Microvia Stress Mechanics under Reflow Temperature Profiles
Substrate Z-axis thermal expansion above Tg drives low-cycle fatigue and target pad separation in HDI microvias during SAC305 lead-free reflow profiles.
SAC305 Lead-Free Reflow
Attachment Process
Soldering operations using the standard tin silver copper alloy require a specific thermal cycle to ensure reliable joint formation. Sac305 lead-free reflow involves heating the assembly to a peak temperature between 235 and 250 degrees Celsius to melt the metal. This alloy is the most common replacement for leaded solder because it offers a balance of mechanical strength and wetting performance.
Phase Transition
The material remains solid until it reaches its melting point of 217 degrees, at which point the liquid solder begins to flow and interact with the copper pads. During this liquid phase, the tin reacts with the copper to form an intermetallic layer that provides the mechanical bond. If the temperature is too low, the solder does not fully wet the surface, while excessive heat can lead to the growth of a brittle intermetallic layer.
The cooling rate after this stage is controlled to create a fine grain structure within the solder joint.
Joint Integrity
Proper execution of the cycle prevents the formation of voids and ensures the joint can withstand the vibration and thermal cycling of its intended environment. Finished assemblies are inspected using optical or x ray methods to verify the fillet shape and the absence of bridges.