In order to achieve high-quality results from a selective soldering process some issues has to be regarded in the design of the soldering pads on a PCB. These issues are based on the experience of application engineers and are mainly empirically determined recommendations.
The recommendations in this articel regarding the pad design and the overall PCB design as well.
- The distance C’ between pads should fit with the component size as shown in Fig. 1.
This can prevent a misalignment of the components during the soldering process. The reason for this behavior is that the component tends to align with the edges of the pad floating on the molten solder.
- The pad area have to fit to the solder paste volumen and the component size as shown in Fig. 1.
If the area is too small the solder junction will not form a good meniscus as defined for most of the solder junctions.
In Tab. 1 and Tab. 2 there are some values for a good pad design with respect to typical SMD resistors sizes:
|Size||L [mm]||W [mm]||D [mm]||Dc [mm]||P [W]|
Tab. 1 Dimensions of some typical SMD resistors
|0201||0,375 ± 0,05||0,275 ± 0,02||0,3 ± 0,05|
|0402||0,65 ± 0,1||0,55 ± 0,05||0,6 ± 0,1|
|0603||0,98 ± 0,2||0,88 ± 0,1||0,96 ± 0,2|
|0805||1,475 ± 0,2||1,375 ± 0,1||1,2 ± 0,2|
Tab. 2 Dimensions of the pads according to the dimensions in Tab. 1
In order to achieve a suitable PCB design for selective soldering of SMDs, heat conduction of the connection has to be regarded. If the connection area is too large, the heat conduction will be very high and the energy input from the laser must compensate this. In certain cases the laser power can even be too low to melt the solder, caused by a high heat loss at the pad area. So, the design of the PCB has to be reconsidered in order to achieve a stable production process. Thus, Fig. 2 depicts some advices for a feasible design regarding the conncetion of the pad onto the PCB.
Fig. 1 Example of a suitable and a non-sufficient pad design for assmbling SMDs on a PCB using selective soldering, e.g. laser soldering
Fig. 2 Example of good and non-sufficient connection design of the soldering pads on a PCB for the usage of selective soldering, e.g. laser soldering