A Proposed Mechanism and Remedy for Ball-in-Socket and Foot-in-Mud Soldering Defects on Ball Grid Array and Quad Flat Pack Components

摘要

A common source of defects on area array components is the "ball-in-socket" (or "pillowhead") defect. This defect is defined as one or more connections that show physical contact but no wetting or intermetallic connection after reflow. The defect is difficult to detect on x-ray, and can only really be verified on cross section or if the joint in question is in a location accessible to visual inspection. Worse, the assembly may pass electrical test, since there may physical contact between the bulk solder and the metallization on the component lead. The lack of an intermetallic bond results in almost immediate failure in the field, however. The same sort of defect can also occur on large quad flat pack components, with the component lead resting on top of the solder deposit without a metallurgical connection. In this case, the defect is referred to as a "foot-in-mud" defect. The source of these defects is not always obvious, and little has been written about their prevention. This paper presents an in-depth examination of the physical causes of this defect type, along with specific steps that may be taken to eliminate it. There are several potential root causes, but the end result of all is vertical movement of one portion of the component (tilting), resulting in lack of contact with the land during soldering. Formation of an intervening oxide layer prevents soldering, even when the two metal surfaces are brought together. Prevention of these defects relies on good design practices that limit thermal gradients, well-designed reflow profiles and capable reflow equipment. The specific solder paste used can also have an impact on the appearance of this defect, for several reasons including the alloy melting behavior, flux activity and rheology, and printing characteristics.