[Text / high-tech LED reporter Long Zonghui]
"Our solid crystal eutectic machine adopts the method of soft eutectic soldering, and the requirements for chip and bracket materials will be greatly reduced, thus ensuring 99% super high yield of products." Shenzhen Invo Technology Co., Ltd. General Manager Chen Xiaoyu said.
In 2012, as an LED equipment manufacturer, it is definitely a year to preserve its strength. At this time, Chen Xiaoyu believes that the energy production of the solid crystal eutectic machine is a minority. At present, this product has been successfully tested in 6 companies. The yield rate has reached 99%. Compared with the high price of such equipment from foreign manufacturers and the need to customize the market, the price of 380,000 automatic equipment and 280,000 semi-automatic equipment is very competitive.
First to introduce solid crystal eutectic equipment
The waste caused by the conversion of 80% of the energy input from the high-power products of LED lamps into thermal energy not only makes the LED energy-saving level greatly reduced, but also causes the LED product to have light attenuation, luminous efficiency and stability due to excessive LED junction temperature. In addition to providing mechanical and electrical connections to the device, the solder layer of the wafer also requires a good heat dissipation path for the device. At present, the bonding between the chip and the substrate is mainly by silver glue bonding. At present, the commonly used silicone resin and epoxy resin-based glue have obvious difference in thermal conductivity compared with metal, and the metal alloy welding method is marketed. More and more respected.
At present, the most representative metal alloy welding methods are gold tin eutectic and silver tin alloy. The gold-tin alloy welding method is also called hard eutectic method. This alloy is mainly produced by repeated lamination, so it is very difficult to control in terms of uniformity. The hard eutectic welding method has the advantages of high mechanical strength, small thermal resistance, good stability, high reliability and less impurities. However, the equipment cost is extremely high. At present, the sales price of ASM is about 2 million to 3 million, and it needs special order. "This kind of eutectic method is superior in quality, but the price is very expensive." Chen Xiaoyu said that because Wowo mainly uses silver-tin eutectic welding method, because its welding mechanical strength is relatively small compared with gold tin, it is also It is called soft eutectic soldering. The thermal conductivity of silver-tin alloy is about 53 W/mÂ·K, which is second only to the thermal conductivity of gold-tin alloy (Au80Sn20) of 57.3 W/mÂ·K. However, the gold plating/silver plating thickness, high temperature resistance and process difficulty of the bracket are greatly reduced. Solving the failure requires an overall solution
It is understood that the welding failure mode is mainly divided into two cases: ohmic contact failure and thermal stress failure. Poor ohmic contact will increase the thermal resistance of the device and uneven heat dissipation, affecting the distribution of current in the device and damaging the thermal stability of the device. It is mainly caused by the weak soldering or voiding of the Ag-Sn soldering layer, which causes a current-intensive effect, resulting in irreversible destructive thermoelectric breakdown in the vicinity. The ohmic contact failure is mainly a mechanical profit failure, and the final performance is the weld surface crack or wafer peeling. The soldered surface of the LED is composed of materials with different properties. Due to the different coefficients of thermal expansion, the compressive or tensile stress existing between the interfaces of different materials when they must be bonded together may accumulate in the cavity to cause the solder to form cracks or even The silicon chip cracked and eventually caused the device to fail.
In order to control the bad products, Chen Xiaoyu believes that it must be improved from the following points:
First, the quality of the chip and the way it is saved. The wafer production process often requires backside steaming. At the eutectic temperature, Si will penetrate the gold layer and oxidize to SiO2. This layer of SiO2 will cause uneven weld infiltration and lead to a decrease in weld strength. The protective gas N2 during welding must ensure a sufficient flow rate, preferably by adding part of H2 for reduction. In addition, the preservation of the chip should also be paid enough attention not only to pay attention to the temperature and humidity of the environment, but also to consider the solderability in the future. For long-term unused chips, it should be placed in a nitrogen cabinet.
Second, thermal control. Although the Ag-Sn eutectic point is about 230 Â°C, there will be some loss in the heat transfer process, so choosing a slightly higher temperature can effectively avoid the surface oxidation or PPA yellowing failure of the stent. In order to ensure the welding quality, it is necessary to measure the temperature of each zone of the eutectic furnace in time during the welding process, and the temperature of the welding surface must be monitored if necessary.
Third, the basic quality control of the bracket. The substrate of the stent is contaminated, with localized oil stains or oxidation which directly affects the wettability of the welded surface. Therefore, in the eutectic operation, we must pay attention to the operation specification, and it is forbidden to directly contact the welding surface of the material by hand.
Fourth, thermal stress control. The failure caused by thermal stress is a slow gradual process and is not easy to detect. Generally, the larger the thickness of the wafer, the smaller the stress, so the wafer should not be too thin. In addition, if the support or substrate does not match the thermal properties of the wafer, it will also cause great mechanical stress. Before welding, the bracket or substrate must be preheated at 200 Â° C. If necessary, it can be slowly cooled under N 2 protective atmosphere, and part of the stress can be eliminated during the cooling process.
Fifth, the substrate gold layer is too thin. When the gold plating layer of the substrate is thin and not dense enough, even under the protection of nitrogen, when the Au-Si eutectic temperature is reached, the plating layer will be severely discolored, thereby affecting the welding strength. Experiments have shown that for a 1 mm Ã— 1 mm chip, the thickness of the gold plating on the substrate is greater than 2 Î¼m to obtain reliable eutectic soldering. Generally, the larger the chip size, the corresponding increase in the gold plating layer.
It can be seen from the above various failure modes that in addition to the quality problems of the eutectic solid crystal device, the causes of the chip, the bracket, the auxiliary materials, and the personnel operation may cause the product to fail. â€œInvestors provide customers with an overall failure solution. In addition to eutectic solid crystal machines, we also offer eutectic solders â€“ silver solder paste and eutectic furnaces.â€ Chen Xiaoyu said.
LED eutectic solid crystal requires high flowability of solder. The finer the solder particles, the more convenient for automated equipment operation. In addition, the more uniform the solder melts during soldering, the uniformity of solder joints printed on the bracket, and the void ratio after soldering. The time, temperature and firmness of welding can be greatly guaranteed. The baking phase after eutectic solidification also has high requirements on the temperature change curve, time and temperature. "Our eutectic furnace is precisely adjusted by visualization, and it is completely baked according to the temperature curve, every 6 minutes, and Guarantee the product's 99% yield level." Chen Xiaoyu introduced.
On the whole, after using eutectic solid crystal technology, the luminous flux of the product will increase by about 5% under the same color rendering index; the brightness will not decay with time; the junction temperature will drop by more than 2 Â°C; the thermal conductivity is 5 More than twice; the shearing thrust is more than twice that of the silver paste; the welding time is only 1/15 of the silver paste. "At present, our upgraded eutectic solid-crystal equipment has a yield of over 99%, and the production capacity can reach 5k-6k/H." Chen Xiaoyu said.
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