Welding technology in the assembly of electronic products play a very important position. General welding divided into two categories: one is mainly used in through-hole electronic components and PCB welding - wave, the so-called wave soldering (wavesoldering) that is, the soft solder melting solder, the electric pump or electromagnetic pump jet peak solder into the design requirements, glue stick so pre-equipped with electronic components of the PCB through the solder wave crest to achieve components with the PCB solder pad or pin end of mechanical and electrical connection between the soft solder welding.
The other is mainly used in surface mount components and PCB welding - Reflow (reflowsoldering), also known as reflow soldering,glue gun reflow soldering is the so-called pre-assigned by re-melting the paste to the PCB pad like soft solder solder, surface mount components to achieve end or pin and PCB solder pad connection between the mechanical and electrical soldering, in order to achieve a certain reliability of the circuit functions. With surface mount components in electronic products widely used, reflow soldering surface mount technology to become the main technology. Its main feature is the process: welding with flux will clean the metal surface (oxide removal), so good on the solder wetting; supply molten solder to wet the metal surface; in solder and weld metal intermetallic compounds formed between ; also can micro-welding.
Is the pre-reflow soldering in PCB area (pad) and the appropriate form of release amount of solder paste release surface mount components, solder reflow using an external heat source to meet the requirements for welding of a group or by spot welding process. Reflow soldering and wave soldering compared with the following features:
1, no reflow soldering as the components that need to direct immersion in molten solder, so the components are subject to thermal shock is small;
2, reflow, the site only need to solder on the cast, a significant reduction in the use of solder;
3, solder reflow can control the discharging amount, to avoid defects such as bridging the generation;
4, when the components affixed to put a certain deviation from the position, due to the role of surface tension of molten solder, cast in the correct position as long as the solder, reflow soldering can automatically correct this minor error, so that components fixed in position;
5, can be localized heating source, which can be used in the same substrate for different reflow soldering process;
6, the solder is generally not mixed with impurities, the use of solder paste for reflow solder can be formed to maintain the right.
Reflow soldering techniques are classified according to heating are: vapor phase reflow, infrared reflow, infrared hot air reflow, laser reflow, hot air reflow soldering and heating tools and so on.
Principles and reflow temperature profile:
From the temperature curve (see Figure 1) of reflow principle: When the PCB into the heating zone (dry zone), the solder paste in the solvent, the gas evaporate, while the flux in the wetting pad solder paste, component side head and pin, solder paste softening, collapse, covering the pad, the pad, pin and oxygen components isolated; PCB into the insulation area, the PCB and components are fully preheated to prevent sudden PCB heating up too fast into the welding zone PCB and component damage; when the PCB into the welding zone, the temperature rose rapidly to achieve the molten solder paste, liquid solder pad on the PCB, components and pin ends wetting, diffusion, overflow or back mixing of the solder joint; PCB into the cooling zone, so that solder solidification, to complete the soldering.
Temperature curve is the key to ensuring the quality of welding, the actual temperature curve and solder paste temperature curve slope and the peak heating temperature should be basically the same. 160 ℃ heating rate control in front of 1 ℃ / s ~ 2 ℃ / s, if the temperature slope too fast, on the one hand the components and PCB heat fast, easy to damage the components, could easily lead to PCB deformation; the other hand, Paste in the solvent evaporation rate too fast, easy to spill the metal components, resulting solder ball. Peak temperature is generally set higher than the melting temperature of solder paste 20 ℃ ~ 40 ℃ or so (for example, the melting point of solder paste Sn63/Pb37 183 ℃, the peak temperature should be set at about 205 ℃ ~ 230 ℃), back to the (re) flow time for the 10s ~ 60s, the peak temperature is low or back (re) flow time is short, make the welding is insufficient, solder paste does not cause serious melting; peak temperature is too high or back to the (re) flow for a long time, resulting in oxidation of metal powder, affect the welding quality, or even damage to components and PCB.
Reflow profile and return according to the principle of reflow machines currently on the market is generally simple four-zone reflow machine, there is a large six, eight or even twelve-zone reflow machine, and reflow model QHL320A machine with programmable temperature control section 20, the equivalent of 20 zone reflow machine, which will return the temperature curve segments, then a more precise temperature control, more fitting ideal reflow profile to achieve the perfect welding.
Where good welding quality protection? QHL320A reflow machine in addition to full compliance with the control outside the reflow temperature profile, but also allows users to truly understand the principles of reflow soldering. QHL320A reflow machine with a large transparent window, users can be through a transparent window throughout the entire welding process control, solder paste can be observed simultaneously in the whole state of the welding process changes, easy to find problems in the welding process, through the parameters adjustments to improve in order to ensure good welding quality. At the same time QHL320A reflow soldering machine for small desktops, all-static welding, effectively prevent a large multi-zone reflow machine to send the resulting Crawler small vibration, the vibration may be in the flow of molten solder welding area state of the small pitch IC (Ru ≤ 0.5mm) and components (such as 0603,0402 and 0201, etc.) affect the welding, leading to drift components, solder balling, solder bridges welding defects, and all Jingzhi welding Ze possible to completely avoid the above deficiencies.
Set back (again) reflow soldering temperature curve based on:
1, according to the temperature curve using the paste to set. Solder paste with different metal content have different temperature curve should be provided in accordance with the temperature of solder paste manufacturers set a specific product curve back (again) reflow soldering temperature profile;
2, according to PCB material, thickness, whether the plywood, size, etc.;
3, based on surface mount boards equipped with components of the density, size and availability of components of BGA, CSP and other special components set.
4, according to the specific equipment, such as: the length of the heating zone, heating the source material, back to the (re) construction of reflow oven and heat conduction and other factors set.
Hot air reflow and infrared reflow machine is very different: infrared radiation conduction reflow machine is mainly based, the advantage of high thermal efficiency, the temperature gradient large, double-sided PCB, welding, easy to control the temperature; its defects is uneven temperature in the same piece of PCB, the color of the devices, materials and sizes, the temperature is different, in order to make the device color depth large volume of solder joints around the welding temperature components, welding temperature must be increased. Hot air convection reflow machine is mainly is mainly conduction, the advantage of even temperature, good welding quality; disadvantage is that PCB upper and lower oven temperature and the length along the direction of welding difficult to control the temperature gradient.
Set of hot air reflow and infrared reflow with their respective advantages and large reflow technology and the advantages of China Aviation Industry Group, Innovation and Technology Development Co., Ltd. Beijing Qingyun introduced the use of infrared heating and forced air circulation with a small way QHL320A-SMT Precision desktop reflow machine. Collection of the company's strong military aviation technical strength, eight years of professional research, development and production process experience, independent research and development for all types of enterprises, research institutes, research and development, small batch production using the full set of complete SMT placement of small, welding equipment: SMT reflow small precision desktop machine (● QHL320 classic ● QHL320A enhanced ● QHL360 lead-free type), track on-line reflow soldering machine, manual screen printing units, anti-static vacuum suction pen, put stickers precision IC units, aircraft components and so on. Another BGA placement positioning system, semi-automatic placement machine, high precision screen printing units, semi-automatic dispenser, temperature tester, electronic insulation box (store solder paste used), magnifying glass and other auxiliary equipment.