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Research on vacuum sintering process of low temperature solder

Commonly used chip mounting technologies mainly include solder sintering and conductive adhesive bonding. The conductive adhesive bonding process has the advantages of simple operation and low cost, but its thermal conductivity is poor. For chips with high power and need to dissipate heat, the solder sintering process is still mainly used. . Due to the complex structure of component products, which usually include non-chip assembly processes such as connectors, circuit boards, and surface-mount resistor-capacitive component installation, two temperatures of 217 ℃ (Sn96.5Ag3Cu0.5) and 183 ℃ (Pb63Sn37) are generally used. Gradient, and the chip assembly process needs to separate the temperature gradient from the non-chip assembly process to ensure the reliability of the previous process assembly. Power chips are usually sintered onto a matching carrier chip with better heat dissipation using a gold-tin solder sintering process, and then the carrier chip is soldered into the box with low-temperature solder to achieve better heat dissipation effects.

Indium-lead-silver solder (In80Pb15Ag5), as a low melting point electronic soldering material, can create a temperature gradient with Pb63Sn37 solder, and has high thermal conductivity and strong fatigue resistance. Therefore, it is used in the power carrier chip assembly process of component products. Be widely used.
The traditional manual sintering method has the following problems: (1) The wettability of the solder is poor, and the carrier and box body need to be tinned first and then sintered, which is very inefficient; (2) Due to product size limitations, sufficient friction space cannot be reserved when the carrier is put into the box. As a result, the carrier and the enameled tin on the box body cannot be fully integrated, and quality problems such as the carrier falling off occur later; (3) Many products require sintering more than a dozen or even twenty things, and manual rubbing requires sintering one by one. Due to the long melting time, The first sintered carrier solder dries out, and quality problems such as carrier falling off may occur later.
Based on the above reasons, there is an urgent need to study the vacuum sintering process based on 154°C low-temperature solder to achieve low void rate vacuum sintering based on 154°C low-temperature solder.

Factors affecting the vacuum sintering process of low-temperature solder

1. Metalization of the back of the carrier and box
The indium element in solder at 154°C can form an alloy with gold, and the corrosion effect of indium on gold is much smaller than that of tin, and will not cause a “gold brittle” effect. Therefore, indium-containing solder can be suitable for welding thick gold materials. The material plating generally used for welding adopts a nickel-gold two-layer metallization structure. For the more commonly used molybdenum-copper carriers, the nickel layer can form a barrier layer to prevent the diffusion of copper elements into the gold-plated layer, ensuring the purity of the gold-plated layer and ensuring Soldering quality with solder. Since the box body usually needs to be suitable for soldering with tin-containing solder at the same time, the gold-plated layer of the box body is generally not larger than 0.5 microns, which requires the thickness of the gold-plated layer of the carrier to be thicker to ensure the percentage of gold elements in the indium-lead alloy. The thickness of the gold-plated layer of the carrier Generally above 1 micron, a thicker gold plating layer is conducive to the nucleation process of the alloy of indium and gold, improving the solder strength and welding quality of the solder and carrier.

2. Surface condition of raw materials
Normally, the non-chip assembly process of component products precedes the chip assembly process. Therefore, when the power carrier is put into the box, the box has gone through multiple welding processes. Flux residues in the previous process or the introduction of other pollution sources will cause damage to the surface to be welded. Sticky dirt will affect the wettability of the surface, resulting in poor spreading of the molten solder state, which will have a serious impact on the welding effect. Therefore, it is necessary to clean the surface of the box body to be welded before welding to remove dirt on the surface and ensure the welding quality.

3. Vacuum sintering process curve
Vacuum sintering refers to using an alloy with a lower melting point than the melting point of the material to be welded as solder under a certain degree of vacuum. The solder is melted by heating, and the liquid solder is filled into the gap of the welding contact surface by capillary action. The liquid solder and the metal to be welded diffuse and dissolve to form an intermetallic compound, which is finally cooled to form a highly reliable weld.

The vacuum sintering temperature curve generally consists of four parts: preheating zone, heat preservation zone, peak zone, and cooling zone. The preheating area is mainly used to preheat the raw materials, improve solder activity, and thus ensure the bonding effect; the insulation area is mainly used to ensure that the raw materials are heated evenly and reduce the impact of temperature differences. At the same time, it can be used under the action of flux or reducing gas. Remove the oxide film on the surface of raw materials; the peak area is mainly used for the eutectic reaction of solder to generate intermetallic compounds to achieve welding between raw materials; the cooling area is mainly used to control the cooling rate to avoid the impact of too fast or too slow cooling on the metal to metal The grain growth of the compound eventually leads to problems such as a decrease in bonding strength. By setting an appropriate temperature curve and performing the vacuuming process at an appropriate temperature point, it is possible to ensure that bubbles or flux in the solder are extracted, thereby achieving low void rate sintering.