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Resin plugging process has been more and more widely used in the PCB industry in recent years, especially in products with high layer counts and large PCB thicknesses. It is hoped that the use of resin plugging can solve a series of problems that cannot be solved by the use of green oil plugging or press-fit resin.However,due to the characteristics of the resin used in this process, there are a lot of difficulties that need to be overcome in order to achieve a good quality of the resin plugged holes.
Origin of Resin Plugging
With the continuous updating of electronic product technology,the structure and mounting method of electronic chips are also improving and changing.The development is basically from components with inserts to highly dense IC modules with solder joints arranged in a spherical matrix.The following diagram shows the evolution of components:
Origin of Resin Plugging
In the PCB industry, many processes have been widely used in the industry,and people are basically not concerned about the origin of certain processes.In fact,as early as the ball matrix arrangement of the electronic chip has just been on the market,people have been for this kind of small chip mounted components,looking forward to from the structure of the finished product to reduce the size.
In the 1990s, a Japanese company developed a resin to plug the holes directly, and then plated copper on the surface, mainly to solve the green oil plug holes prone to air blowing problems.Intel applied this process to Intel's electronic products, giving birth to the so-called POFV part factory (also called Viaonpad) process.
Application of Resin Plugging
Currently,the resin plugging process is mainly used in the following products:
1.Resin plugging with POFV technology
The principle of POFV resin plugging technology is to plug the via holes with resin and then copper plating is applied to the hole surface.
The advantage of POFV technology is to reduce the distance between holes and reduce the board area.Solve the problem of lead-in and wiring,and increase the density of wiring.
Resin plug holes for POFV technology
2.Inner HDI Resin Plugging
The technical principle of inner HDI resin plugging is to use resin to plug the buried holes of inner HDI, and then press the holes.This process balances the conflict between controlling the thickness of the dielectric layer for lamination and the design of the inner HDI plughole.
If the inner HDI vias are not filled with resin, the PCB will burst during thermal shock and be scrapped.If the vias are not filled with resin,multiple sheets of PP need to be laminated to satisfy the filling requirement. However, the thickness of the dielectric layer between layers will be thicker due to the addition of new sheets of PP.
Inner HDI resin plug holes are widely used in HDI products to meet the design requirements of thin dielectric layers for HDI products.For blind vias with buried holes in the inner HDI layer, the addition of inner HDI resin plug holes is often required due to the thin dielectric design of the intermediate bond.
For some blind hole products, because the thickness of the blind hole layer is more than 0.5mm, the press-fit filler cannot fill the blind holes,so it is also necessary to carry out resin plugging to fill the blind holes to avoid the problem of no copper in the blind holes in the subsequent production process.
HDI Resin Plug Hole
3.Through hole Resin Plugging
In some communication products,because the thickness of PCB reaches more than 3.2mm,people in order to either improve the reliability of the product,or to improve the reliability of the green oil plugging holes brought about by the cost allows,but also the use of resin to plug the through-hole.This is a major product category in which the resin plugging process has been promoted in recent times.
3.1Resin Plugging Processes
The three types of resin plug holes introduced above have different manufacturing processes as follows:
(3.1.POFV type products have different equipment in different factories and the process is also different)
1）Opening → Drilling → PTH/plating → Plugging → Baking → Grinding → PTH/plating → Outer circuit → Anti-soldering → Surface treatment → Moulding → Electrical testing → FQC → Shipments
2) material opening→drilling→copper sinking→plate electroplating→plate electroplating (copper thickening)→resin plugging→polishing→drilling through holes→copper sinking→plate electroplating→outer layer pattern→pattern plating→etching→resist soldering→surface treatment→moulding→electrical testing→FQC→shipment
3.2.Two processes for inner layer HDI resin plug hole type products: Grinding and non-grinding.)
Grinding process:
1）Opening → Buried hole inner layer pattern → AOI → Pressing → Drilling → PTH / Plating → Plugging → Baking → Grinding → Inner Layer Circuit → Brownish → Pressing → Drilling Laser Drilling / Mechanical Drilling → PTH / Plating → Outer Layer Circuit → Anti-soldering → Surface Treatment → Moulding → Electrical Test → FQC → Ship out.
2）Opening→Buried hole inner layer pattern→AOI→Pressing→Drilling→Immersed Copper→Board Electricity→Board Electricity Thickened Copper)→Resin Plugging→Polishing→Inner Layer Pattern→AOI→Pressing→Drilling Through Hole→Immersed Copper→Board Electricity→Exterior Layer Pattern→Pattern Plating→Etching→Weld Resist→Surface Finishing→Moulding→Electrical Measurement→FQC→Shipping
No Grinding：Opening → Buried Hole Inner Layer Pattern → AOI → Pressing → Drilling → PTH/Electroplating → Inner Layer Circuit → Browning → Plugging → Flattening → Baking → Pressing → Drilling (Laser Drilling/Mechanical Drilling) → PTH/Electroplating → Outer Layer Circuit → Resistance Soldering → Surface Processing → Forming → Electrical Measurement → FQC → Shipment
3.3.Outer layer through-hole resin plug hole type
1）Opening → Drilling → PTH/Plating → Plugging → Baking → Grinding → Baking → Outer Layer Circuit → Resistance Welding → Surface Treatment → Moulding → Electrical Measurement → FQC → Shipment
2) material opening→drilling→copper sinking→plate electrolysis→plate electrolysis with thick copper)→resin plugging→baking→grinding→baking→outer layer pattern→pattern plating→etching→weld resist→surface treatment→forming→electrical testing→FQC→shipment
4.Resin Plugging Process Special Features
From the above process, we obviously find that the process is different. Our general understanding is that products with ‘resin plugged holes’ followed by ‘through-hole drilling and copper plate electrodes’ are considered to be POFV products. If the process immediately following the ‘Resin Plug’ is ‘Inner Layer Graphics’, then we consider it to be Inner Layer HDI Resin Plug products. If the process immediately following the ‘resin plug hole’ is an ‘outer layer pattern’, then we consider the product to be an inner layer HDI resin plug hole product.
The above different types of products are strictly defined in terms of process, so you can't go through the wrong process.
5.Improvement of resin plugging process
(1) For products with resin plug holes, in order to improve the quality of the product, people are constantly making adjustments to the process to simplify the production process and improve the yield rate.
(2) Especially for the inner layer HDI hole plugging products, in order to reduce the scrap rate of the inner layer circuit after sanding, people adopt the process of plugging the holes after the circuit, complete the inner layer circuit first, and then pre-cure the resin after plugging the holes, and then make use of the high temperature of the pressing stage to cure the resin.
(3) In the beginning, for the inner layer of HDI plug holes, people use UV pre-curing + thermosetting ink, now more often directly choose the thermosetting resin, more effective to improve the thermal efficiency of the inner layer of HDI resin plug holes.
6.Resin Plugging Process Conditions
(1) resin plug holes in the holes or tens of thousands, and to ensure that there can not be a hole is not saturated. This one in ten thousand defects will lead to the chance of scrap, will certainly require in the process of rigorous thinking and standardisation.
(2) Good hole plugging equipment is an inevitable requirement. Currently used in resin plugging holes in the screen printing machine can be divided into two categories, namely, vacuum plugging machine and non-vacuum plugging machine.
Vacuum Plugging Machines and Non-Vacuum Plugging Machines
7.Non-vacuum Plugging Process for General Screen Printing Machines
1) The selection of screen printing machine should consider the maximum cylinder pressure, screen lifting pipeline, the stability of the knife holder and the level.
2) The squeegee for screen printing needs to use 2CM thickness, 70-80 degree hardness squeegee, of course, must have strong acid. Of course, it must be resistant to strong acid and alkali.
3) The screen for screen printing can be either silk screen or aluminium sheet. The control is based on the requirements of the hole plugging process conditions, select the appropriate mesh size and the size of the window for the hole diameter.
4) Resin plugging holes used in the pad plate has a variety of details, but often overlooked by engineers. Gasket plate not only plays the role of air-conducting, but also plays a supporting role. For dense holes in the area, we drilled the pad plate after the whole area is empty, in this position, the pad plate bowing or deformation, the worst support for the plate, which will cause the location of the hole plugging saturation is very poor. Therefore, in the pad production time, to find ways to overcome the problem of large areas of empty space, the best practice is to use 2mm thick pads, only drilling pads of 2/3 depth.
5) in the printing process, the most important thing is to control the printing pressure and speed, in general, the larger the aspect ratio, the smaller the aperture of the board, the slower the speed required, the greater the pressure requirements. Control of slower speed for the improvement of the plug hole bubble for the best results.
8.Vacuum hole plugging process of vacuum resin hole plugging machine
Due to the high price of vacuum resin hole plugging machine, as well as its equipment use and maintenance of technology confidentiality, now can use this technology of PCB manufacturers are very few.
VCP vacuum resin hole plugging machine hole plugging technology is mainly it has an ink folder and two can be traversed plug control head, plug hole head has a lot of small holes. After the equipment is vacuumed, a piston pushes the ink in the ink folder to the small holes in the plug hole head. The two transverse plug hole heads first clamp the PCB, and then the ink is filled into the through holes or blind holes on the PCB through the many small holes in the plug hole head. The PCB is hung vertically in the vacuum chamber, and the traversing plug heads can be moved downward until the holes inside the board are filled with resin. The pressure between the plugging head and the ink can be adjusted to satisfy the requirement of hole filling, and different sizes of plugging heads can be used for different sizes of PCBs. After the hole plugging is completed, you can use a spatula to scrape off the hole plugging ink and then add it into the hole plugging ink folder and reuse it.
At present, there is another type of vacuum hole plugging machine is printed with the help of silk screen, the use of CCD alignment system alignment, its operation is similar to ordinary silk screen printing, but there is an additional process of vacuum hole plugging. This type of hole plugging machine plug holes in the best results, but because of the expensive investment in equipment, has not yet been widely used.
The use of vacuum hole plugging machine is undoubtedly the best way to solve the problem of resin bubbles, and the choice of hole plugging ink is basically not limited by the process. However, since the whole board surface is covered with resin, it is very difficult to remove the resin. It is necessary to use good sanding machine together.
9.Sanding after resin plugging
(1) non-woven grinder or belt grinder is an indispensable equipment for resin plugging, on the one hand, the equipment is required to be able to effectively remove the resin on the board surface, on the other hand, it is also required that the roughness of the copper surface should not be scuffed, scratches and other problems. On the other hand, the roughness of the copper surface should not have scratches and scratches.
10. Preventive and improvement measures for resin plugging NG
1) Select suitable hole plugging ink and control the storage conditions and shelf life of the ink.
2) Regular inspection of the process to avoid the emergence of holes in the patch bit hole. Even if we can rely on excellent hole plugging technology and good screen printing conditions to improve the yield of plugging holes, but one in 10,000 can also lead to product scrap, sometimes just because of a hole in the hole caused by no soldering pads on the hole and scrap is a pity.It is a pity that sometimes there is no soldering disc on the hole just because of a hole, so the only way to find out the location of the hole and repair it is to inspect it.Of course,the problem of checking the hole of the resin plug hole has been explored for a long time, but it seems that there is no good equipment to solve this problem yet. There are many different ways to make manual inspection and judgement more accurate.
3) Selection of suitable resin, especially the selection of Tg and expansion coefficient, suitable production process and suitable debinding parameter, can avoid the problem that the solder pads and the resin are detached after being heated.
4) For the problem of resin-copper delamination, we found that when the thickness of copper on the hole surface is more than 15um, the problem of resin-copper delamination can be greatly improved.
Resin Plugging Technology
With the increasing sophistication of resin plugging technology and the effective solution of stubborn problems such as air bubbles, resin plugging technology is being promoted continuously. For example, the blind holes of HDI are filled with resin plugging holes, and the inner layer of HDI buried holes of stacked HDI structure is VIP process, and so on.
At present,in the industry standard (IPC-650), it seems that there is no requirement for the copper thickness above the hole for resin plugging, the potential risk is that once the copper thickness above the hole plating of the resin plugging hole is thin, after the surface treatment of the inner layer of HDI circuits, after browning treatment, the thin copper above the hole will be drilled by laser drilling through the hole may be possible, and in the electrical test can not be determined to be problematic. It is not possible to determine the problem during electrical testing. However, the quality of this thin layer of copper in terms of its resistance to high pressure and so on is really worrying.
According to our experimental data, if we can guarantee that the copper thickness above the buried hole is more than 15um, which meets the Hoz's requirement of finished copper thickness, there will not be any quality abnormality in general. Of course, if the customer has higher conductivity requirements, it is another matter.
After years of development, the technology of resin plugging holes has been gradually accepted by many users and has been playing an indispensable role in some high-end products. Especially in the blind buried holes, HDI, thick copper and other products have been widely used, these products are involved in communications, military, aviation, power, networking and other industries. As a manufacturer of PCB products, understand the process characteristics of the resin plug hole technology, application methods, we need to continue to improve the resin plug hole product process capabilities, improve product quality, and solve the related technology problems of such products.