinner than the thickness of the lower layer due to the creation of a channel during the pressing of the plate. The thickness of the dielectric layer. Therefore, the number of such holes should be reduced as much as possible during circuit design.

There are many processes for CO 2 laser blind hole manufacturing, and each has its own advantages and disadvantages. The Conformal mask method is the most mature CO 2 laser blind hole production process in the industry. This processing method uses a pattern transfer process to etch lines on the surface of the copper foil layer to etch and laser processing.A small window with the same aperture size is then processed with a laser beam larger than the aperture size to be processed according to the coordinate program of the copper film. This processing method is mostly used in the process of subtractive manufacturing of multilayer multilayer boards. SYE uses this process to make CO 2 laser blind holes.

9.conformal mask

1. Conformal mask is the pre-preparation process for laser drilling. It is divided into two parts: Conformal mask1 and Conformal mask2.

2. Conformal mask1 is to etch the copper foil corresponding to the PAD between the outer periphery of the mother board and the blind hole (laser hole) of the outer layer of the daughter board by making lines on the copper foil on the upper and lower sides of the daughter board, and the mother board is etched at the same time The upper part corresponds to the copper foil at the target position of the automatic exposure machine set on both sides of the sub-board for use in the production of Conformal mask2 and laser drilling.

3. Conformal mask2 is to etch a window slightly larger than the laser hole at the position of each laser hole on the copper foil on the upper and lower sides of the board by making lines for CO 2 laser processing.

10. LASER DRILLING

Since the laser drilling of the HDI board with the resin to burn the resin to form a connected blind hole is drilled by a laser, when the laser is drilled from top to bottom, the energy gradually decreases, so as the hole diameter continues to deepen, The diameter of the hole keeps getting smaller. The hole diameter of the laser hole is generally 4-6mil (0.10-0.15mm). According to IPC6016, the hole with a hole diameter of <=0.15mm is called a micro-via (micro-via).

If the hole diameter is greater than 0.15 mm, it is difficult to drill the hole at one time, but a spiral drilling is required, which results in a decrease in the efficiency of drilling. The cost has risen sharply. At present, laser drilling generally uses three guns to form holes, and the drilling speed of laser holes is generally 100-200 per second. And with the shrinking of the hole diameter, the speed of drilling is obviously accelerated.

For example: when the hole diameter is 0.100 mm, the drilling speed is 120 per second. When the hole diameter is 0.076 mm, the drilling speed is 170 holes per second.

11. Laser drill Metallization of holes

Since the laser drilling of HDI board is drilled by laser, the high temperature during laser drilling burns the hole wall. The scorched slag is attached to the hole wall, and the second layer of copper will be oxidized due to the high temperature burning of the laser. Therefore, after drilling, the micro-holes need to be pre-treated before electroplating. Because the plate has a relatively small hole diameter and is not a through hole, it is difficult to remove the coke residue in the hole. It is necessary to rinse with high-pressure water when removing pores.

For the stacked form of 2-stage HDI, special blind hole plating and COPPER FILLING technology are required, so the cost will be greatly increased, so it is currently only used for the design and production of some high-end products.

12. Third Sub-inner dry film

After the metalized blind holes, a second Conformal mask1 will be performed. Then, the production of the secondary outer layer graphics will be started, that is, the inner layer dry film process will be returned to the 2nd and 7th layer graphics. The finished circuit will be sent to the blackening process for oxidation treatment. The PCB will then be laminated for the third time.

The laminated board will undergo the third blind hole etching copper 1 and the second blind hole etching copper 2 production. This is to prepare for the second laser drilling. From the above, it can be seen that for the second HDI need to go through many times of alignment, so the alignment error also accumulates, which is one of the reasons for the larger rejection rate of the second-order HDI.

As far as the difficulty of production is concerned, the order from simple to difficult for the various designs of the second-order HDI board is as follows:
1. There are 1-2 layers and 2-3 layers of holes. 2. There are only 1-3 layers of holes. 3. There are 1-2 layers and 1-3 layers of holes. 4. There are 2-3 layers and 1-3 layers of holes. 5. There are 1-2 layers, 2-3 layers, 1-3 layers of holes.

Note:
1. When designing HDI holes, it is necessary to adopt a symmetrical design as much as possible. Only one side is listed above, and the other side is the same.
2. The holes mentioned above are all HDI holes

13. The second laser drilling
14. Mechanical drilling (drilling through holes)

15. De-drilling and sinking copper (PTH)
Metal blind holes and through holes together
 
At this point, the special process of HDI ends and the normal process of transferring to the ordinary board is ended.

16. Outer layer DRY FILM & PATTERN PLATING (DRY FILM & PATTERN PLATING)

The transfer of outer layer graphics is similar to the principle of inner Printed on the board.

The difference between the outer dry film and the inner dry film is:

⒈　 If the subtractive method is used, the outer dry film is the same as the inner dry film, and the negative film is used as the board. The cured dry film part of the board is the circuit. The uncured film is removed, and the film is retreated after acid etching, and the circuit pattern remains on the board because of the protection of the film.

⒉　If the normal method is adopted, the outer dry film is made of positive film. The cured part of the board is the non-circuit area (base material area). After removing the uncured film, pattern plating is performed. Where there is a film, it cannot be electroplated, and where there is no film, copper is plated first and then tin is plated. After the film is removed, alkaline etching is performed, and finally the tin is removed. The circuit pattern remains on the board because it is protected by tin.

17. Wet film (Solder Mask) WET FILM SOLDER MASK

1. Concept: The solder mask process is to add a solder mask on the surface of the board. This solder mask is called Solder Mask or solder mask ink, commonly known as green oil. Its function is mainly to prevent undesirable tinning of conductor lines, prevent short circuits between lines due to moisture, chemicals, etc., break circuits caused by poor operations in the production and assembly process, insulation, and resistance to various harsh environments, to ensure The function of the printed board.

2. Principle: At present, this layer of ink used by PCB manufacturers basically uses liquid photosensitive ink. The production principle is partly similar to the transfer of line graphics. It also uses film to block the exposure and transfer the solder mask pattern to the PCB surface. The specific process is as follows:

Pre-treatment-> coating-> pre-bake-> exposure-> development-> UV curing-> thermal curing

Associated with this process is the soldmask file. The process capabilities involved include the solder mask alignment accuracy, the size of the green oil bridge, the production method of the vias, the thickness of the solder mask and other parameters. At the same time, the quality of the solder mask ink will also have a great impact on the later surface treatment, SMT placement, storage and service life. In addition, the entire process takes a long time and has many manufacturing methods, so it is an important process in PCB production.

At present, the way of designing and manufacturing vias is a problem that many design engineers are more concerned about. The apparent problem caused by solder mask is a key item for PCB quality inspection engineers to check.

18. Optional Immersion Gold (IMMERSION GOLD)

Electroless nickel/gold plating is a surface treatment method for the exposed parts that need to be plated after the solder mask is applied to the printed circuit board. Due to the development of science and technology, the line width and spacing on the PCB have become smaller, and the surface packaging has increased. This requires good coplanarity and flatness of the land or solder pad, and requires that the PCB cannot be bent. The chemical Ni/Au surface coating can meet the above requirements. In addition, since the gold on the surface is relatively stable and protective, its storage time is similar to that of lead-tin.

Since this nickel/gold coating is produced after the solder mask is applied to the printed circuit board, only chemical nickel/gold can be used to achieve selective coating. As the surface plating layer of the PCB, the thickness of the nickel layer is generally 5μm, and the thickness of the gold is generally between 0.05-0.1μm. As the non-solderable layer, the thickness of Au cannot be too high, otherwise it will cause brittleness and weak solder joints. If it is too thin, the protection is not good. The disadvantage is that the solderability is poor, and the defect of black disk is prone to occur.

19. Character ( C/M PRINTING )

20. Milling shape (PROFILING)

So far, the PCB we have made has always been in the form of PANEL, that is, a large board. Now that the production of the entire board has been completed, we need to separate the delivery graphics from the board according to (UNIT delivery or SET delivery). At this time, we will use CNC machine tools to perform processing in accordance with the pre-programmed program. The contour edge and strip milling will be completed in this step. If there is a V-CUT, a V-CUT process needs to be added. The capability parameters involved in this process include shape tolerances, chamfer dimensions, internal corner dimensions, and so on. When designing, it is also necessary to consider parameters such as the safety distance from the graphic to the edge of the board.

21. Electronic Test (E-TEST)

Electronic testing refers to the electrical performance testing of the PCB, which is usually referred to as the "on" and "off" testing of the PCB. Among the electrical test methods used by PCB manufacturers, the two most commonly used are bed of needle test and flying probe test.

(1) Needle beds are divided into general network needle beds and special needle beds. The general needle bed can be used to measure PCBs with different network structures, but its equipment is relatively expensive. The special needle bed is a needle bed specially formulated for a certain type of PCB, and it is only applicable to the corresponding type of PCB.

(ii) The flying probe test uses a flying probe tester, which uses two moving probes (multiple pairs) to test each network separately Continuity. Since the probe can move freely, the flying probe test is also a general test.

22.OSP< /span>

Organic Solderability Preservative (OSP), also known as anti-oxidation flux, Entek. In this method, after the PCB completes all the manufacturing processes, and after electrical testing and first appearance inspection, after OSP treatment, a heat-resistant organic solderable film is obtained in the exposed copper pads and through holes. The thickness of this organic heat-resistant solderable film is between 0.3 and 0.5 μm, and the decomposition temperature can reach about 300°C.

OSP technology has been quickly promoted and used due to its high thermal stability, compactness, hydrophobicity and many other advantages.

The main advantages are:
1. It can overcome the problem of small line width and spacing, and its coating surface is very flat.

2. The process is simple, easy to operate, less pollution, easy to operate, maintain and automate.

3. Low cost and good weldability.
The disadvantage is that the protective film is extremely thin and easy to scratch, so be very careful during production and transportation. In addition, its solderability only relies on the protective film, once the film is damaged, the solderability is greatly reduced. Therefore, the time it is placed is also very short.

At present, ENIG+OSP has been widely used in the design and production of high-precision circuit boards. Using ENIG's good protection and OSP's good solderability is a solution to replace HSAL in lead-free production. However, the cost is higher due to the mixed use of the two methods.

23. Final check (FINAL AUDIT)
24. Packaging (PACKING)