The Reflow Soldering stage consists of pasting the surface mount components to printed circuit boards (PCBs). A sticky mixture of powdered solder and flux, called the solder paste is used to stick the components. It is a simple process where the components/PCB/solder paste are pre-heated to form acceptable solder joints. But one must be careful to avoid any damage due to overheating. In addition to that, there are other aspects one must be mindful of for an effective reflow soldering process:
Suitable Reflow Soldering Machine:
A wide range of reflow soldering machines can be found in the market. Each has a distinct character depending on the required line speed and design/material of the PCB assemblies to be processed. Be sure the selected oven is of a suitable size to handle the production rate. Based on the repeatability of the process, the load factor of the oven must be checked.
Based on the design of the PCB assembly choose the possible machine with available options, (a) based on the conveyor type, which could be a mesh conveyor or an edge conveyor (b) with closed-loop control for speed of convection fans and (c) Automatic control of conveyor and centre-board-support widths.
Acceptable Reflow Profile:
Each assembly needs to be considered separately in order to create an acceptable reflow profile. The type of solder paste, PCB material & its thickness, number of layers, amount of copper within the PCB, and number & type of surface mount components, can affect how the reflow oven is programmed.
To attain an acceptable reflow profile, initially, a sample assembly is created by connecting multiple thermocouples in a number of locations which is passed through high temperatures and then the range of temperatures across the PCB is measured. The main aim is to transfer enough heat into the assembly to melt the solder and form the solder joints without causing any damage to components or PCB.
PCB/Component Footprint Design:
For an assembly to reflow smoothly, it is important to have a balanced footprint design. Make sure the size of tracks connecting a component footprint to the other is the same in size. Also, adding copper balancing to waste areas is recommended to avoid panel wrapping.
Carefully printed PCB using a well-designed Stencil:
Consistent deposit of solder paste onto the PCB is integral to an effective reflow soldering process. Any fault at this stage can lead to undesired results. An effective stencil design can avoid errors and help proceed with an effective reflow soldering process.
Repeatable Placement of Surface Mount Components:
Even a minute variation in the placement of the components can be disastrous. The processes must be equipped enough to provide repeatably. For this one must ensure a well-maintained pick-and-place machine with component packages taught in the correct way. The placement of surface mount components must be repeatable and reliable.
Good quality PCB, components and solder paste:
Lastly, it is very important to ensure good quality products are being used during the process. Any compromise with the quality can lead to undesirable results.
Always remember an effective reflow soldering process is key to achieving your desired PCB. Be mindful of the above aspects and always go with a trusted brand like Gelco EMS when approaching PCB relating solutions!