Modern electronics products like tablets and mobile phones are very tiny, yet, serve too many functions. Do you ever wonder how they can fit to a very small container? These devices are called surface-mounted devices (SMD). They are created in a PCB assembly line thru Surface-Mount Technology, or SMT.
There are two soldering techniques that can connect SMDs to PCBs. Reflow soldering, though, is the popular choice. This article will discuss the steps, or “zones”, needed to connect the electronics components onto the printed circuit board, by using the reflow soldering technique.
Reflow soldering is a soldering technique that melts a solder alloy to a point that it flows like a liquid. At the start, it uses a sticky mixture of powdered solder and soldering flux. This mixture is used to create a temporary connection between the component pins and PCB pads.
The PCB assembly is then placed to a machine called the reflow oven. The solder paste in the enclosed environment will liquefy. The PCB is then cooled down, to a point that the solder will now solidify, and a permanent joint is now formed.
There are zones that the SMT-type PCB will undergo in order to complete the reflow process. These are preheat, thermal soak, reflow and cooling zone.
The preheat zone is often the most time-consuming of the four zones. It allows the PCB to be heated to a specific temperature. The ramp-rate is established here. This rate determines whether the change in temperature is too fast for the PCB to experience thermal shock. A thermal shock must be prevented because it can create cracks in the board. These cracks can damage the board and render it useless.
The solder that was applied before this zone could spatter if the ramp-rate is too hasty. Preheating is done once the paste’s solvent starts to evaporate.
Thermal soak zone
This zone runs between 1 and 2 minutes. It is tasked to remove solder pastes and to activate the flux. The reduction of oxygen within the joints to be soldered will also start here.
The temperature in this zone should also be regulated. Too much heat may cause the solder to spatter. Change of the copper surface’s color may also happen. The paste may also evaporate vigorously.
Too little heat may also mean ineffective flux activation. A flux that is not activated may not “glue” the components together. A thermal equilibrium of the entire PCB assembly must first be achieved in order to progress to the reflow zone.
This zone is concerned on how long it would take to achieve the maximum temperature. “Time above reflow” and “time above liquidus” (TAL) are determined here.
The peak temperature, or the maximum allowable temperature must be from 20 to 40 degrees Celsius above liquidus. In order to determine the limit, search within the PCB assembly for the lowest thermal tolerance among components. This component is the most vulnerable to high temperature. As a standard guideline, 5 degree Centigrade is subtracted from the thermal limit, or maximum temperature, of the component. This value is found on the datasheet of the electrical component.
It is very important monitor the temperature of the oven to keep it from exceeding the limit. Temperatures beyond 260 degrees Centigrade may cause permanent damage to the internal circuitry of SMT components. Temperature that is too low may prevent the adequate reflow.
This zone is tasked to gradually cool the reflowed board, and to solidify its solder joints. Proper cooling must be done to prevent the formation of unwanted metals, or thermal shock to the board’s components.
Typical cooling temperatures are between 30 and 100 degrees Celsius. A faster cooling rate is suggested than a gradual cool down. This will create a fine-grained solder that is compact and strong.
This article discussed how to start a reflow solder. It discussed the zones that are needed, and the basic conditions needed to reflow properly. Reflow solder is necessary in the PCB assembly of SMT components. Always keep in mind that SMT is the future of electronics. It must be understood if you want to plan for your electronics business’ future.