Principle and basic process of reflow oven
1. What is reflow soldering
Reflow soldering is in English. Reflow is to remelt the solder paste pre-distributed on the printed board pads to realize the mechanical and electrical connection between the solder ends or pins of the surface mount components and the printed board pads. Brazing. Reflow soldering is to solder the components to the PCB board, and reflow soldering is to mount the components on the surface. Reflow soldering relies on the effect of hot airflow on the solder joints. The colloidal flux physically reacts under a certain high temperature airflow to achieve SMD soldering; the reason why it is called "reflow soldering" is because the gas circulates in the soldering machine to generate high temperature to achieve soldering. Purpose.
The principle of reflow soldering is divided into several descriptions:
A. When the PCB enters the heating zone, the solvent and gas in the solder paste evaporate, and at the same time, the flux in the solder paste wets the pads, component ends and pins, and the solder paste softens, collapses, and covers the solder The pads isolate the pads and component pins from oxygen.
B. When the PCB enters the heat preservation area, make the PCB and components fully preheated to prevent the PCB from suddenly entering the welding high temperature area and damaging the PCB and components.
C. When the PCB enters the soldering area, the temperature rises rapidly so that the solder paste reaches a molten state, and the liquid solder wets, diffuses, diffuses, or reflows to the PCB pads, component ends and pins to form solder joints.
D. The PCB enters the cooling zone and the solder joints are solidified; when the reflow soldering is completed.
The working principle of dual-track reflow soldering
The dual-rail reflow oven can double the capacity of a single dual-rail oven by processing two circuit boards in parallel at the same time. Currently, circuit board manufacturers are limited to handling circuit boards of the same or similar weight in each track. Now, the dual-track dual-speed reflow oven with independent orbital speeds makes it a reality to process two more different circuit boards at the same time. First, we need to understand the main factors that affect the heat transfer from the reflow oven heater to the circuit board. Under normal circumstances, as shown in the figure, the fan of the reflow oven pushes gas (air or nitrogen) through the heating coil. After the gas is heated, it is delivered to the product through a series of holes in the orifice.
The following equation can be used to describe the process of heat energy transfer from the airflow to the circuit board, q = heat energy transferred to the circuit board; a = convective heat transfer coefficient of the circuit board and components; t = heating time of the circuit board; A = heat transfer surface area ; ΔT = the temperature difference between the convection gas and the circuit board. We move the relevant parameters of the circuit board to one side of the formula, and move the parameters of the reflow oven to the other side, and the following formula can be obtained: q = a | t | A | | T
Dual-track reflow soldering PCB has been quite popular, and it gradually became more and more popular. The main reason for it to be so popular is that it provides designers with extremely good elastic space, so as to design more compact, compact and low-cost product. As of today, dual-rail reflow soldering boards generally have the upper side (component side) soldered by reflow, and then the lower side (lead side) is soldered by wave soldering. A current trend is toward dual-track reflow soldering, but there are still some problems with this process. The bottom component of the large board may fall off during the second reflow process, or part of the bottom solder joint may melt, causing reliability problems in the solder joint.
2. Introduction to reflow soldering process
The reflow soldering process is a surface-mounted board, and its process is more complicated, which can be divided into two types: single-sided mounting and double-sided mounting.
A, single-sided mounting: pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → inspection and electrical test.
B, double-sided placement: A side pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → B side pre-coated solder paste → placement (divided into manual placement and machine automatic placement) Mounting) → reflow soldering → inspection and electrical test.
The simplest process of reflow soldering is "screen printing solder paste-patch-reflow soldering. Its core is the accuracy of the screen printing. For the patch, the yield rate is determined by the machine's PPM. The reflow soldering is to control the temperature rise and the highest Temperature and falling temperature curve.
Reflow soldering process requirements
Reflow soldering technology is not unfamiliar in the field of electronic manufacturing. The components on the various boards used in our computers are soldered to the circuit board through this process. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is easier to control. There is a heating circuit inside this device, which heats the nitrogen to a high enough temperature and blows it to the circuit board where the component is already attached, so that the solder on both sides of the component is melted and then bonded to the motherboard.
1. Set up a reasonable reflow soldering temperature curve and do real-time testing of the temperature curve on a regular basis.
2. The welding should be carried out in accordance with the welding direction of the PCB design.
3. Strictly prevent the vibration of the conveyor belt during the welding process.
4. The welding effect of the first printed board must be checked.
5. Whether the soldering is sufficient, whether the surface of the solder joint is smooth, whether the solder joint shape is half-moon shape, the condition of tin balls and residue, the condition of continuous soldering and virtual soldering. Also check for changes in the color of the PCB surface, etc. And adjust the temperature curve according to the inspection results. The welding quality should be checked regularly during the whole batch production process.
Contact Person: Mr. Ivan Zhu