Influence of reflow welding in the hottest protect

2022-08-18
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Influence of protective atmosphere reflow soldering on the performance of plastic ball grid array solder joints

with the development of computer technology, the working frequency of chips is required to be higher and higher, there are more and more pins, the arrangement of components on boards is more and more dense, the production batch is larger and larger, and the replacement cycle is shorter and shorter, This puts forward high requirements for electronic packaging technology. Bgaballgridarray packaging is a new electronic packaging technology that has developed rapidly under this requirement in recent years. BGA packaging devices produce spherical contacts as pins on the bottom of the substrate in the form of array. Therefore, when continuing experiments on a large number of experimental chips, the operation efficiency can be improved. The number of pins is much higher than that of square flat package (QFP). In terms of assembly technology, BGA has better characteristics than QFP devices, which is mainly reflected in the fact that BGA devices have less strict requirements for mounting accuracy; There is no pin deformation of devices like QFP; It has good coplanarity, large lead out foot spacing, and the defects of "bridging" are significantly reduced. In addition, due to the short pin, the lead inductance and capacitance are small, which improves the working frequency and electrical performance of the circuit. PBGA is a common BGA packaging type. It is packaged in plastic. Its manufacturing cost is low, and its electrical and thermal properties are good. It is commonly used on computer motherboards and other board cards. The chip carrier of PBGA is an ordinary printed circuit board substrate. The silicon chip is connected to the carrier by wire pressure welding or inverted core chip, and then molded with plastic, The eutectic solder ball array is connected to the lower surface of the carrier. Although BGA packaging has a series of advantages, there are still potential reliability problems. From the perspective of assembly process, since the PBGA pins are not exposed, most of them can only use the cleaning free process, and there are certain difficulties in quality inspection. In order to improve the process quality, protective atmosphere reflow welding can be used. However, At present, many boards and cards are not completely assembled by protective atmosphere reflow welding. How different are the quality of boards and cards produced by nitrogen protection reflow welding and those produced without protective atmosphere? Because the equipment investment and production cost of providing nitrogen are very different from the purity of nitrogen, so what is the best purity of protective atmosphere, In this paper, there are a lot of pores and impurities between the PBGA Solder Joint and the copper pad obtained by static bending and high-speed bending deformation flow welding, and the defects of the solder joint after nitrogen protection reflow welding are significantly reduced. This shows that the * great advantage of nitrogen protection reflow welding is that it greatly reduces the defects at the welding interface, thereby improving the strength and reliability of the solder joint. The solder ball components of spherical array packaging include pb/sn (90/10) with high melting point, There is also eutectic component sn/pb with low melting point (63/67> PBGA packages all use eutectic composition solder balls with low melting point. The reflow soldering process of eutectic solder balls is slightly different from that of high melting point solder balls: the reflow soldering of eutectic solder balls does not need to print a 'layer of eutectic soldering fig.8schematicofadutilefracture paste in advance, but only needs to apply a layer of flux. In the subsequent reflow soldering process, the eutectic solder balls will melt and weld with copper pads when the temperature reaches and exceeds eutectic At the temperature point, the solder ball melts and rapidly extends to the whole pad. In this process, the impurities and pores between the interfaces will be pushed to the edge. These defects at the edge become the crack source. (a) It should be seen that there are many pores and impurities at the upper right edge, which is the starting position of the crack. The impurity distribution at the whole edge is not seen from the fracture, but only at the beginning of the crack. In fact, during the crack propagation process, the copper tin master alloy is very brittle, and the crack basically propagates along the master alloy. Therefore, the propagation area seen on the fracture is the master alloy layer, not the part where impurities and pores are concentrated. The defect rate of the fracture surface was statistically measured by laser microscope and image analyzer. The results showed that there was a high defect rate in the reflow welding without protective atmosphere. With the reduction of oxygen content in nitrogen protected atmosphere reflow welding, the defect rate at the solder ball interface also decreases slowly. 5 conclusion the welding interface performance of solder joints plays a very important role in the reliability of electronic packaging and assembly range parts. By studying the static and high-speed deformation fracture behavior of PBGA Solder Joints obtained by different reflow soldering processes, the following conclusions can be drawn: using high-speed deformation test for PBGA Solder Joints, different fracture modes from static deformation can be obtained, and high-speed deformation test can show the brittleness of solder joints. In the nitrogen shielded reflow welding process, with the decrease of oxygen content (from 1000ppm to 50ppm), the energy required for PBGA Solder Joint Cracking and the increase of oxygen content, the lower the oxygen content, the better the mechanical properties of the solder joint. The area diagram of static and high-speed deformation and fracture of PBGA Solder joint is between PCB and solder ball. There is brittle Fig.9 copper tin master alloy layer in this area. For high-speed deformation and fracture, the fracture presents typical cleavage fracture with brittle peeling fracture/www shows that the brittle characteristic crack extends along the thin medium Si gold layer. There are a lot of pores and impurities in PBGA Solder Joints obtained by convection reflow welding with compressed air, while the defects of reflow welding joints protected by nitrogen are significantly reduced. Crack source. J film technology (ion plating, diamond-like carbon film) and pore and impurity defects are mainly distributed at the welding interface of PBGA Solder joint. When the solder ball melts, these defects can be selected by users to be more suitable for their own product equipment and pushed to the edge of the pad. Therefore, the distribution density of defects at the edge is higher than that at the center. In the process of high-speed deformation and fracture, The defects at the edges become the back cover. 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