TO laser multi-chip eutectic placement process.
Gao Xiaowei, Zhang Yuan, Hou Yixue, Liu Yanli.
The Second Research Institute of China Electronics Technology Group Corporation).
Abstract:Based on the principle of low melting point alloy welding, the automatic eutectic placement machine equipment was introduced, the fluid protection of the eutectic welding hot table atmosphere was studied, the multi-chip eutectic welding test was carried out, and the relevant accuracy of the welding product was measured. Through the analysis of the quality inspection of the welding effect and the accuracy measurement results after eutectic welding, the process test achieved the expected effect and met the current product marketization indicators. The research conclusions provide a strong guarantee for improving the quality and reliability of 5G chips themselves, and provide a reference for promoting the rapid development of the semiconductor packaging industry.
0 Introduction. With the rapid development of 5G applications, larger transmission capacity and faster transmission rate support have become the goals pursued by the optical device module and optical communication industry, which can be said to be the lifeblood of optical communication, and optical chips are the key to optical modules [1]. Surface mount technology is the most important process in the packaging process of microelectronic components, and packaging and testing is the last link in the marketization of microelectronic products [2]. Therefore, surface mount technology occupies a very important position in the microelectronic packaging process, and multi-chip eutectic chip is one of the key process links. The multi-chip eutectic chip process directly determines the quality and service life of the product, which has a great impact on the product performance index.
1 Eutectic welding.
1.1 Eutectic welding principle.
Eutectic welding is also known as low melting point alloy welding. The soldering process of eutectic soldering refers to gently rubbing the chip on a gold-plated base at a certain temperature and pressure to wipe off the unstable oxide layer at the interface, so that the contact surfaces are melted and a liquid phase is formed from the solid phase of the two [3] . After cooling, when the temperature is below the eutectic point, the grains formed by the liquid phase combine with each other to form a mechanical mixture.
Eutectic alloy technology is widely used in the electronic packaging industry, such as chip-to-substrate bonding, substrate-shell bonding, etc. [4] . It has the advantages of high thermal conductivity, low resistance, high reliability, high shear force after bonding, and good heat dissipation [5-6]. For electronic components with high heat dissipation requirements, most of them will use eutectic alloys to complete eutectic welding.
1.2. Automatic eutectic mounter.
The automatic eutectic placement equipment is used for process placement test. The equipment adopts cam drive, connecting rod linkage, precision rack and pinion, linear motor, high-precision module and guide rail, with multi-axis motion control technology and multi-camera visual positioning technology to ensure continuous high-speed and high-precision operation of the equipment. The principle is based on the heating function of the pulse power supply under the protection of nitrogen atmosphere, so as to realize the real-time control of the heating temperature and heating time, so as to complete the one-time eutectic welding of the thermal sink pad and solder on the tube base. The equipment realizes the complex process of multi-chip eutectic soldering, realizes real-time controllable temperature time, completely replaces the process of manual solder, and improves the intelligence of the multi-chip eutectic industry. At the same time, the high-speed and high-precision operation not only improves the production efficiency, but also reduces the possibility of virtual welding of the product, so that the consistency of the finished product has been greatly improved. The structure of the automatic eutectic chip placement equipment is shown in Figure 1, and the process flow of multi-chip eutectic chip is shown in Figure 2.
2 Eutectic welding hot table fluid**.
The symcrystalline welding hot table of this equipment adopts pulse heating method, that is, the joule heating generated when the pulse current passes through the high resistivity material to achieve the effect of rapid heating. Pulse heating is an instantaneous heating method, which is only heated when the solder is melted, and multi-segment temperature curves can be set according to different solders to realize real-time control of heating temperature and heating time, which can meet the multi-chip eutectic process. A thermocouple is connected to the hot stage, which is used to feedback the temperature of the hot table to the temperature control system in real time, so that the temperature of the hot table is consistent with the set temperature.
Due to the need to complete a number of complex placement processes at the eutectic hot stage, it is impossible to have a stable atmosphere to protect the welding environment, which directly leads to the solder must be welded in the atmospheric environment during soldering, there is a risk of oxidation, and the solder climbing effect is not good. In view of the problem of poor welding effect, a relatively closed space is designed, and nitrogen is continuously filled at the same time, and the flow rate is controlled to form a relatively stable atmosphere and protective environment at the fixed place of the hot bench pipe seat. After the structure of the three-dimensional model is simplified, the software is used to analyze the fluid of the simplified eutectic hot stage. Figure 3 is shown in Figure 3 for single-sided inlet fluid** and Figure 4 for double-sided inlet fluid**.
Through the analysis of the first result contour, it can be seen that the nitrogen distribution in the single side of the hot table cover is uneven, and the nitrogen flow rate at the lead posts on the left and right sides of the hot table tube seat eutectic welding is also quite different, and the overall atmosphere environment is not ideal, which affects the eutectic welding effect, and the same conclusion is also reached through follow-up tests, and the welding effect is not good. Double-sided air intake** result cloud diagram analysis shows that the distribution of the hot table cover is relatively uniform, the nitrogen flow rate at the lead posts on the left and right sides is basically no difference when the eutectic welding of the hot table tube seat, and the conditions can be basically the same when the left and right sides are eutectic welding at the same time, the welding conditions are ideal, and the follow-up process test verifies that the welding effect meets the requirements of customers.
3. Multi-chip eutectic soldering test.
3.1 Test material.
The rapid development of 5G technology has higher and higher requirements for laser performance, especially with the increase of transmission speed, the heat generated by photoelectric signal conversion has also increased significantly. In order to improve the heat dissipation capacity of the laser itself, a new model header has been designed for 5G applications in the industry. The heat dissipation capacity of the header is improved by increasing the fan table of the pipe header, increasing the size of the pad, and replacing the gold wire bonding with direct soldering. In this test, a new type of TO5605 tube holder is used, and the matching cushion block is used as the test material, and the solder is gold-tin solder (AU80SN20), which is widely applicable in the eutectic industry.
3.2 Welding test requirements.
1) The surface of the solder is smooth and bright after welding, and there is no oxidation problem;
2) The height of the lead post must reach more than 1 4 of the height of the lead post;
3) After the heat sink pad is welded, the solder overflow around it is uniform;
4) The accuracy of the x, y direction of the heat sink cushion block patch is within 15 m;
5) The rotation angle accuracy of the heat sink pad patch is 1°.
3.3. Test and measuring instruments.
In this test, a fully automatic eutectic placement machine was used for eutectic welding, and the measuring instruments were a two-dimensional image measuring instrument and a microscope.
3.4 Welding test method.
The feeding method of the tube base is to the array tray and the cushion block blue film platform is used. The welding method is to clamp the tube seat upside down and clamp at the eutectic table, first pre-weld the heat sink cushion block on the fan table of the pipe seat, then place two pieces of solder, use a special motion mechanism to put the cushion block and the solder in the fixed position of the fan table, and apply static pressure through the nozzle, and at the same time eutectic, so that the cushion block is welded on the fan table. The cushion circuit and the pipe leg are welded together, and the subsequent soldering of the chip can be directly completed on the heat sink if it is needed, and the chip is not soldered in this test. The TO5605 header welding is shown in Figure 5.
3.5 Influencing factors of welding process.
The main factors affecting the effect of multi-chip eutectic chip process of TO laser are nitrogen atmosphere environment, nitrogen flow rate, basic constant temperature of eutectic hot stage, welding eutectic temperature, solder and socket cleanliness, welding pressure, nozzle pick-and-place position accuracy, nozzle pick-and-place rotation accuracy, camera calibration accuracy, tube holder clamping consistency and the coordination of each timing of the motion mechanism.
3.6. Determination of welding process parameters.
According to the size of the material and the size of the pipe holder, different specifications and models of bakelite nozzles are selected, and the aperture of the cushion nozzle is 05 mm, the solder nozzle aperture is 02 mm。
The test was completed under the protection of nitrogen atmosphere, by setting different inlet flow rates to determine the most suitable nitrogen flow rate for this welding, when the air intake volume was higher than 7 ml min, the nozzle would blow the cushion and solder away when placing the cushion and solder, resulting in the failure of the whole welding. When the air intake is less than 5 ml min, the welding environment cannot be closed due to each moving mechanism, and there is a risk of oxidation after welding when the nitrogen flow rate is insufficient. Therefore, the optimal nitrogen flow rate is 5 7 ml min, and the welding was completed in this test under a nitrogen protective atmosphere of 6 ml min.
Before the start of the test, set a basic constant temperature to the eutectic hot table to ensure that the preheating of the tube seat is completed, and there will be no melting phenomenon when placing the cushion block and solder at the same time, and it will not cause the blockage of the nozzle, because the melting point of the gold-tin solder (AU80S20) is 280, in order to prevent the solder from partially melting and the solder sticking to the nozzle, the constant temperature temperature is set to 270 in this test.
The optimal eutectic temperature suitable for the TO5605 header was determined by setting different gradient eutectic temperature segments, and it was found that there were four situations at different temperatures
1) The temperature is too high and the solder has been burned dry;
2) The surface is smooth after welding, and the tin climbing effect is better;
3) The welding effect is slightly worse, and the tin climbing is not ideal;
4) The solder is not melted and the welding is not completed.
Therefore, the eutectic temperature of the pulsed power supply was selected by testing 418 when the welding effect was the best.
3.7. Welding test and result analysis.
The tube holder is ultrasonically cleaned before the test, and the common product soldering test is completed on the automatic eutectic placement machine after cleaning. Through microscopic observation and analysis, it can be seen that the welding surface is smooth and bright, there is no bad situation of oxidation, and the climbing height reaches more than 1 4 of the height of the lead post. The solder overflow is uniform under the heat sink to meet the welding requirements. After eutectic soldering, a side view of the pin being welded to the spacer is observed by microscopy, as shown in Figure 6. After the co-product welding, the front view of the pin and the pad is observed by the microscope, as shown in Figure 7.
3.8. Welding accuracy test.
The two-dimensional image measuring instrument was used to randomly inspect 10 welded pipe seat cushion blocks, and the X-direction accuracy, Y-direction accuracy and rotation angle accuracy of the cushion block after welding were measured, and the measurement accuracy results are shown in Table 1.
Through the analysis of the accuracy measurement results of the sampling tube seat, it can be seen that the X-direction accuracy is within 10 m, the Y-direction accuracy is within 13 m, and the rotation angle accuracy is within 1° after the cushion block is welded, which meets the requirements of the welding test, meets the requirements of the current market-oriented mass production, and provides an accuracy guarantee for subsequent chip placement.
4 Conclusion. Through the analysis and research of the automatic multi-chip eutectic placement machine and the TO socket process, it is concluded that the automatic multi-chip eutectic placement machine can realize the automatic eutectic welding of various specifications and models of TO socket through pulse heating. Through the analysis of the fluid environment of the eutectic welding hot table, the double-sided air intake can ensure the stable and continuous protection of the atmosphere environment at the hot table welding of the TO tube base, and provide an effective guarantee for the consistency of the left and right lead post welding. Through the observation and analysis of the TO socket microscope after welding, it can be seen that the welding surface is smooth and bright, there is no problem of poor oxidation, and the climbing height reaches a quarter of the height of the lead post, which meets the current market standards and requirements. The accuracy of the x, y and angle of the welded socket is 15 m, and the angle accuracy is 1°, which meets the needs of the current market-oriented mass production of the optical communication industry. This provides a strong guarantee for improving the quality and reliability of 5G chips themselves, and provides a reference for promoting the rapid development of the semiconductor packaging industry.