Measuring Intermetallic Phase Growth in Lead Free Solder Joints
Accurate intermetallic phase growth measurement requires bevel-free metallographic polishing, etching chemical selection, and kinetic modeling to set supplier liability limits.
Cu6Sn5 Scalloped Morphology
Meaning
Initial solidification of the molten solder onto a copper substrate produces a characteristic crystal structure that determines the primary bonding strength of the electronic interconnect. This Cu6Sn5 scalloped morphology, known as the eta phase, is the first intermetallic compound to form during the reflow soldering process when liquid tin reacts with the solid copper surface. It is easily identified in cross sections by its rounded, wave like appearance, which provides a large surface area for the metallurgical bond.
The size and density of these scallops are influenced by the peak temperature and the duration of the liquid state during reflow. A well defined layer indicates that proper wetting has occurred, which is essential for the electrical and mechanical integrity of the joint. In lead free solder alloys like SAC305, this phase is the dominant intermetallic component and its structural stability is a primary focus of reliability testing.
Growth Morphology
Formation of the eta phase occurs through a dissolution and precipitation mechanism where copper atoms dissolve into the molten solder and then react to form crystals upon cooling. The Cu6Sn5 scalloped morphology develops because the growth rate is higher at the peaks of the crystals than in the valleys between them. This results in a non-planar interface that looks like a series of rounded hills when viewed under a microscope.
As the reflow time increases, the individual scallops grow larger and eventually merge, although the rounded top surface usually remains visible. The presence of alloying elements like nickel or silver can modify this shape, sometimes making the interface flatter or more refined. A fine-grained scalloped structure is generally preferred because it provides better resistance to crack propagation than a coarse structure with large, brittle crystals.
Reflow Dynamics
Molten solder interaction with the copper pad is the most critical stage for the development of the intermetallic structure. During the reflow cycle, the time above liquidus determines how much copper is consumed and how thick the Cu6Sn5 scalloped morphology becomes. If the temperature is too low or the time is too short, the scallops may be small and disconnected, leading to poor adhesion and potential joint failure.
Conversely, excessive heat leads to overly large scallops that can become brittle and prone to cracking under mechanical stress. The cooling rate also plays a significant role, as rapid cooling tends to freeze the structure in a finer state, while slow cooling allows for more crystal growth. Manufacturers carefully calibrate their reflow ovens to achieve a balance between ensuring a complete reaction and avoiding the overgrowth of the intermetallic layer.
Joint Failure
Mechanical integrity of the solder joint depends heavily on the thickness and uniformity of the scalloped layer. While the Cu6Sn5 scalloped morphology is necessary for the bond, it is also a brittle material compared to the soft tin matrix of the solder. Under conditions of mechanical shock or drop testing, cracks often initiate at the base of the scallops or along the interface with the copper substrate.
As the electronic device ages, the scalloped appearance may gradually flatten out as the layer undergoes solid state diffusion and transforms into a more planar structure. This transition is often accompanied by the growth of the underlying Cu3Sn phase, which further complicates the reliability of the joint. Cross sectional analysis is the primary method used to verify that the scalloped structure meets the quality standards of the industry.
A layer that is too thick or has irregular growth patterns is often a sign of poor process control during the assembly of the printed circuit board.