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Ultra-fine pitch development trend on new requirements for solder paste
As electronic products move toward shorter, smaller, lighter, and thinner directions, packaging devices are shifting toward smaller, thinner, faster, more convenient, and more reliable structural trends. Under this market trend, 01005 devices and 0.35MM pitch chips smaller than 0201 The device has been promoted and applied, which means that the difficulty of SMT assembly will be greatly improved. The miniaturization of the pad size and pitch will make the soldering and soldering of the solder paste significant more difficult. Now, the No. 4 powder solder Paste will be difficult to meet the demand in the 01005 device and the 0.35mm fine pitch chip printing. How to improve the quality and improve the quality? The reliability of solder joints puts new demands on the application performance of solder paste. a new generation of fine powder---5 powder lead-free solder paste will become the trend of the industry.
The paper mainly tests and evaluates the printing and soldering performance of No. 4 powder and No. 5 powder lead-free solder paste on high-end smartphones and special test boards. Through various data collection, the differences are analyzed and a new generation is No. 5 powder solder paste can fully meet the printing requirements of 0.35mm ultra-fine pitch chip and 01005 pad setting and the welding requirements under nitrogen atmosphere, so as to select more suitable solder paste and stencil for welding on finer pad in The future. Holes provide a reference.
1. The difficulty of soldering process caused by the trend of device package miniaturization
1.1 The trend of miniaturized device packaging
CHIP component (passive component) and pad size change (unit: mm) Table 1
As can be seen from the above table, when the component size is changed from 0402 to 01005, the pad area will be reduced by 85%.
BGA component (active component) pitch and pad size change (unit: mm) Table 2
As can be seen from the above table, when the BGA pitch is changed from 0.5 to 0.3, the pad area is reduced by 50%.
1.2 Pad size reduction corresponds to stencil opening requirements
Area ratio and aspect ratio of different stencil thicknesses on different device pads (opening ratio 1:1) Table 3
Area ratio and aspect ratio (BGA circular opening) obtained by different template thicknesses on different BGAs Table 4
1.3 The thickness of the stencil is inversely proportional to the area ratio (AR). Therefore, when the stencil opening is reduced, the thickness of the stencil must be reduced in order to obtain a standard area ratio.
1.4 The finer the pad size and pitch, the higher the difficulty of soldering tin paste, and the volume of tin can be reduced by multiples.
Increased welding difficulty.
1.5 Reducing the pad size and pin pitch. To achieve the standard tin-to-tin area ratio, it can only be achieved by thinning the thickness of the stencil, but this will greatly reduce the volume of tin under the solder paste, no doubt, this It will affect the welding; for example, the No. 4 powder solder paste is used to produce high-end smart phone products with high assembly density. This product has multiple 0.4mm pitch BGAs and large structural parts and plane frames, with 0.1mm thickness. The stencil, the 0.8mm pitch BGA obtained AR=0.625<0.66, which caused the bad soldering or tinning in the BGA printing, but the 0.08mm thick stencil and the 0.4mm pitch BGA. AR=0.781≧0.66, the ideal tin effect can be obtained, but for the structural parts and the plane frame with a large amount of tin, it is easy to cause false soldering because of insufficient tin content. How can the two problems be effectively solved? Can improve welding yield? The selection of No. 5 powder solder paste has become a trend in the development of the industry.
No. 4 powder particle state and particle size distribution (standard particle size range 20-38UM) Table 5
No. 5 powder particle state and particle size distribution (standard particle size range 15-25UM) Table 6
Using the particle size of the microparticle image analyzer to detect the particle size of the powder, it was found that the powder diameter of the No. 4 powder was distributed at 30-35 UM, while the powder diameter of the No. 5 powder was distributed at 18-25 UM, indicating that the powder diameter of No. 5 powder It is 10 UM less than the No. 4 powder diameter.
2.2 The relationship between the diameter of the particles and the welding of the stencil
Five-ball law: The thickness of the steel mesh is generally not less than four times the diameter of the solder paste particles, preferably five times. The minimum rectangular opening for a steel mesh is generally no less than five times that of solder paste particles. For the smallest circular window opening is generally not less than eight times the solder paste particles. To ensure such a relationship, we can ensure a better weld quality.
Relationship between tin powder particles and stencil openings Table 7
It can be seen from Table 7 that with the two commonly used stencil opening methods, No. 4 can only satisfy the "three-ball law", and No. 5 powder can satisfy the "five-ball law", so that good printing and printing can be obtained. Welding quality.
2.3 Differences in surface oxygen content between No. 4 powder and No. 5 powder
As can be seen from the above table, the finer the tin powder, the higher the oxygen content on the surface, indicating that more solder paste is needed to remove the oxidation during soldering.
BGA and SIM card devices Table 8
It can be seen from the amount of tin in the BGA that the amount of tin under the 0.1mm steel mesh is significantly increased; the SIM card (the steel mesh is locally thickened to 0.1mm), and there is no significant difference with the tin content of the No. 5 powder 0.1mm steel mesh.
CHIP and QFP devices Table 9
Test board pad design Remarks: 1 and 2 are OSP, 3 and 4 are nickel gold
2.5.2 Printing under tin molding comparison
2.3.2 Welding comparison on the test board
No. 4 powder solder paste is well soldered on the 01005 device pad and 0.2mm pad. Most of the solder joints have a grape ball phenomenon and do not meet the IPC standard. The No. 5 powder solder paste is completely cold welded. It shows that the finer the tin powder, the easier it is to oxidize when subjected to high temperature, and it cannot meet the welding requirements under air conditions.
There are tiny voids in the BGA solder joints of No. 4 powder and No. 5 powder solder paste. There is no significant difference between primary reflow and secondary reflow, but they all meet the IPC-7095A standard.
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