Ball grid array solder joint reliability under impact test
by Zheming Zhang
Ph.D. Mechanical Engineering
xxiii, 141 p. : ill. ; 30 cm
Ball grid array (BGA) is one of the most important packages in industry due to the high interconnection density. Therefore, reliability is the focal point. Because one of these interconnections is failed, the whole package cannot be used. When solder joint reliability problems are discussed, competition exists between external loading and the strength of the sample, is always needed to be well studied. Among the different kinds of external loadings  , the high speed impact loading condition was well researched in this study. Because the intermetallic compound (IMC) layer formed by new solder material is easily to break at high speed load condition, and also in real application, many electronic devices or board level assemblies would suffer impact, such as being dropped and hit in dai...[ Read more ]
Ball grid array (BGA) is one of the most important packages in industry due to the high interconnection density. Therefore, reliability is the focal point. Because one of these interconnections is failed, the whole package cannot be used. When solder joint reliability problems are discussed, competition exists between external loading and the strength of the sample, is always needed to be well studied. Among the different kinds of external loadings  , the high speed impact loading condition was well researched in this study. Because the intermetallic compound (IMC) layer formed by new solder material is easily to break at high speed load condition, and also in real application, many electronic devices or board level assemblies would suffer impact, such as being dropped and hit in daily use and delivery. Therefore, some tests were conducted to simulate these conditions. Drop test  was one of them. By varying different acceleration speeds and impact time intervals, the solder joint strength between the solder balls and copper pads was evaluated. However it was a board level test, which can only be used to benchmark the quality of different package designs after mounting. Researchers also want to know the solder joint quality before surface mounting as a preliminary check. From the academic point of view, the mechanism of solder joint failure needs to be considered, meaning some methods should be developed to test normal stress and shear stress of solder joints. Thus, single ball pull  and single ball shear  test were used to check the solder joint normal stress and shear stress respectively, and is widely accepted by industrial standard organizations and international companies.
Based on prior researches  , the solder joint failure mode in the traditional static shear test  is not similar as what has happened in drop test  . The failure often happens inside the solder ball in the static shear test, but occurs at the intermetallic compound layer in the drop test because the IMC layer existing between solder ball and copper pad was considered as brittle material and easily broken at a high strain rate. In order to simulate the failure mode under real drop conditions, some new methods should be considered and some new test devices are produced.
In order to research the solder joint strength under different load speeds, one single solder ball impact tester (BIT)    was developed. It can provide high speed impact load to a single solder ball. Two technical issues need to be considered. First, the small solder ball diameter is about 250-300 μm and in order to achieve high speed impact condition, the impact test should finish at higher than 1.0 m/s shear speed (maximum capability: 4.0 m/s). Therefore, the time interval of this test is less than 100 μs. In order to get 100 data points during the test, the sensor should collect each data point, every 1 μs and requires a highly sensitive load sensor. Second, the distance between two solder balls is less than 500 μm, which is not large enough for the impact tip to accelerate, so some solder balls need to be removed to increase the acceleration distance for the impact tip to speed up before this test.
Some tests were done to disclose the mechanism of solder joint strength after finishing fabrication of the BIT. The shear rate effect  had a great influence to solder joint strength and its failure mode. By increasing of shear speed, the ductile-brittle (D-B) transition zone would be presented. However, rarely has research discussed the mechanism behind this.
The mechanism of D-B transition is considered as a competition between solder strength and IMC strength. In order to confirm this hypothesis, the material property of both solder and IMC needs to be understood. However, few researchers have discussed the material property of IMC, due to testing difficulties  . The IMC layer was usually less than 10 μm and formed by metal diffusion. Therefore, it’s difficult to make pure IMC bulk material to test the mechanical property under high speed test conditions.
Due to the thin IMC layer, a novel method was introduced to measure its mechanical property. In these samples, all tin material was consumed by copper diffusion. Finally, only thin IMC layers remained between two copper plates. A shear test followed, and a crack would happen in the IMC layer. Using this method, the failure stress of IMC was tested.
From the test results, we clearly found that the failure stress of solder material would increase and the failure stress of IMC material would decrease when the test speed increased. And it means there will be a crossing point area in which range the D-B transition often happened in the single solder ball shear test. Before this transition zone, the solder material would dominate the solder joint strength, and after this transition zone, the IMC material strength is the dominate factor.
After obtaining the failure stress of IMC and solder material, parameters which can affect the strength of these two materials would be discussed. From further experiments, it is known that the solder composition affects the solder material’s strength. And it is proved by following experiments. On the other hand the surface structure affects the IMC property. Experiments of four pad designs are introduced, with different pad structures, the strength of IMC was altered by changing the thickness of IMC layer, the mechanical interlocking, and the defects at the interface. These two parameters would finally affect solder joint reliability. The model provided in this research was finally proven by experiment. It provides a guideline for developing new lead-free materials and surface treatment in electronic packaging industry.