Pinsheng Electronics Co.,Ltd
With the constant development and innovation of electronic products and technologies, the design concept of electronic products is gradually moving towards the light and short, and the design of printed circuit board (PCB) is also developing towards the direction of small aperture, high density, multi-layer number and thin circuit.
However, with the increase of the thickness of the circuit board layer and the decrease of the pore diameter, the ratio of pore thickness to diameter of the product increases obviously, and the difficulty of PTH processing increases gradually, which easily leads to the phenomenon of no metal in the hole.
In view of such problems, this paper introduces the specific reasons for the occurrence of non-metal phenomenon in the hole during PTH electroplating through the aspects of abnormal potion, special design and production operation, and determines the prevention and improvement measures according to different reasons to ensure good PTH and guarantee the subsequent effects of deep hole electroplating.
2 circuit board PTH principle
PTH is also known as electroplated through hole. Its main function is to deposit a layer of thin copper on the base material of the insulated hole by chemical method to provide conductive layer for subsequent electroplating, so as to achieve the conductive effect of the inner and outer layers.
The main process of PTH is shown in figure 1 below:
Figure 1 PTH flow chart
The chemical reactions involved are as follows:
Activation: Pd2++2Sn2+ - [PdSn]2+ - forms an unstable complex in solution
[PdSn]2+ - Pd+Sn4++Sn2+ - most complexes are reduced to metal palladium
SnCl2+H2 - Sn(OH)Cl+HCl - hydrolysis of SnCl2 into alkaline stannate precipitation in water washing after activation
With SnCl precipitation, Pd nuclei were also deposited together on the surface of activated substrate
Copper deposition: when HCHO+OH- - goes to H2+HCHOO- - Pd as a catalyst, the step reaction can be carried out
Cu+H2+OH- - Cu+2H2O -- copper ions are reduced to copper under alkaline conditions
The structure of PTH back hole is shown in figure 2 below:
FIG. 2 schematic diagram of electroless copper plating and deposition
3 cause analysis of no copper in PTH hole and improvement measures
Due to the high aspect and aspect of deep hole electroplating products, it is difficult to exchange potion in the hole during the PTH process, and metal free phenomenon in the hole can easily occur.
In the PTH process hole of the circuit board, the shape of the metal free section is obvious. The phenomenon of metal free in the hole caused by different reasons is similar, which requires careful analysis and differentiation to identify the real cause of the defect. According to different section forms, the metal free in the PTH hole can be divided into three categories, as shown in table 3.1:
3.1 abnormal liquid medicine
No metal defects in PTH hole caused by abnormal potion were mainly manifested as poor PTH loading at the star point in the hole. The defects were shown in the figure of electrical copper wrapping plate. The defect diagram is shown in figure 3 below:
FIG. 3: abnormal PTH hole metal-free schematic diagram
A. The insufficient palladium ion content in the activator leads to the inability to form enough colloidal palladium deposition on the surface of the substrate during the activation process. In the subsequent copper deposition process, the lack of palladium ion catalysis leads to poor copper deposition in the hole wall, resulting in no metal defects in the hole.
B. Infiltration of tiny air bubbles into the activation cylinder causes hydrolysis of colloid palladium in the cylinder, resulting in loss of activation function of the activation cylinder, and no copper layer can be deposited in the hole.
C. The PH value in the solution is too low, because chemical copper sinking can only be carried out under the condition of strong attenuation. When PH is too low, the reduction capacity of formaldehyde decreases, which affects the reaction rate of copper sinking, resulting in poor copper sinking.
D. Insufficient complexing agent in the copper sinking cylinder leads to the formation of copper hydroxide precipitate in part of copper ions. There are not enough copper ions in the copper cylinder to react and deposit to the inner wall of the hole, resulting in the adverse occurrence of copper sinking.
3.1.2 improvement measures
In the PTH production process, for the activation cylinder and the copper cylinder, each component in the cylinder should be kept within the normal process concentration range to ensure the orderly chemical reaction.
In addition, the PH and temperature in the cylinder also affect the copper sinking effect inside the bore wall, which should be continuously monitored.
The hydrolysis of colloidal palladium in the activated cylinder is very easy due to the influence of small bubbles.
3.2 special design classes
The morphology of non-metal chips inside the hole caused by such reasons is mainly manifested as the obvious defects, as shown in the figure of electric copper wrapping plate, and at the same time, the inner copper is electroplated and thickened at the defects, as shown in figure 4 below:
FIG. 4 schematic diagram of metal free hole in special design
For deep hole electroplating products, the aspect ratio is usually large. In this case, the exchange rate of potion in the hole decreases significantly, resulting in that the copper deposit layer in the center of the hole is often not dense enough.
In this case, if the hole in the plate is shown in figure 5 design, are in the process of plate after PTH electric, the inner layer copper sank A place due to the large aspect ratio of copper layer dense enough, compared with B in plate electric potential after the power supply is higher than B, because B with C phase conduction through the inner layer copper, so although the orifice is closer to A more C, but the potential is still high, copper ion absorption ability less than C.
In the process of plate electroplating, the solution exchange rate in the hole is slow, and the copper ions in the hole are relatively less.
Based on the above two points, in the process of the electricity board, is originally A copper ions is relatively small, the copper ion absorbability for potential reasons fall again, as A direct result of the copper plating layer thickness is not enough, in A subsequent process (outer patterns and electroplating) before processing hole open circuit caused by the damage of copper, metal causing hole without defect.
FIG. 5. Diagram of thickness difference of copper sink in hole
3.2.2 preventive measures
In view of such design problems, under the condition that the design manuscript is not changed, the copper sinking and plate electrical parameters can be adjusted appropriately to ensure that the copper thickness at point A is sufficient to avoid the loss of copper opening in the subsequent process.
The main method can extend the original copper sinking time or re-enter the plate from the presoaking cylinder after completing the first copper sinking, so as to guarantee the thickness of the copper sinking layer.
It is also possible to make the plate electric with small current for a short time (8ASF*30min) after the first copper sinking. After the plate electric, the plate electric is made again from the oil cylinder incoming plate, and then the hole copper is thickened to enough at the plate electric.
In addition, the plate current density can be appropriately reduced, the plating time can be extended, and the copper ions in the hole can be guaranteed enough and the plating layer can be uniform under the condition of unchanged copper deposit.
3.3 production operation class
The main reason for the non-metal in PTH holes caused by production operation is the abnormal and illegal operation of the equipment. The section features mainly include the residual foreign body in the hole, the defect in the hole, and the diagram of copper clad plate, etc. The defect diagram is shown in figure 6 below:
FIG. 6 metal-free diagram of production operation hole
Production operation caused by hole without metal mainly for PTH to burr and PTH process caused by abnormal activation and sink copper cylinder device, main exception: PTH to burr period of abnormal high pressure water washing post-processing, in hole drilling cuttings copper powder and other foreign matter cannot discharge in time, causing subsequent PTH process, hole potions to exchange, leading to poor hole sink copper;
In addition, in the process of PTH, the vibration and gas cap of the activated cylinder and the copper sinter cylinder are stopped abnormally or the vibration amplitude and frequency are not qualified, which will cause the bubble in the hole to be unable to be discharged in time, affect the exchange of potion in the hole, and lead to abnormal copper sinter, thus causing no metal defects in the hole.
3.3.2 improvement measures
For high aspect ratio products, it is important to ensure the equipment is in normal working state during PTH and pre-treatment deburring process. In addition, for such deep hole electroplating products, the process parameters should be changed to ensure the cleaning of the hole and sufficient liquid medicine exchange.
The pre-treatment deburring can use normal deburring followed by extra high pressure swing wash to clean the product hole again.
In addition, the electric vibration amplitude of PTH activation cylinder and copper sinking cylinder can be appropriately increased when processing high aspect ratio products, and the original 2.5m/s can be increased to 3m/s, so as to guarantee no bubble residual in the hole during PTH process.
In view of the characteristics of thick plate, small hole size and high aspect ratio of deep hole electroplating products, in order to eliminate metal defects in the hole during PTH process, proper adjustments should be made to equipment, process parameters and process flow to ensure PTH quality in the hole.
Specific measures are as follows:
A. After PTH went to the burr, conduct high pressure swing wash again to remove foreign objects such as drilling debris in the hole.
B. In the PTH process, ensure that the PH parameters of all cylinders, especially the activated and copper bath solution and thermometer, are within the normal process range.
C. In the PTH process, the electric vibration amplitude of the activation cylinder and the copper cylinder shall be appropriately increased from the original 2.5m/s to 3m/s.
D. After one PTH, enter the plate again from the pre-dip cylinder and sink the copper once more;
Or after the first PTH, the plate is electroplated with 8ASF and 30NIN, and the plate is poured again from the oil removal cylinder.
Ensure the copper layer uniformity inside the hole.
E. The PTH rear plate shall adopt a small current for a long time to ensure sufficient copper ions in the hole to ensure the plating effect of the hole wall.
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