With the continuous miniaturization of thick-film electronic products, the density of thick-film printed circuits has also been increasing. It is often required to print on both sides of the substrate and connect them together. Usually, the connection of the printed circuits on both sides of the substrate is achieved by using solder circuits or wire clamps to bypass the edge of the substrate. This method requires additional conductor layers and insulating layers to be printed on both sides of the substrate, which not only makes the manufacturing process more complex but also wastes space. A better solution is to drill through holes in the substrate at the connection points of the circuits on both sides to achieve connection. This can not only improve the integration of the product, reduce the product size, and lighten the product weight, but also shorten the connections and reduce the number of solder joints. Thus, it can shorten the production cycle, reduce the product cost, and improve the stability and reliability of the product. 
1. Characteristics of thick film through-hole printing 
Thick film through-hole printing is carried out on an aluminum oxide substrate with pre-drilled holes using screen printing. These holes are usually drilled using a laser. When the batch size is large, using mold holes is very economical. Generally, there is no unified standard for the diameter of the holes, ranging from 0.1mm to 1mm. The commonly used diameters are 0.3 to 0.6mm. When the power requirement of the circuit is high, slot-shaped holes or multi-hole connections may also be used. The specific requirements for the holes are that there should be no burrs around and the inner wall should be smooth. The basic technical requirements for thick film through-hole printing are to conduct screen printing on the substrate with holes, that is, while the printing process is taking place, a vacuum is applied on the other side of the substrate to allow the paste to pass through the holes and be attracted close to the holes or reach the bottom surface. Not only is it required that the edges of the thick film graphics printed be clear, the thickness uniform, the surface smooth, and there be no obvious marks, but also it is required that the paste around the holes be distributed evenly, smooth, and without forming burrs. After printing one side, it is dried and sintered, and then the other side is printed again to make the paste on both sides of the substrate connected in the holes. 
2. Thick Film Through-Hole Printing Method 
Thick film through-hole printing requires a special printing machine. The characteristic of this printing machine is that it has two sets of vacuum systems, one is adsorption, that is, through the negative pressure of the vacuum, the substrate is fixed on the printing machine tabletop to prevent the substrate from moving during printing; and after the scraper printing, the substrate can be correctly peeled off from the screen and not stick to the screen. Therefore, the repetition accuracy is high, and the surface of the printed Saturn is smooth and complete. This is basically the same as the conventional thick film printing, except that the contact area of the adsorption is small, so the flatness of the printing machine tabletop and the substrate is required to be relatively high. Usually, the flatness requirement of the substrate is within ±0.15% of its long side size to ensure that the substrate is well adsorbed on the printing tabletop during printing without displacement. The second set of vacuum system's function is attraction, that is, through it, the conductor material is sucked into the through-hole to connect the circuits on the front and back sides. In practice, there are many schemes for through-hole printing. It can be printed and vacuum suction materials simultaneously; or it can be printed first and then perform vacuum suction materials; some only print the slurry on the hole wall or fill the entire small hole with slurry; some make the slurry penetrate the entire hole or make the slurry flow through the hole by more than half; etc. During printing, the penetration degree of the slurry is affected by many factors, such as the type and mesh of the screen, the viscosity of the slurry, the printing speed, the pressure of the scraper, the suction force of the vacuum, the duration of the vacuum, and the air flow speed through the hole. If to increase the penetration degree of the slurry into the through-hole, the viscosity of the slurry, the speed of the scraper, the working angle of the scraper and the gap of the screen can be reduced; or to achieve this by increasing the pressure of the scraper, the suction force of the vacuum, the duration of the vacuum and the air flow speed through the hole. According to the size of the substrate, the diameter of the through-hole, the number of through-holes and the viscosity of the slurry, suitable through-hole printing parameters are designed. On the DEK-J printing machine in Japan, the main printing parameters of the through-hole are usually represented by V1, V2, VE and S1, S2, S3. V1 is the time delay of the first attraction, after V1, the electromagnetic valve opens for the first attraction; V2 is the time delay of the second attraction, after V2, the electromagnetic valve opens for the second attraction; VE is the time delay after the printing ends, after VE, the electromagnetic valves for both attractions are closed. S1 is the starting position of V1, S2 is the starting position of V2, S3 is the starting position of VE, and there are five numbers 1, 2, 3, 4, 5 for selection. 1 represents the starting position of S begins from pressing the "START" key, 2 represents the starting position of S begins from entering the printing tabletop, 3 represents the starting position of S begins from scraping material with the scraper, 4 represents the starting position of S begins from the end of scraping material with the scraper, 5 represents the starting position of S begins from returning to the printing tabletop. During trial printing, first adjust the size of the vacuum degree for attraction. The vacuum degree cannot be too large, otherwise, the slurry will be all attracted to the reverse side of the substrate, causing the reverse side of the substrate to be contaminated by the slurry; while there is no slurry on the through-hole wall, it cannot achieve the purpose of positive and negative connection; but if the vacuum degree is too small, the slurry will cover the top of the hole to form burrs, and cannot be attracted into the through-hole. Therefore, they must be adjusted to an appropriate degree. Secondly, adjust the size of V1, V2, VE and S1, S2, S3, and find the best delay and starting position. The size of V1, V2, VE is related to the following factors: printing speed, the size of the vacuum, the printing stroke, the size of the substrate, the diameter of the through-hole, the number of through-holes, the viscosity of the slurry, etc. After adjusting all parameters appropriately, perform printing, dry and sinter, and measure the conduction rate of the through-hole. If the conduction rate is 100%, then carry out batch printing; otherwise, continue to adjust the printing parameters until the conduction rate of the through-hole in the trial printing reaches 100%, then can carry out batch printing. Because the variables in thick film through-hole printing are numerous and there is no fixed pattern, therefore, in the production process, it is necessary to conduct repeated exploration to find the best printing solution. 
Requirements for equipment in thick film through-hole printing 
a) The through-hole printing parameters should be easily set and ensure the accuracy of repeated printing. b) The vacuum pump directly connected to the printing table sometimes causes air turbulence. By using an independent vacuum tank connected to the printing table, the phenomenon of air turbulence can be effectively eliminated, ensuring the stability of each material suction. c) It is difficult to effectively control the air flow with a manual valve. The influence of human factors is significant. Therefore, a programmable electromagnetic valve should be used, which can easily set the through-hole printing parameters and control the air flow. d) The pointer of the analog vacuum gauge will shake in the air turbulence and is difficult to read. A digital pressure gauge should be used. In addition to being installed on the printing table, a digital pressure gauge should also be installed on the independent vacuum tank as the vacuum pump starter. 
The application of 4-hole process in mass production 
The thick film through-hole printing process is used by the company for the mass production of products, and the conduction rate has reached 100%. The typical application scenarios of the products are as follows: a) The printed product DC28Q5—1 has 26 holes on each piece, among which 10 holes have a diameter of Φ1.2mm and 16 holes have a diameter of Φ0.4mm. The production batch is 630 pieces of products. After repeated adjustments of the printing parameters, the conduction rate of the through-holes after firing is 100%. b) The printed SI3225 product has 27 holes on each piece, with a hole diameter of Φ0.3mm. By adjusting the through-hole printing parameters, the conduction rate reached 100%. c) The printed SI3210 product has 21 holes on each piece, with a hole diameter of Φ0.3mm. After adjusting the appropriate through-hole printing parameters, the conduction rate reached 100%. d) The printed GP0003 product has 354 holes on each piece, with a hole diameter of Φ0.8mm. After adjusting the appropriate through-hole printing parameters, the conduction rate reached 100%. Through the mass production of these four batches of products, it has been proved that this through-hole process can meet the needs of actual production and can be applied in production in batches. 5 Conclusion 
The successful trial of thick film through-hole printing technology can further reduce the product size, increase the integration of circuits, and also reduce the number of single-sided circuit layers without shrinking the product size, thereby improving the reliability and yield of the product. After adopting through-hole printing technology on products with cross-wiring, cross-wiring can be eliminated, reducing the operational difficulty of the product and shortening the production process. When printing resistors, using the through-hole printing technology enables consolidation of the same pattern, reduces the variability of resistor printing, and thus greatly improves labor productivity.