Table 1 Comparison of reflow soldering and wave soldering processes and applications Main wave soldering application scope and features 1. Pure SMD welding process, saving time and quality assurance; 2. Where SMD pin spacing is 0.635mm and below, Only by this method can welding standards be achieved, with good quality and pass rate; 3. Various types of SMD can be welded, especially suitable for welding high-profile or unsealed devices. The soldering of a small number of plug-in components SMT mixed PCBs often uses manual repair welding.
1. When SMD and plug-in components are mixed, this method can save time and cost, solder joint strength and quality are good; 2. Only when the lead spacing is more than 0.635mm, SMD, low profile, tightness This method can be used only if it is good; 3. It is especially suitable for welding of passive chip components SHIP, SOT and mixed board with a large proportion of inserted components. The efficiency is high, and it can be connected to the production line, and the degree of automation is low.
With the development and production of Xiahua's clear digital color TV and inverter color TV, surface mount technology (SMT) and double-sided mixed multilayer printed boards are becoming more mature, traditional through holes, and our unloaders have been from 1994 to the present. It has been 8 years before I changed one. It is not the cause of spiral wear, but the fatigue fracture of the spiral shaft.
In the mixed SMT wave soldering, since the SMD pad has no through holes, the gas generated by the flux and the adhesive heated by the molten tin cannot be discharged through the through holes, and tends to stay near the pad, which prevents the molten tin from contacting the end face of the part Or the pad on the PCB. At the same time, due to SMD's immersion in melting tin waves, its own three-dimensional obstacles have caused shadowing effects and shadow effects (see). Therefore, the use of traditional smooth wave soldering will not completely wet the pad and form a full solder joint. Its defects are mainly manifested in SMD leakage welding (unwelded, empty welding) or pinholes.
In addition, for high-density component solder joints and quad flat package (QFP) lead pitch is less than 0.65mm wave soldering, continuous soldering problems are also inevitable.
In this regard, in the equipment selection of the wave soldering machine, especially in the selection of the wave nozzle structure, it is necessary to allow the PCB to undergo a specially designed solder wave (such as oscillating shock wave, spray wave, etc.) before the stable wave peak, so that the molten tin can be smooth Break through its shadow effect and wet the pad. The realization of this program includes multi-wave soldering machine, oscillating wave soldering machine, bubble tin soldering machine and jet soldering soldering machine, etc. See, the effect of using dual wave soldering machine (see) is very good.
The first wave is a turbulent, vigorous and upward three-row dense fountain-like tin column wave, which can penetrate deep into the dead corner and drive away the gas accumulated in the shadow part by the evaporation of flux and achieve initial welding. The second wave is a standard adjustable and stable wide tin wave. It really completes the welding work at the end, and also reduces the bridge short circuit to a minimum. Both wave peak motors use variable frequency speed regulation. But SMD continuous welding also appears in SMT wave soldering, especially QFP devices. This is mainly because the first sprayed column wave wets the solder pads while flushing out the flux, making the secondary wave peak easily cause excessive soldering when soldering, and it is easy to form a bridge at the pin and short circuit.
For this reason, some wave soldering machines are equipped with a hot air knife (HAK) processing device after the second wave soldering (just after the component is removed from the solder). It uses a bunch of high-temperature and high-speed nitrogen or air to blow away too much solder to reduce bridging and continuous welding, but this requires stricter operation and more complicated control, so it must be used selectively. In summary, the selection of a dual wave soldering machine with hot air knife (HAK) that can be controlled independently is still the first choice for SMT wave welding. Of course, it is better to be able to weld under an inert gas such as nitrogen.
2Color parameters of wave soldering process parameters on welding quality Although most SMDs can meet the requirements of solder resistance and heat resistance, and the problem of metal fusion on the end face of the component is basically solved, we should still use the short insertion wave soldering process (that is, use Double wave soldering, the primary and secondary jets can not be too far away), to avoid the thermal discontinuity of the components, resulting in secondary thermal shock, resulting in the damage of the components or the displacement and chip removal of the failed components on the welding surface . Based on SMT mixed circuit board wave soldering, several main process parameters have some discussion on the influence of welding quality.
2.1 Preheating temperature The setting of preheating has a lot to do with the choice of flux. Generally speaking, because the flux is washed by a jet, the preheating temperature is higher than that of ordinary PTH substrates, which is 120 ~ 160 It is appropriate (too low flux activation is not sufficient, easy to produce tin beads, too low solder flux is easy to deteriorate, poor wettability, phenolic PCB can be 120 ~ 140 弋, epoxy PCB can be 140 ~ 160T. 2.2 soldering temperature is not in flux Within the scope of deterioration and under the premise that the SMD is not too hot, the solder temperature should be better (limit temperature 270) to speed up the solder flow rate and reduce lap and leakage. Generally, it can be pressed: tin-lead alloy melting point + 60 ~ 65 The value can be determined by taking Sn60Pb40 as an example. It can be set to 250 ~ 255 (new tin can be set to 250, and old tin can be appropriately 255). 2.3 Welding angle 0 (angle between drive direction and crest level) Angle 0 will be It affects how much the solder of the solder joint is pulled back when it is detached from the solder, which determines the possibility of lap short circuit. Generally, it is 4 ~ 6 (too small is easy to cause continuous solder bridge, too large is easy to cause thin tin).
2.4 The depth of dip welding is used to prevent leakage welding. When spraying, use a deeper one (limited to no tin beads, if necessary, use tin bar protection). In order to prevent continuous welding, it is better to set a shallower point, spray The flow speed can be appropriately increased. Generally speaking, the depth of the single-sided pressure solder is 1/2 ~ 2/3 of the board thickness, and the depth of the double-sided pressure solder is 2/3 of the board thickness 2.5 Transmission speed In SMT wave soldering, in order to effectively overcome the shielding effect of SMD and The continuous welding of component pads should be as slow as possible to relieve the thermal shock of PCB and SMD, so that the temperature rise is less than 3T: / s, and it can be selected from about 1.1 to 1.5m / min. The setting of this parameter has an effect on the above parameters and must be revised if necessary.
Through several years of exploration, we found that during the soldering process of binary Sn / Pb alloy solder, the Cu content increased the fastest and had the most significant impact, while the other element contents did not change much. Therefore, the Cu content in the solder is the bottleneck that restricts the continued use of the solder due to solder pollution. Regular scientific sampling and testing of the Cu content (the warning value in SnPb alloy solder is 0.3%) is the key indicator for deciding whether to replace tin.
3.2 Selection of flux The purpose of using flux is to remove the oxide on the surface of the object to be soldered. When soldering, it can be activated at a temperature below the preheating temperature and the melting point of the solder, and the metal solder surface is chemically cleaned (dissolved oxide), so that The solder can wet and spread on the surface of the metal to be soldered. At present, most of the fluxes used on printed circuit boards are organic compound fluxes, which can be divided into rosin and non-rosin. The ideal flux should be very stable at room temperature and active at soldering temperature, and its residues are at room temperature. Under the non-corrosive, rosin flux is more in line with this requirement. In this type of flux, it can be divided into non-active flux (R), medium-active flux (RMA), active flux (RA) and super-active flux (RSA). The first two are no-clean fluxes, and the surface tension is relatively small; the latter two are fluxes that must be cleaned, and the surface tension is relatively large. For SMT wave soldering, low residues should be used, and the solid content should be 3% lower), no halide, RMA type no-clean flux with low surface tension. The application method of flux should use ultrasonic spray or air spray. 4PCB design and the influence of components on the welding quality. For the soldering quality of SMT mixed circuit, the quality of printed circuit board (PCB) design, the solderability of components Sex, solder resistance and SMD arrangement are very important. In the design of PCBCAD, generally pay attention to the following issues: 4.1 Distribution of components The distribution of components should place all large devices (such as PGA, BGA, QFP, PLCC, etc.) on the same side (A side), and Try to distribute them as evenly as possible, and the B surface (welding surface) is preferably a chip component, such as CHIP, SOT components.
4.2 Arrangement of components on PCB The components on PCB should be arranged like this. For the 1C parts of the fine pitch component arrangement schematic spacing, after the 1C pin (the last two solder legs in the direction of tin flow, one on each side), add two tin steal pads to reduce the possibility of bridge welding Sex.
We compared the welding effect of SMT mixed circuit board on our DW-300 short-plug double wave soldering machine in Xiamen Huacai Power Plant (the heat capacity of the tin furnace has been increased to stabilize the tin temperature and at the same time facilitate the removal of oxides). The technical parameters of the double wave soldering process are compared with the original single wave traditional process technical parameters, and the SMT welding defects are significantly reduced (see Table 2). In order to solve the QFP pin bridging, the process parameter adjustment was adopted to accelerate the reflow speed of the solder when the solder was separated from the instant. The main process parameters are as follows (different equipment, solder flux, process parameters are slightly different): properly adjust the baffle of the wave mouth and increase the welding angle e. The solder is Yun Tin EAOS-60A, the flux is Shenzhen Tongfang TF-96 -5 (spray type).
The outlook of SMT soldering technology can be seen from Table 2. The quality of wave soldering as a SMT mixed PCB can be obtained by varying the correct equipment selection, reasonable selection of solder flux and optimizing process parameters, but its elimination After all, the defects are limited. From a certain perspective, the wave soldering method still has certain limitations, and it is difficult to achieve zero defects. In order to adapt to the development of SMT soldering technology, its soldering method and solder flux are also under continuous research and exploration.
6.1 Perforation Reflow Soldering Process (PIHR) PIHR is to align the nozzle (needle) hole with the positioning system, and then use the scraper to coat the solder on the solder tray on the PCB pad to carry out solder distribution leakage printing, after turning After the board and the components are inserted, a new process of soldering through-hole lead components by reflow soldering is finally used. This process can avoid the adverse effects of thermal effects on PCBs and components, save energy, and has developed rapidly in recent years.
6.2 Selective soldering process Selective soldering is to use the centrifugal pump system to press the molten solder into the delivery pipeline and the nozzle, forming a certain flow and shape through the nozzle to wet the PCB pad and the component to be soldered. To achieve selective welding. This process technology is often controlled and positioned by a programmable controller (PLC), which can perform multi-point welding at the same time. The process is relatively tolerant, with good consistency, and can be integrated with fully automatic production.
6.3 New development of selective de-bridge technology The selective de-bridge technology is an improvement on our highly recommended wave soldering equipment with a hot air knife, which can be controlled by a microcomputer and select areas that are prone to bridge defects for de-bridge Handle without destroying other good solder joints. If necessary, a soldering bracket must be installed on the PCB.
6.4SMT welding process materials are generally developing towards environmentally friendly products. The commonly used SnPb alloy solders are developing to lead-free, low lead and low temperature alloy solders, such as 99.3Sn / 0.7Cu, 95Sn / 5Sb, etc. Although the flux is developing toward no-clean, non-volatile organic compounds (VOC), rosin-free and other environmentally friendly materials. But the flux-free wave soldering process is an effective way to solve PCB cleanliness. Only flux-free wave soldering can completely solve the problem of PCB residual pollution. This technology is one of the main topics of the current wave soldering technology process, which has been used abroad. It is believed that this process will be adopted in China in the near future.
7 Conclusion Although the reflow soldering process plays an important role in SMT assembly, but for the assembly of through-hole components and the welding of through-hole and SMT mixed circuit boards, wave soldering is still the most effective automation, High-yield online welding method. Using special wave soldering equipment (such as HAK nitrogen-filled double wave soldering machine), using high-quality flux solder, optimizing and adjusting the welding process technical parameters, can minimize welding defects.
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