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Analysis and Countermeasures in metallized hole coating of multilayer PCB
 

1 Preface
The quality of metallized hole is closely related to the quality and reliability of multilayer board. The metallized hole plays an important role in the electrical interconnection of multilayer printed circuits. The quality of copper coating on hole wall is the core of PCB quality, which not only requires the coating to have proper thickness, uniformity and ductility, but also requires that the coating can not break at 288 ℃ thermal shock for 10 seconds. Because the thermal shock fracture of copper coating on hole wall is a fatal defect, it will cause open circuit between inner circuits and between inner and outer circuits; The light will affect the intermittent conduction of the circuit, and the serious will cause the multi-layer board to report waste.
At present, the defects of metallized hole coating often appear in PCB production mainly include: copper layer cavity in metallized hole, tumor, thin coating in hole, pink ring and poor connection between hole wall and inner copper ring of multilayer board. The vast majority of these defects will lead to product scrap, causing serious economic losses and affecting the delivery date.

Causes and Countermeasures of main defects in metallized porous coating
We first briefly review the manufacturing process of multilayer printed circuit board.
Etching blackening lamination of blanking plate
Decontamination and pitting treatment of borehole metallization
Whole plate plating pattern plating and film removal
Hot air leveling of silk screen characters by etching silk screen solder mask
This paper will analyze the main defects and causes of metallized hole coating from the aspects of drilling process, resin contamination and pitting treatment process of hole wall, electroplating and multilayer plate lamination process, and expounds how to optimize the process parameters, carry out strict process and production management, so as to ensure the quality of metallized hole coating.

2.1 drilling procedure
Most of the coating holes are accompanied by hole wall defects caused by poor drilling quality, such as hole burr, rough hole wall, substrate pit and epoxy resin greasy dirt. As a result, the copper coating on the hole wall is hollow, the substrate of the hole wall is separated from the coating or the coating is uneven. Next, the causes of hole wall defects and the measures taken will be described

2.1.1 generation and removal of orifice burr
Burr is always inevitable in the process of drilling copper clad laminate, no matter what kind of drill bit and drilling process parameters are used, whether manual drill or CNC drill. The influence of hole burr on the quality of metallized hole has never been paid attention to, but it is a factor that can not be ignored for the quality of metallized hole of high reliability printed circuit board.
First of all, the burr of the orifice will change the size of the aperture, resulting in the smaller size of the entrance of the aperture, affecting the insertion of components. Secondly, the burr of copper foil that protrudes or dents into the hole will affect the distribution of power line during electroplating in the process of hole metallization, resulting in thinner coating thickness and stress concentration at the hole, so that when the copper coating at the hole of finished PCB is subjected to thermal shock, it is easy to fracture due to the axial tensile stress caused by the thermal expansion of the substrate.
The traditional deburring method is to use 200 (400) water sandpaper to grind carefully. Later developed to use silicon carbide abrasive nylon brush mechanical polishing brush. However, with the continuous development of PCB technology and the popularization and application of 9 (18 micron) ultra-thin copper foil, great changes have taken place in deburring technology in PCB processing. It is reported that foreign countries have begun to use liquid sand blasting grinding method to remove the burr.
Generally speaking, the mechanical brushing method is very effective for deburring the hole of copper clad laminate whose thickness of copper foil is more than 18 microns. However, during the operation, the particle size of SiC abrasive in the brush roller and the pressure of the brush plate must be strictly controlled, so as to avoid excessive pressure and too coarse abrasive to expose the substrate. In the nylon brush roller used for deburring, the particle size of silicon carbide abrasive is generally 320 (380 ×).
Modern double-sided deburring machine has four rollers, half at the top and half at the bottom, which can remove the burr on both sides of copper clad laminate at the same time. When deburring the hole on the same side, the rotation direction of the two rollers is opposite. One rotates clockwise and the other anticlockwise, plus the axial swing of each brush roller, so that the burr of the orifice is evenly affected by the brush force in all directions of the plate surface, so that it is completely removed. Deburring machine must be equipped with high pressure jet water flushing section.
The liquid sand blasting grinding method is to use a special equipment to spray silicon carbide abrasive on the plate surface with the help of water jet force, so as to achieve the purpose of deburring.

2.1.2 influence of hole wall roughness and substrate pit on coating quality
In the good condition of electroless copper plating system, the hole wall with poor drilling quality is easy to produce copper plating layer cavity.
Because on the smooth surface of the hole wall, it is easy to obtain a continuous electroless copper coating, while on the rough borehole wall, due to the poor continuity of electroless copper plating, pinholes are easily generated; Especially when there are holes in the hole wall, even if the electroless coating is very complete, but in the subsequent copper plating, because of the folding phenomenon of the electrodeposited coating, the copper plating layer is not easy to be uniform. At the holes, the coating is easy to be thin, or even can not be plated with copper, resulting in plating holes.

2.1.3 causes of greasy epoxy resin
As we all know, PCB drilling is a very complex process. Under the mechanical force of the cutting edge of the drill bit, including shear, extrusion, tearing and friction, the substrate produces elastic deformation, plastic deformation, fracture and separation with the substrate to form holes.
Most of the mechanical energy is converted into heat energy. Especially in the case of high-speed cutting, a lot of heat energy is produced, and the temperature rises abruptly. When drilling, the temperature of the bit is above 200 ℃. The glass transition temperature of the resin in the substrate of printed circuit board is much lower. The softened resin is affected by the drill bit and greasy on the cross section of the copper foil on the wall of the cut hole, forming greasy dirt.
It is difficult to remove the greasy dirt, and once there is a certain amount of greasy dirt on the copper foil section, the interconnection of multilayer boards will be reduced or even destroyed.

2.1.4 ways to avoid drilling defects and improve drilling quality
Hole burr, rough hole wall, substrate pit and epoxy resin greasy dirt and other defects can be removed or weakened by strengthening the following aspects of process and quality control, so as to improve the quality of drilling.

2.1.4.1 bit quality control
The quality of the bit itself plays a key role in the quality of drilling. The grain size of tungsten carbide alloy material must be very fine and reach submicro level. Tungsten carbide alloy has no sparse hole and can be wear-resistant. The diameter tolerance of drill shank and cutting edge is within 0 (0.005mm). The concentricity tolerance of the whole drill part, drill tip and shank is within 0.005mm. The geometric shape of the drill bit has no defect, that is, there should be no break when observed under a 40x magnifying glass.
A good bit should have another characteristic, namely symmetry. Both sides of the bit must be the same in size and shape. Poor symmetry will cause wear of bit edge. In order to ensure the quality of the drill bit, the supplier must be strictly selected and the drill bit manufacturer with reliable quality should be selected. In addition, the incoming drill bit shall be inspected, and the unqualified products shall not be used in production.

2.1.4.2 bit shape selection
The main shapes of the drill are generally divided into common type, taper type, shovel type and special type. For small holes, especially for multi-layer plate holes, the latter three types are best used. The latter three types are characterized by relatively short blade length, generally about 0.5mm, which can significantly reduce the heat of drilling and reduce contamination.

2.1.4.3 length of drill row groove
The length of the groove plays an important role in chip removal. If the groove of chip removal is too short, the cuttings can not be removed smoothly, and the resistance of drilling increases, which is easy to cause the bit to break and contaminate the hole wall. Therefore, in general, the length of the chip removal groove of the bit should be 1.15 times of the thickness of the laminated plate (including the upper cover plate) plus the depth of the bit into the lower base plate, that is, at least 15% of the length of the chip removal groove should be left outside the plate.

2.1.4.4 control drilling process parameters
The drilling process parameters mentioned here include the number of blocks per stack and the number of drills / feed ratio.

2.1.4.5 life control of bit
As the situation of each factory is not completely consistent, the service life of the bit is also different, which should be determined according to the specific experimental results. When the quality index of the hole, there is one? When it falls close to the tolerance limit, it is necessary to replace the drill bit, turn it over or discard it. In general, the maximum number of holes allowed for multilayer board is 1500, and the multilayer board generally does not turn over the drill bit.

2.1.4.6 use of upper cover plate and lower base plate

2.1.4.6.1 use of upper cover plate
The upper cover plate used in drilling can play the following roles:
1) Prevent the pressure foot from damaging the board surface;
2) Prevent the burr on the entrance surface;
3) Improve the accuracy of aperture;
4) It can reduce the probability of bit breaking.

If the upper cover plate is not used, one side of the bit may collide with the glass cloth after the bit penetrates the thin copper foil, resulting in a larger cutting force on one side of the bit, and the bit will tilt, thus affecting the positioning accuracy. Moreover, after penetration, when the bit returns, the force is uneven, and the bit is easy to break. Generally, the upper cover plate can be made of 0.2 (0.4mm) thick hard aluminum foil.

2.1.4.6.2 use of bottom plate
The use of the bottom plate can prevent the bit from touching the worktable; It can also prevent the burr on the outlet surface.
In addition to flatness, the lower base plate also needs to have a certain hardness and no oil pollution, so as to prevent partial hole burr and hole wall contamination.
American laminate company has a kind of corrugated plate bottom plate. When the drill bit penetrates the thin aluminum foil, it rotates at high speed in the air. When lifting the drill, due to the venturi effect, a stream of air cools the drill bit effectively, greatly reducing the drilling temperature.

2.1.4.7 depth control of bit entering into the lower base plate
The depth of the bit into the bottom plate should be moderate. If the drilling depth is too deep, the part of the chip removal groove left outside the plate is small, which is not conducive to the discharge of debris, and it is easy to plug the hole, so that the bit is easy to break in the hole. Secondly, if the drill is too deep, it is possible to drill to the drilling platform and damage it. In addition, generally for the 1.5 (1.7) thick backing plate, in order to improve the use efficiency and reduce the production cost, most of them are used twice, and each side is used once. If the drilling is too deep, the backing plate will be drilled through, which is easy to produce burr. For the bit larger than 0.6 mm, the drilling depth of the bottom plate should be 0.75 mm; For the bit less than 0.6 mm and more than 0.3 mm, the penetration depth of the bottom plate is 0.6 mm; For the drill bit less than 0.3mm, a thick paper can be put on the bottom plate for trial drilling to determine the depth of drilling. Once these parameters are determined, these data are incorporated into the drilling program.
 & nbsp; 2.2 resin contamination and pitting treatment process of hole wall & nbsp;
First of all, it should be pointed out that pitting and decontamination are two interrelated but independent concepts and processes.
The so-called concave corrosion refers to the process of controlling the removal of non-metallic materials from the hole wall to a specified depth in order to fully expose the inner conductive surface of the multilayer plate.
The so-called decontamination refers to the process of removing the molten resin and drilling cuttings on the hole wall.
Obviously, the process of pitting is also the process of decontamination. However, the decontamination process does not necessarily have pitting effect.
Although people choose high-quality substrate, optimize multilayer laminate and drilling process parameters, but the hole wall epoxy contamination is still inevitable. Therefore, the multi-layer board must be decontaminated before hole metallization. In order to further improve the reliability of the connection between the metallized hole and the inner conductor, it is better to carry out a concave etching treatment while removing the contamination. The hole of multilayer board after concave etching treatment not only removes the epoxy resin fouling layer on the hole wall, but also makes the inner conductor protrude in the hole. After realizing the hole metallization, the inner conductor and the hole wall layer can be reliably connected in three-dimensional space, which greatly improves the reliability of multilayer board. The etch depth is generally required to be 5 (10 μ m).
There are four ways to remove resin contamination on the pore wall, namely plasma, concentrated sulfuric acid, chromic acid and potassium permanganate. Because potassium permanganate has many advantages in removing resin contamination: it can produce micro uneven resin surface, unlike concentrated sulfuric acid corrosion resin to produce smooth surface; Unlike chromic acid, which is easy to produce resin over corrosion and make glass fiber protrude from the hole wall, and is not easy to produce PINK ring, these are the advantages of potassium permanganate to remove resin contamination, so it is widely used at present.
In order to achieve a balanced corrosion rate by decontamination and pitting treatment of potassium permanganate, it is necessary to do a good job in process management and maintenance
2.2.1 select the best process parameters
Taking the solution of ammett company as an example, the parameters are as follows:
1) Swelling agent securiganth P: 450 (550ml / L, the best 500ml / L).
PH calibration solution: 15 (25ml / L, the best 23ml / L).
Or sodium hydroxide: 6 (10g / L, the best 10g / L).
Working temperature: 60 (80 ℃, the best 70 ℃).
Processing time: 5 minutes 30 seconds.
2) Potassium permanganate kmno4:50 (60g / L, the best 60g / L).
NaOH: 30 (50g / L, the best 40g / L).
Working temperature: 60 (80 ℃, the best 70 ℃).
Processing time: 12 minutes.
3) Reducing agent securiganthp: 60 (90 mL / L, the best 75 ml / L).
Sulfuric acid H2SO4: 55 (92g / L, the best 92g / L).
Glass etchant: 5 (10 g / L, the best 7.5 g / L).
Working temperature: 50 ℃.
Processing time: 5 minutes.

2.2.2 the concentrations of KMnO4, k2mno4 and NaOH should be determined once a week. If necessary, the concentrations of KMnO4 and NaOH should be adjusted.
2.2.3 if possible, continuous electrolysis should be carried out to oxidize k2mno4 to KMnO4.

2.2.4 observe the surface color of PCB after KMnO4 resin decontamination. If it is purplish red, it indicates that the solution state is normal; If it is green, it means that the concentration of k2mno4 in the solution is too high. At this time, electrolysis regeneration should be strengthened.

2.3 electroplating process

2.3.1 chemical coarsening of plate treatment before electroplating

2.3.1 chemical coarsening of plate treatment before electroplating

In order to ensure the adhesion between electroless copper coating and copper foil, a micro roughening (micro etching) treatment must be carried out on the surface of copper foil before electroless copper plating (copper deposition). The treatment method generally adopts chemical etching, that is, through the micro etching effect of chemical roughening solution, the surface of copper foil presents a rough surface with high surface activation energy.
There are three kinds of micro etching solution commonly used in PCB copper plating process: ammonium persulfate (NH4) 2S2O8, hydrogen peroxide H2O2 and sodium persulfate (Na2) 2S2O8. The former solution is unstable and easy to decompose, so the micro etching rate is not easy to be constant; H2O2? Although the H2SO4 system is easy to use and even can be added automatically to adjust the concentration, it needs H2O2 stabilizer and wetting agent. The price is not low, and the micro etching rate is low, generally 0.5 (0.6) μ m/Min; The (Na2) 2s2o8-h2so4 etching solution has a higher and more stable micro etching rate, which can provide a more suitable micro rough surface, so as to ensure that the electroless copper coating will not break under thermal shock.
The etching rate of micro etching solution must reach 0.7 (0.9) in order to make the copper coating on the hole wall not fracture or crack after thermal shock at 288 ℃ for 10 seconds μ M / min, (Na2) 2s2o8-h2so4 system can achieve this goal.
Process formula:
(Na2) 2S2O8: 60 (80g / L, the best 70g / L).
H2SO4:15ml/L。
Cu2+:1(20g/l。
Working temperature: 30 (50 ℃).
Processing time: 2 minutes.
1) Before daily production, analyze the concentration of (Na2) 2S2O8 and adjust it if necessary.
2) Measure the etching rate every day before production, and use 18 μ The etching rate can be determined quickly and simply by recording the etching time of the copper foil. If necessary, add (Na2) 2S2O8.
3) When the amount of dissolved copper is more than 20g / L, change the solution. When the cylinder is just opened, the etching rate is small and the concentration of (Na2) 2S2O8 can be increased appropriately.
2.3.2 optimization of electroplating process parameters
For the small hole plating with high density and fine lines, high level (14 layers, 20 layers) and large plate back aperture ratio (6 (10:1), the biggest difficulties are: the plating solution is difficult to exchange in the hole, and the uniformity and dispersion of plating are poor. Therefore, it is necessary to optimize the electroplating process parameters.
1) The ratio of H2SO4 to Cu2 + should be at least 10:1.
Cu2+:10(13g/l。
H2SO4:190(220g/l。
Cl-:30(50ppm。
[H2SO4]:[Cu2+]=17(20:l
Temperature: 22 (26 ℃).
2) Low cathode current density and long plating time are selected.
3) On the basis of ensuring three kinds of agitation modes (cathode movement, compressed air agitation and circulation) of copper plating bath, a vibration device was installed on the running rod.
With the vibration device, the cathode not only swings back and forth, but also vibrates up and down, which will inevitably promote the exchange of plating solution in the small hole, so as to improve the dispersion performance of the plating solution, even if the thickness difference between the hole and the center of the hole becomes smaller.
Using the above three measures, the uniformity of the coating and the deep plating ability of the small hole are greatly improved, the thin coating on the hole wall and even the formation of the coating cavity are avoided, and the hole metallization quality of the small hole is improved.

2.4 lamination process of multilayer plate & nbsp;
With the development of information technology revolution, the number of printed circuit layers has increased, the wiring density has increased, the structure has diversified and the size tolerance has decreased. Therefore, the lamination process has become the key to the production of multilayer boards.
The lamination process mainly includes the treatment and lamination of the inner laminate. Among them, the following two aspects play an important role in the hole metallization quality of Multilayer PCB

2.4.1 the curing effect of laminates shall be complete
Here refers to the laminated material, mainly refers to the inner laminate single chip and multilayer printed board after the lamination process.
1) After cutting the single inner plate, the number of each laminated plate should be controlled according to the thickness of the single plate, and the pre drying treatment should be carried out in horizontal position;
2) After the lamination process, the multilayer printed circuit board should be cured after drying, and must be carried out before the drilling process, not after drilling.
Without the above two processes, the curing effect of laminate material is not sufficient, which will easily produce epoxy contamination and affect the drilling quality. In addition, when drilling the incomplete cured laminate, a large number of chips with strong viscosity will fill the chip removal slot of the drilling hole, which can not be ruled out and may eventually cause the bit to break.

2.4.2 optimize the lamination process to reduce the occurrence of pink circle
The so-called Pink Ring refers to the phenomenon that the oxide film on the copper surface of the hole ring has changed color or been removed due to chemical reaction at the junction of the hole wall and the inner copper ring, revealing the natural color (pink) of copper.
With the increase of the number of PCB layers, the possibility of peeling at the junction of inner copper foil and hole increases; With the decrease of pore size, the difficulty of hole cleaning increases, and the possibility of chemical penetration corrosion along the interface of each layer of hole wall increases. Therefore, the more layers, the smaller the hole, the more pink circle phenomenon will occur.
Pink circle is often found in the late stage of PCB production, which affects the product quality of multilayer board. First of all, it will affect the adhesion between layers; Secondly, the solution infiltrates along the direction of the glass fiber, which reduces the insulation resistance between the pads close to each other and leads to short circuit in serious cases; In addition, due to the smaller contact area of the copper ring, the small defects allowed by the metallized hole, such as coating blisters and voids, may lead to the increase of hole line resistance or even open circuit.
In the process of PCB production, the inner surface treatment, lamination, curing, drilling, pitting, chemical copper deposition, copper plating and other processes may lead to pink rings. The key lies in whether the blackening layer is firmly combined with the substrate, and the strength of the corrosion resistance of the blackening layer. In the process of PCB production, it has to withstand vertical mechanical impact force and horizontal chemical etching force, so there must be enough bonding force between layers to resist the harm of these two effects. The main reasons for the formation of pink circle are low peel strength and poor corrosion resistance of blackened layer.
1) Improve the binding force between blackening layer and substrate.
The surface of copper is blackened to form a layer of oxide (black copper oxide or red cuprous oxide or a mixture of them) to further increase the specific surface and improve the bonding condition between copper foil and substrate.
The bonding ability between blackening layer and substrate is related to blackening process, crystal structure and thickness of oxide layer.
2) Improve the corrosion resistance of blackening layer.
The thickness of oxide layer can be reduced. A layer can be removed by mechanical method or by chemical reduction. The reducing agents available are formaldehyde / sodium hydroxide, sodium superphosphate, sodium borohydride, etc. After reduction, not only peel strength but also acid corrosion resistance of blackened layer are enhanced

2.5 other factors causing defects of hole metallized coating and corresponding countermeasures

2.5.1 voids in metallized coatings due to the presence of bubbles.
Generally speaking, the existence of bubbles in the pores may hinder the deposition of plating solution or activation solution. Finally, the coating cavity in the metallized hole is formed. There are two kinds of bubble entrapment: external introduction and internal generation.
2.5.1.1 way of bubble introduction:
The introduction of foreign bubbles may be when the board enters the slot, or when it vibrates or swings into the through hole.
The introduction of inherent bubbles is caused by hydrogen produced by the side reaction in the chemical copper precipitation solution
2hcho + 2cu2 + 4OH? Cu + 2hcoo? X 2H2O x H2 ↑
Or it is caused by hydrogen produced by cathode or oxygen produced by anode in electroplating solution
Cathode side reaction: 2h + + 2e → H2
Anode side reaction: 2H2O - 4E → O2 ↑ + 4H+

2.5.1.2 cavitation characteristics of metallized porous coating caused by bubbles:
The voids in the metallized hole coating caused by bubbles are often located in the center of the hole. Through the metallographic section, it can be seen that they are symmetrically distributed, that is, there is no copper in the same width range on the surface of the opposite hole wall.

2.5.1.3 possible process of bubble cavity:
The possible processes of bubble cavity are chemical copper deposition, whole plate electroplating and pattern electroplating.

2.5.1.4 methods to avoid bubbles entering the hole:
Vibration and collision are the most effective ways to avoid bubbles entering the hole. At the same time, it is very important to increase the plate spacing and the cathode moving distance.
Air agitation and impact or vibration of activation tank have little effect on preventing bubbles from entering the hole. In addition, it is also very important to increase the wettability of chemical copper deposition and avoid bubbles in the pretreatment tank.
The surface energy of the plating solution is related to the size of hydrogen bubbles before they run out of the hole or burst. It is obviously hoped that the bubbles will be excluded from the hole before they become larger, so as not to hinder the solution exchange and cause coating defects in the hole.

2.5.2 voids in metallized coatings caused by organic dry film.

2.5.2.1 cavity characteristics of metallized porous coating caused by organic dry film:
The hole of metallized coating caused by organic dry film is often located at the orifice, that is, near the plate surface, about 50 (70 μ M wide, 50 (70 μ m。 Edge voids may be located on one or both sides of the board, which may cause complete or partial open circuit.

2.5.2.2 causes of dry film resist penetration
For the hole covered by organic dry film, the air pressure in the hole is 20% lower than the atmospheric pressure. When the film is applied, the air in the hole is hot. When the air is cold to room temperature, the air pressure decreases. As a result, the pressure difference causes the resist to slowly flow into the hole until it develops.
There are three main factors that lead to the flow speed and depth of dry film resist: there is water or water vapor in the hole before the film is applied; Small hole with high thickness diameter ratio; Film and development time is too long.
The main reason is that water vapor stops in the hole. Water can reduce the viscosity of the resist and make it flow into the hole quickly. The hole with high thickness diameter ratio is easy to have cavity problem, which is because it is difficult to dry. The resist in the small hole is also difficult to develop. Longer time before development also makes more resist flow into the hole.

2.5.2.3 causes of orifice cavity:
As the resist enters into the hole and is not removed during development, it hinders copper and tin electroplating. When the resist is removed from the film, the lower copper layer is exposed. Therefore, once etched, the copper layer is etched away, forming a coating cavity.

2.5.2.4 measures to avoid orifice cavity:
The best and simplest way to avoid the hole is to increase the degree of drying after surface treatment. If the hole is dry, there will be no hole. No matter how long the storage time and poor development, it will not cause hole.
After increasing the drying time, the time between the film and the development should be as short as possible, but the problem of stability should be considered. If the following conditions occur, the hole may occur (not before)
1) After installation of new surface treatment equipment and drying equipment;
2) The function of drying section of surface treatment equipment is abnormal;
3) Production of high thickness diameter ratio small hole plate;
4) The film and development time is long;
5) Resist change or change thickness of dry film;
6) The pressure difference of vacuum film sticking machine is larger.

2.5.3 voids in metallized coatings caused by other factors.
The existence of solid matter (dust, cotton) or organic fouling will also hinder the deposition of plating solution or activation solution, and eventually lead to hole metallization coating.

3 conclusion
The causes of coating defects of metallized holes in multilayer printed circuit boards can be traced back to the drilling process, and can also occur during lead / tin plating. Sometimes, a kind of coating defect is often caused by the interaction of various process conditions. They may act at the same time or in sequence. Therefore, it is possible to find out the root cause accurately by careful analysis along the technological process and metallographic sectioning technique when necessary. On this basis, by optimizing the process parameters and carrying out strict process and production management, the purpose of improving the quality of metallized hole coating can be achieved.

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