The lead-free reflow soldering process has much higher requirements on the PCB than the lead-based process. The heat resistance of the PCB is better, the glass transition temperature Tg is higher, the thermal expansion coefficient is low, and the cost is low.
Lead-free reflow soldering requirements for PCB.
In reflow soldering, Tg is a unique property of polymers, which determines the critical temperature of material properties. During the SMT soldering process, the soldering temperature is much higher than the Tg of the PCB substrate, and the lead-free soldering temperature is 34°C higher than that with lead, which makes it easier for the thermal deformation of the PCB and damage to components during cooling. The base PCB material with higher Tg should be properly selected.
During welding, if the temperature increases, the Z-axis of the multilayer structure PCB does not match the CTE between the laminated material, glass fiber, and Cu in the XY direction, which will generate a lot of stress on the Cu, and in severe cases, it will cause the plating of the metallized hole to break and cause welding defects. Because it depends on many variables, such as PCB layer number, thickness, laminate material, soldering curve, and Cu distribution, via geometry, etc.
In our actual operation, we have taken some measures to overcome the fracture of the metallized hole of the multilayer board: for example, the resin/glass fiber is removed inside the hole before electroplating in the recess etching process. To strengthen the bonding force between the metallized hole wall and the multi-layer board. The etch depth is 13~20µm.
The limit temperature of FR-4 substrate PCB is 240°C. For simple products, the peak temperature of 235~240°C can meet the requirements, but for complex products, it may need 260°C to be soldered. Therefore, thick plates and complex products need to use high temperature resistant FR-5. Because the cost of FR-5 is relatively high, for ordinary products, composite base CEMn can be used to replace FR-4 substrates. CEMn is a rigid composite base copper-clad laminate whose surface and core are made of different materials. CEMn for short represents different models.
Post time: Jul-22-2023