MVCs refer to the most vulnerable components (MVCs) during reflow soldering such as liquid dielectric aluminum electrolytic capacitors, connectors, DIP switches, LEDs, transformers, PCB (Printed Circuit Board) substrate material etc. Lead and lead-free components differ from each other in terms of their capability to withstand reflow soldering.
Because the peak temperature in lead reflow soldering won't exceed 230°C, heat resistance of MVCs should be set to be at 240°C, including all the soldering tools manufactured by industrial manufacturers, soldering equipment and supporting materials used for soldering all of which are designed and selected based on heat resistance of 240°C.
Peak temperature in lead-free reflow soldering can be as high as 250°C, so the lowest heat resistance of MVCs has to be set to be at least 260°C. As a result, all the soldering tools manufactured by industrial manufacturers, soldering equipment and supporting materials used for soldering have to be designed and selected based on heat resistance of 260°C.
Equivalent to lead wave soldering, lead reflow soldering shares the same commonly-used solder paste ingredient, that is, Sn37Pb eutectic solder paste and Sn36Pb2Ag solder paste.
Ingredient of alloy in solder paste applied for lead-free reflow soldering mainly includes:
a. SAC305 solder paste. As one of the most wide-ranging elements applied in modern industry, it features a melting temperature from 217°C to 220°C.
b. SAC387 solder paste. As eutectic composition of SnAgCu alloy, SAC387 features its melting temperature at 217°C on which it is able to complete solid-liquid transition. Due to its low melting temperature, it's primarily applied in some specialty products, military applications for example.
As far as simple products are concerned, peak temperature range during lead reflow soldering is from 205°C to 220°C. When it comes to complex products such as some IC packages, however, peak temperature may be as high as 225°C, which is indicated in the figure below.
As far as lead-free reflow soldering is concerned, if lowest peak temperature is 235°C in practical reflow soldering, the highest peak temperature will be determined by temperature difference (ΔT) on PCB board which, however, is determined by PCB size, PCB board thickness, PCB layer count, component layout, copper layer distribution, component size and thermal capacity. Those large and thick PCBs with large and complex components assembled on feature a typical ΔT that is as high as 20°C to 25°C. As a result, peak temperature should be minimized to prolong preheating and reflow soldering time, as is indicated in the following figure.