1. What is IMC?
IMC stands for Intermetallic Compound, which is often translated into Chinese as [Intermetallic Compound] or [Intermetallic]. It is important to note that IMC is a chemical composition rather than an alloy or a pure metal. As IMC represents a molecular structure, energy is required for its formation, which explains why solder paste must be heated during the soldering process. In this context, only pure tin (Sn) in the solder paste interacts with the copper base (OSP, I-Ag, I-Sn) or nickel base (ENIG) through a diffusion reaction at elevated temperatures, leading to the formation of a robust interfacial IMC.
2. What is the difference between [alloy] and [Intermetallic Compound]?
1. The interface metal compound is formed by combining more than two metal elements in a “fixed ratio.” This results from a “chemical reaction” and constitutes a pure substance. Examples include compounds like Cu6Sn5, Ni3Sn4, and AuSn4.
2. An alloy, in contrast, is a mixture of two or more metals with a non-fixed ratio that can be adjusted as needed. It simply requires a uniform blending of different elements. Thus, one could say that men and women together represent alloys, while the children born from their union represent compounds. Does this metaphor resonate?
3. Why does “Solder Paste” contain other metal components? Pure tin has a high melting point of 232°C, which poses challenges for general PCB assembly and soldering, as current electronic components cannot withstand such temperatures. Therefore, solder primarily consists of tin, with added alloys to lower the melting point, enhancing mass production efficiency and energy savings, while also improving the toughness and strength of solder joints. For instance, adding silver and copper produces SAC305, lowering the eutectic point to 217°C; combining copper and nickel yields SCNi with a eutectic point of 227°C. An intriguing question arises: why does the eutectic point of two high-melting-point metals decrease significantly when mixed in a specific ratio? Interested readers can consult the binary equilibrium metallographic diagram of tin-lead for insights.
4. I often encounter the chemical formulas Cu6Sn5, Ni3Sn4, Cu3Sn, AuSn4, Ag3Sn, and PdSn4 in IMC. What are their formation and locations? Copper-based PCBs undergo surface treatments like OSP, I-Ag, I-Sn, HASL, and solder paste, which during high-heat reflow, generate the beneficial IMC Cu6Sn5. However, if the PCB remains in the reflow furnace for too long, the inferior IMC Cu3Sn can develop over time. Nickel-based surface treatments, such as ENIG, ENXG, and ENEPIG, will form the desirable IMC Ni3Sn4 when combined with solder paste in a high-heat reflow environment. Gold, silver, and palladium can also form compounds like AuSn4, Ag3Sn, and PdSn4 with tin, but these roaming IMCs can weaken solder joint strength. The primary role of gold and silver on the solder pad is to protect the underlying nickel and copper from corrosion. An optimal thickness of gold and silver is essential; too thick can weaken solder joint strength, while too thin may fail to provide adequate protection.
5. What is the strength of various IMCs? Remember that soldering is a chemical reaction. For instance, in copper-based solder pads, effective soldering generates the benign η-phase (Eta) Cu6Sn5, which grows thicker with soldering heat and aging. Over time, malignant ε-phase (Epsilon) Cu3Sn can develop from Cu6Sn5. Generally, copper-based joints exhibit better strength and reliability compared to nickel-based ones. Thicker nickel-based finishes, such as immersion gold and electroplated nickel-gold, tend to have thinner IMCs, leading to potential gold brittleness. The nickel base can form Ni3Sn4 only after AuSn4 migration, but its strength does not match that of Cu6Sn5.
6. Is a thicker IMC always better? A uniformly grown interface IMC is sufficient, as it will naturally thicken with time and heat. However, excessive IMC thickness can compromise strength and induce brittleness, akin to cement between bricks: the right amount holds them together, but too much can weaken the structure. The generation rate of IMC generally correlates with the square of time and temperature. This concludes the overview of IMC and its relationship with PCB soldering strength.
