What is a selective soldering machine?
A selective Soldering machine is a programmable machine used to solder individual through-hole components on a PCB using a solder fountain. This process is called the selective soldering process. The selective soldering machine is ideal for precise, accurate, reliable soldering of through-hole components in a mixed PCB board (with previously reflow soldered SMT components).
The mixed PCB is the PCB (used in some applications) that has both SMT and through-hole components. The selective soldering process is used for mixed PCB because the traditional wave soldering is not apt for the soldering of through-hole components of mixed PCB.
As compared to manual soldering, this machine provides faster (capable of soldering multiple boards per hour), repeatable, consistent, high-quality, cost-effective (reduce labour cost) soldering and thus leads to better results & increased throughput.
The selective soldering machine typically consists of a flux spray system, preheater, solder pot(s), XYZ axis drive unit, PCB handling system/table, power supply system, and the machine is operated & controlled by a computer program. The program controls the entire soldering process that includes, preheat temperature, solder temp, wave height, wave duration, soldering dwell times, location, direction, and more. Some machines come with a live-camera option to show the soldering process on a PC. Also, some machines have more than one flux spray system & solder pot to solder multiple boards simultaneously.
The Selective soldering machines are available in both off-line and online configurations, and also they are available to compatible with both lead and lead-free solder. This machine can be used in mixed PCB prototype development & product manufacturing and also can be used as a through-hole PCB rework station.
Working principle of offline selective soldering machine:
Offline selective soldering process
First, the machine is programmed as per the selective soldering requirement of the PCB. After the machine has been programmed, the PCB that needs selective soldering is loaded into the machine. In the offline machine, the flux sprayer and soldering pot are in a fixed position, and the PCB is moved in the X/Y/Z direction, usually by the servo motor. Initially, the PCB is moved in the X/Y/Z direction to spray flux on the selected area on the PCB to be soldered. Typically, the flux is sprayed by using the servo Jet system. The servo drive drop-jet fluxer can spray single spots or tracks with high precision and volume control.
The purpose of flux spraying is to ensure the area to be soldered is free from oxidation & clean, as well as to prevent the re-oxidation of the metal surface during the soldering. Thereby, the flux enables the creation of an acceptable solder joint.
After flux spraying, the board moves to the preheater stage, where the PCB is gradually heated from both the top and bottom to activate the flux and also reduce the thermal energy that will have to be supplied by the molten solder wave. Then, the preheated board is moved to the soldering stage.
The soldering stage consists of solder pot(s) (a bowl with a heating element) to produce molten solder by melting the solder (easily meltable adhesive material). Then, a solder wave fountain is created by using the solder nozzle. The electromagnetic solder bump is used to provide a consistent solder wave fountain. The solder pot that creates the solder wave fountain is on the Z-axis (fixed), and the PCB is moved in the X&Y direction to perform the selective soldering operation. The solder nozzle (solder fountain) applies solder to the passing components pin(s) on the underside of the PCB.
The solder pot (s) is automatically filled as needed. The nitrogen gas is supplied to the selective soldering process to prevent the formation of oxides within the liquid solder for ensuring higher quality solder joints. After the soldering process, the PCB is unloaded from the machine and inspected by the machine operators. The board is inspected to check whether the PCB has been soldered to IPC standards.
Working principle of online selective soldering machine:
The online selective soldering machine additionally has an in-line conveyor system. Here, also, first, the machine is programmed as per the selective soldering requirement of the PCB. After the machine has been programmed, the PCB that needs selective soldering is placed onto the conveyor line, and the clamp is adjusted to make sure that they are held firmly in place. The conveyor moves the boards into the machine and to the fluxing stage. In this machine, the PCB is held in place with the side clamps, and the machine sprays flux on the selected area on the PCB. Then the board moves to the preheating stage.
After preheating stage, the preheated board is moved to the soldering stage. In the soldering stage, the PCB is held in place, and the solder pot (solder wave fountain) moves X/Y/Z direction to perform selective soldering. The solder pot (s) is automatically filled as needed, and also nitrogen gas is supplied in the process to ensure perfect soldering. After the soldering process, the PCB is unloaded from the machine and inspected by the machine operators.
Key specification details of selective soldering machine:
Solder pot capacity: Represents the solder pot capacity, usually expressed in Kg.
PCB size: It is the maximum size of the PCB that can be handled by the machine and is represented in length mm x breadth mm.
PCB thickness: It is the PCB thickness that can be handled by the machine and is represented in mm.
IR heater power: Represents the power rating of infrared (IR) heater, expressed in Watts.
Flux volume: Represents the maximum flux volume that can be handled by the machine and is represented in litre.
N2 consumption: Represents the N2 consumption of the machine and is represented in litre.
N2 pressure: Represents the required pressure level (in bar) of N2 to ensure perfect soldering.
Position accuracy: It is a measure of the exact positioning capability of the machine to move PCB/flux & solder pot system when soldering. It is measured in ± X mm.