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60 SMT Magazine • June 2014 THe TROuBLe WITH BgA SOLDeR JOInTS continues arTICLe Figure 1 shows a simplified illustration of the structural relationships. A static, oriented signal with simple ohmic resistors is assumed. The chip's internal conditions (wire bonding, etc.) are considered OK and thus not shown here. During reflow, the solder of the balls and the solder paste will melt, and through a chemical reaction an intermetallic zone will be formed between molten solder and board surface. An- other intermetallic zone exists between ball and chip. It is built up during manufacture of the BGA chip and must be checked by the BGA manufacturer. From an electrical point of view, the line resistance of the ball and the interme- tallic zones is essential. Normally, the resistance between signal source and sink should remain stable in the milliohm range. But all theory is grey, and in practice system- atic and random errors occur and lead to heav- ily altered parameter values. Even shiny solder joints can't guarantee solder joint integrity. Soldering defects arise from quality defects of the elements to be soldered, but also from deviating soldering profiles. The error patterns may differ widely. They range from visible de- formations of the joint in the sense of insuffi- cient or excess solder, where the electrical con- tact may be given, to visually perfect solder joints with random or even no contact. With regard to evaluation of a BGA solder joint, the standard IPC-A-610E [1] plays an impor- tant role. It sets up acceptability requirements for electronic assemblies and identifies criteria for BGA components. In a production environ- ment system solutions are necessary, which are capable of verifying compliance of the solder joints with that standard. This helps to avoid structurally unstable solder joints, which may break under mechanical load and lose electrical conductivity. It should be noted, however, that many defects, which are related to the form of the solder joint, will show electrical effects only under extreme conditions. In contrast, failures in the intermetallic zone are particularly devilish and hard to recognise. "Head-in-pillow" and "black pad" are widely known phenomena of that kind. With the first effect, the solder doesn't fuse with the solder paste, so a sort of barrier layer will be build up. The visual appearance of the solder joint, how- ever, normally doesn't reveal that. This effect is mainly caused by contamination of the ball surface. On the other hand, black pad is more re- lated to board issues. Here, the ball reacts with the solder paste, but below it, a layer is built up with reduced or entirely missing conductivity. This phenomenon is mainly caused by quality defects of the surface of the board's pads. Table 1 gives an overview of the fault categories dis- cussed so far. As Table 1 shows, there are a number of fault scenarios, and all of them must be controlled to ensure the required production quality. What's more, in practice the typical problems vary be- tween different manufacturers, and sometimes figure 1: Static reference model illustrating corresponding mechanical and electrical design.