Solder paste printing is a key process in the current electronic manufacturing process. The quality of solder paste printing directly affects the performance of the product. This article from the leakage template, printed circuit board, solder paste, printing presses to proceed with the four aspects, to explore the quality control of solder paste printing.
Keywords: solder paste leaky template printed circuit board printer
I. Introduction
With the rapid development of electronic manufacturing, the current electronic assembly process is Surface Mount Technology (SMT).
A new printing process, solder paste printing technology, has emerged. Although its printing method is a kind of screen printing - the stencil printing, but the printing medium is not ink, but the electronic functional material - solder paste. The purpose of solder paste printing is not to obtain a pleasing tone and level, but to accurately dispense the solder paste to obtain the best solder paste deposition thickness, which will lay the foundation for the next step of component hot melt welding. Below, first briefly introduce the relevant knowledge of hot-melt welding, and then explore the quality control of solder paste printing.
Second, hot welding
In surface mount technology, hot melt welding fixes the surface mount component to the printed circuit board. The first procedure is to apply the solder paste and adhere the component, that is, to print the solder paste on the component placement point of the printed circuit board. Due to the sticky effect of the solder paste, the component is temporarily fixed. The second process is to perform hot melt welding (abbreviated in Figure 1), usually in the gas phase or in the infrared furnace. The solder paste is liquefied after heating to the melting point, and is spread under the action of gravity and surface tension. After cooling, the components are Printed circuit boards are connected together. The third step is to remove the flux and clean it.
Third, the quality control of solder paste printing
There are many factors affecting the quality of the solder paste printing process. Among them, the leakage template, the printed circuit board, the solder paste, and the printing press constitute the four basic contents of the solder paste printing. These four aspects are related and interact with each other and determine the entire printing process. the quality of.
1. Optimization of leaky template
The drain template is the basic tool for solder paste printing and is used to limit the location of the solder paste on the printed circuit board surface pads. It is a key factor affecting the printing quality. It is important to design and make a good leaky template.
1) Material of the leaky template
The effect of the material of the drain template on the printing quality is mainly that the material is easily deformed when the blade applies pressure, so that the distribution of the solder paste is biased. When making the leaky template, try to use a material with small deformation, usually made of stainless steel or brass plate.
2) The thickness of the leaky template
The thickness of the drain plate determines the amount of solder paste deposited on the printed circuit board. Good solder paste printing must have a suitable amount of solder paste deposited. Excessive thickness of the drain plate can cause excessive deposition of the solder paste. The consequence of this is the short circuit and bridging between the fine pitch components (abbreviated in Figure 2); too thin a drain plate can result in insufficient solder paste deposition, resulting in soldering strength. Not enough, prone to failure (see Figure 3 omitted). Table 1 shows the values ​​of the drain template thickness for different lead spacings.
3) The size of the opening of the drain plate
The size of the opening of the drain plate is one of the main factors that affect the quality of the welding print. The size of the opening of the drain template has a certain relationship with the pad size, and is usually slightly smaller than the pad size by 10% to 20%. If the opening is too large, short-circuiting and bridging between components can easily occur; if the opening is too small, welding strength tends to be insufficient. Table 2 shows the size of the die plate opening for different pad widths.
4) Open template shape and hole wall finish
A good shape of the opening and a smooth hole wall will make the solder paste print well. A good process (such as laser cutting) should be used as far as possible to make the leakage template, so that the opening shape is small and large. In this way, solder paste can easily escape from the drain plate during printing. In addition, the open hole wall should be as smooth as possible to reduce the adhesion between the solder paste and the hole wall, making it easy to escape from the drain plate.
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