Layout Tipps & Tricks
Soldering / Soldering Processes - Part 2
In order to make a mechanically and electrically reliable brazed joint, time and temperature are required. Soldering does not produce a metallurgical joint as in welding, but rather creates a joint in which different alloys or metals diffuse into each other. This creates an intermetallic phase (IMP). The goal is to achieve the ideal thickness of this IMP. If it is too narrow, a cold solder joint results; if it is too thick, it becomes brittle.
Here, the layout lays the foundation for the subsequent achievement of an optimal solder joint, especially with regard to heat requirements, connection surfaces and heat input or heat dissipation. There are various methods for soldering, but a nitrogen tunnel is used in all machine soldering processes to achieve the best possible soldering result.
While the last Techtalk dealt with the soldering of THT components and wave soldering, this time we will take a closer look at reflow soldering.
###IMAGE2## This process is used to solder SMT and THR components on the top or bottom side. The solder paste is deposited at the solder joint using stencil printing. All components are automatically loaded by the placement machine and the loaded assembly is transported via a transport system to a reflow (or vapor phase) soldering system.
If components are assembled on both sides, care must be taken to ensure that heavy and temperature-sensitive components are always placed on one side (electrolytic capacitors, light-emitting diodes, SMC diodes, connectors, etc.). Otherwise, these components must be glued in an additional process step.
Design parameters for soldering SMT components
The IPC-A-610 acceptance criteria for electronic assemblies describe how solder joints must look. Maximum side overhang, end overhang, minimum and maximum length, width and height of the solder joint, solder gap thickness, etc. are listed here. However, the values vary from package to package. The developer can mainly influence the solder joint in terms of component connection type, pad size, pad spacing and PCB surface.
As a developer, you should always be critical of pad suggestions in the component data sheet. Tip: Draw in connections in the footprint and perform a "proportional consideration": Can the pins on the area be soldered at all? Is there too much protrusion from the pad? Are the pads too small?
Calculation tools can also help here, such as "Proportional SMD Reference Calculator" from the Fachverband für Design, Leiterplatten- und Elektronikfertigung (FED), "PCB Footprint Expert" or "Footprint Designer" from the ECAD systems. These tools usually generate better finishable footprints than those of the manufacturers.
Tips and tricks from 30 years of experience in electronics production can be found in our EMS Design Guide.
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