Micro-Metallurgical Process ─ A Key Process for Realizing Solder Connection of Electronic Component Packaging
2022-05-10
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Tin-based solder is an important connection material for modern semiconductor packaging and electronic packaging. It can connect the metal surfaces of two electrodes to the pads quickly, and the electrodes become electrical and thermal conductive after soldering. The realization of this connection depends on the process that has critical chemical and physical changes in materials, which is called the micro-metallurgical process.
The characteristic of tin-based solder is that it can be used at a temperature less than 400°C and in a small space, while the pad has a high melting point above 900°C. The metallurgical process that realizes the chemical and physical changes between the pad and solder to produce new metal compounds is determined by the properties of metallic tin. The melting point of tin is 231°C. It forms a tin-based alloy with lead, silver, copper, etc. The tin-based alloy has a melting point lower than 231°C, which becomes molten when the temperature is below 231°C. Tin-based metal can rapidly chemically react with metal elements such as copper, nickel, silver, and gold to produce compounds. In the semiconductor and electronic packaging, a micro-metallurgical process can be completed by placing tin-based solder between two copper, silver, or nickel-gold electrodes under the action of thermal energy and chemical energy of the flux. The tin-based solder reacts with the metal on the two electrodes to form an intermetallic compound called the IMC connection. It is the basic principle of modern semiconductor packaging and the electronic packaging reflow process. Similar to the normal metallurgical process, it is available to apply measures such as nitrogen protection and vacuum reflow to improve the physical state of the micro-metallurgical process. As a result, the chemical and physical properties of the solder joints can be enhanced, such as reducing oxidation and voids and improving the surface finish of solder joints.
The international solder and semiconductor electronic packaging industry has recently gained further understanding and improvements to the micro-metallurgical process. People believe that the alloying of the pads can be conducted together with the micro-metallurgical process of reflow soldering. It can produce alloy composition and metallographic structure of the solder joints different from the traditional solder manufacturing process. The conventional solder manufacturing process includes confirming alloy formulation (determining the alloy composition), forming (manufacturing powder, paste, strip, and wire), solder application (printing, solder paste, wave soldering), solder melting, and solder cooling to form solder joints.
The new process includes the following steps: formulating different alloys (using two alloy components), forming and mixing (typically physical mixing of two different alloy components), applying solder (typically printing the mixed solder paste on the pad), reflow soldering (micro-metallurgical process to achieve a new alloy or a new metallographic structure different from the original alloys), and cooling to form a solder joint.
The new micro-metallurgical process can make the metallographic structure of the solder joints different from that of the original solder alloy. The solder joints obtained have different physical and chemical properties from the original alloy. For instance, it can achieve a new melting point to increase the remelting temperature of the solder. Moreover, the new micro-metallurgical process can produce a better metallographic structure of alloys at the same reflow temperature, thus having higher mechanical strength and reliability.
Fitech produced high-strength high-reliability low-temperature FL170, FL180, and FL200 series solders based on the principle of the micro-metallurgical process. If using these types of solders, the bismuth-rich layer near the IMC base of the solder joints can be reduced at the peak reflow temperature of less than 200°C. The solder joints have a dimple alloy structure similar to the SAC305 alloy, thereby improving the mechanical strength and drop resistance of the solder joints equivalent to 80-90% of that of SAC305. The micro-metallurgical principle accelerates the development of new low-temperature high-reliability solder and solutions for secondary reflow soldering of semiconductor and microelectronic packaging.
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