Do you know how electronic components of mobile phones are attached? A mobile phone relies on the technology called surface-mount technology. SMT devices, or SMDs, require their components to be attached on a printed circuit board through a process called soldering reflow. This article discusses reflow soldering, and how it is used in PCB assembly.
The soldering process is essential in manufacturing electronics products. This process is used to permanently attach electronics components to a PCB. Problems may be encountered when these products undergo PCB assembly through SMT. It is because these boards do not contain any holes, as compared to the boards manufactured via through-hole mounting (THM). THM electronics products can be manually soldered by using a traditional soldering iron. This is not the case for SMDs. This is where soldering reflow comes along.
The term "reflow" refers to melting a solder alloy thru a high but specific temperature. This temperature is usually 217 degrees Celsius. This solid metal must not just be softened, it must flow like a liquid. The solder will not flow, however, if cooled below the reflow point. If again heated, the solder will flow again, thus, the name “reflow”.
“Reflow”, however, is a misnomer. Standard assembly guidelines allow the solder to flow only exactly once. They will not allow the solder to reflow. Allowing it to flow more than once may contaminate the liquefied solder.
Reflow soldering uses solder paste to temporarily attach component pins to its PCB pads. A solder paste contains a sticky mixture of flux and powdered solder. The entire PCB assembly is then baked to a reflow oven. The solder paste in the enclosed environment will melt. Once cooled down, the joint will then be permanently connected. Heat can also be sourced via hot air pencil, or by using infrared lamps.
A reflow oven is the equipment used for soldering reflow. It basically has the following components:
•conveyor belt for moving the PCB from different zones
•heat source and thermometer for transferring and maintaining the heat used for soldering reflow
•an optional Nitrogen pump to reduce oxidation
•an optional set of stencils for soldering specific integrated circuits
The objective of the reflow is to melt the solder while heating the board’s surface. This must be done without damaging the electronic components due to overheating. There are usually four stages, or zones, to reflow soldering:
•preheat – often the longest time to prepare . Soldering paste, or flux, is applied between the board and the component lead. The PCB is then heated to prevent thermal shock. This zone is where the flux starts to evaporate.
•thermal soak (or just soak) – the flux activates and the contacts begin to oxidize (change color). This is done to prepare the components for soldering.
•reflow – the maximum allowable temperature must be maintained here. This zone is the actual soldering part.
•cooling – cools or brings down the heat of the soldering process. This is done to solidify the solder joints.
There is also one automated PCB assembly technique that is available for soldering. The reflow soldering counterpart, the wave soldering technique, also has somewhat similar features and mechanism. The only difference is that the latter uses a “wave” of soldering lead, while the former uses just a tiny “reflow” portion of lead.
Reflow is primarily used for SMDs. It is uncommon, however, to be used for THM boards. Reflow can also be used for a board that contains both SMT and THM features. Reflow, though, can be very difficult to use in a THM-reliant board. It is, therefore, not recommended to use this soldering technique on boards with holes.
Reflow is also complicated to use on double-sided boards. Although, there is a technique to alleviate the problem, it is still difficult to perfect. SMT components are also prone to damage from very high temperatures. Therefore, extreme care must be done to prevent this problem.
Reflow is used with the new SMT devices such as mobile phones and laptops. This soldering technique is complicated, and its equipment is expensive. It, however, is essential to learn this technique because soldering SMT is the future of PCB assembly. It is always beneficial to plan for the future.