In recent years, new electronic Surface Mount technology SMT (Surface Mount wave-tech-nology) has replaced the traditional hole cartridge technology, and control the development of electronic equipment, electronic assembly technology for consensus revolutionary transformation. SMT to improve product reliability and performance, reduce the cost for the target, whether in the consumer electronic products, or in the military tip electronic products field, will make the electronic product important changes occurred.

   Surface mount technology and components is introduced

   The SMT, also called surface mount technology (SMT), is a kind of without PCB drilling in cartridge hole, and assembly components surface directly welded to stick the provisions of printed circuit board position, make components with solder and printed circuit board a mechanical and electrical connection between the electronic assembly technology.

   Need for surface mount electronic products are usually made of printed circuit board and SMT components composition. Printed circuit Board PWB (maritime Board) is containing Wire line and welding plates of one-sided or two-sided multi-layer material. Surface mount components including surface mount components and surface mount devices two kinds big. Among them the SMT components refers to all flake passive components, such as resistors, capacitors, and inductors, etc.; And surface mounted device is an electronic device, the Package is usually refers to various active device, such as a Small appearance wrapper SOP (like), the Small Ball Grid Array wrapper BGA (Grid Array) etc. Start Some components can not be used in SMT, such as part of the connection, transformer, large capacitance, etc.

   Surface mount technology process

   Surface mount DingYi including core and auxiliary two big process. The core technology of by printing, patch and soldering 3 parts, any type of product production are through the three working procedure, each part necessary; Aided process mainly by the "glue" process and optical auxiliary automatic detection technology and other components, not necessary, but according to product characteristics and user demand of the decision.

   A single double-sided printed circuit board of points, electronic products and corresponding divided into single products (printed circuit board side need to assembly components) and double products (both sides of the printed circuit board are pasted on the components) need.

   Printing process the purpose is to make solder through the template and printing equipment, the common function of accurate printing to printed circuit board. Printing process of the main elements involved process solder paste, template and printing system. Solder paste is will elements and printed circuit board connection, realize its conduction electrical and mechanical connection of important material. Mainly by alloy solder paste and flux composition. The welding process of, they were completed work effect welding. Template will be used to print solder paste printed circuit board to accurately, template making method and open hole design of the printing quality has very big effect. Printing system is mainly refers to the printing equipment and printing parameters. The quality of printing equipment for printing, printing equipment much influence on accuracy of precision printing parameters and repeat printing set reasonable matching, is the important guarantee of print exactly. Printing parameters are many, but for the printing effect to have the biggest impact on key parameters have printing speed, squeegee pressure, parting speed and stripping distance, etc. Need to set up these key parameters and the match each other. In order to improve the quality of printing. The printing speed general for 12.7 ~ 203.2 mm/s, specific parameters depend on the scraper pressure and solder paste physical performance. And SMT process requirement printing scraper pressure is 4.448 222 6.672 333 N.

   The purpose is to ensure that patch process all parts are accurate and rapid patch to printed circuit board. Patch technology mainly involves placement machine and its ability to patch. Placement machine patch ability is the important guarantee of accurate patch. The key technology of placement machine including: sports, implement and sheets-feeder high speed. Miniaturization technology; High speed machine visual identification and lighting technology; High speed, high precision intelligent control technology; Parallel processing technology; real-time multitasking Open flexible modular technology and equipment system integration technology.

   Reflow soldering process is by melting pre-assigned to printed circuit board welding plate, and realize the solder paste SMT components of welding surface or pins and printed circuit board welding machinery and electrical connection between disk of welding. Reflow soldering can ensure excellent welding effect. Reflow soldering process of the main technological elements is reflow soldering welding stove and ability, the welding ability mainly reflects in soldering furnace heating system, cooling system, flux management system and inert gas protection system. Among them, the heating system with heating efficiency, temperature control precision, temperature uniformity and stability of relevant; The purpose of the cooling system are: when soldering peak temperature is higher, if not rapidly cooling, substrate out of the oven temperature reflow soldering too high, easy to cause the base board plate bend; Rapid cooling can constitute organization, prevent intermetallic thickening. To improve reliability. Flux in the soldering process of volatile, if not an ideal flux management system in the volatile flux siphoned off and filtering circulation, flux can follow high temperature air into the cooling area, condensed in heat sink and furnace, reduce the effect of cooling and pollution equipment and substrate. When the board of matching using solder paste activity is not good enough or circuit board have super fine spacing components and complex yuan piece, plus substrate DuoCi need a reflow oven, are considering in soldering furnace charge inert gas, reduce people oxidation opportunities, improve the welding activity. General use of inert gas is nitrogen gas. Reflow soldering welding stove ability still need through the editor reflow soldering furnace control program to play. Complete the circuit board patch in through the reflow oven, general experience: preheat stage, insulation stage, stage and cooling stage backflow. Through the reflow soldering furnace control program control, ensure welding quality.

   Aided process used to assist mounted smoothly and actively prevent detection and later detection. Aided process mainly by the "point" process and optical stick auxiliary automatic detection of technology. "Glue" process is through special glue "point will stick" to the required components, the components below or other appropriate protection to ensure that the backflow through DuoCi welding components don't fall off; Reduce components to assembly process in under the stress impact; Protection in the use of complex components in the environment without damage. "Glue" technology process elements include "glue" equipment, special glue and "glue" parameter set. Need reasonable choice of equipment, glue and design good parameter Settings to ensure that process effect. Optical auxiliary automatic detection technology is mainly: one is the use of specialized optical equipment after printing measurement solder paste thickness uniformity and printing accuracy, the system after testing patch in soldering accuracy, before the defective circuit board testing out and timely alarm; 2 it is in after reflow soldering use special optical equipment testing solder joints, will have the solder joint defects detection out and call the police. Circuit board Special optical measurement equipment is mainly there is visible light detection equipment and X-ray testing equipment. The former mainly is the Automatic optical Inspection AOI equipment (same), the latter correlate work is mainly three peacekeeping five d X-ray equipment. The former mainly used in the detection of visual solder joints, and the latter in addition to test the solder joints, visual inspection of the BGA component is not visual solder. Whether to adopt auxiliary technology is according to need pasted on the characteristics of the product to decide.

   Reflow soldering principle and temperature curve

   From the reflow temperature curve (FIG. 3) analysis reflow soldering principle: when PCB into preheat area, solder paste solvent, gas is evaporated, and solder paste of flux wetting welding plate, components end and pins, solder paste softening, morphology, covering the welding plate welding plate, components, pins and oxygen isolation; PCB into heat preservation area, PCB and components get fully preheated. In case of PCB suddenly entered again flow welding area rapid warming and damage the PCB and components; When PCB entered again flow welding area, temperature rise up quickly make solder paste, liquid to melting state of the PCB soldering welding plate, components end and pins wetting, diffusion, ManLiu or mixed solder contact form backflow; PCB into the cooling area, solder joints, complete the whole reflow soldering solidification.

   Reflow soldering process, need to the solder paste solvent evaporation. Remove welding flux of surface oxide, solder melted, and flow and solder cooling, solidification. So, reflow soldering process, the welding temperature main points four temperature area: preheating zone, heat preservation zone, and flow welding area and cooling area. Preheating zone for room temperature to 120 ℃; Heat preservation areas for 120 ℃ ~ 170 ℃; Backflow area for 170 ℃ ~ 230 ℃, the highest temperature of 210 ℃ ~ 230 ℃; Cooling area from 210 ℃ to 100 ℃ to about.

   Temperature curve is the key to ensure welding quality, the actual temperature curve and solder paste temperature curve of warming slope and peak temperature should be identical. 160 ℃ temperature before the speed control in 1 ℃ / s ~ 2 ℃ / s, if the speed of temperature rise too fast, on the one hand, make components and PCB heated too fast, easy cause damage components, PCB deformation; On the other hand, the solder paste solvent evaporation speed too fast. Easy to spatter metal components, produce the solder ball. Peak temperature than general set solder paste melting temperature high 20 ℃ ~ 40 ℃ (for example Sn63 / Pb37 solder paste melting point for 183 ℃, peak temperature shall set up in 205 ℃ ~ 230 ℃), back (again) flow time 10 ~ 60 s, peak temperature low or back (again) flow time short, can make the welding is not full, causing serious solder paste don't melt; Peak temperature is too high or back (again) flow time long, cause metal powder oxidation, influence the quality of welding, and even damage components and PCB.

   Set back (again) flow welding temperature curve on the basis of the solder paste: use temperature curve, according to the PCB's materials, thickness, if produce multilayer prototypes, size; Launch of the components surface mount board density, size and presence of BGA component, such special components; CSP The specific situation of the equipment, such as the length, heating source heating of materials, reflow soldering furnace of structure and heat transfer mode, and other factors.

   Some kind of printed circuit board for equipment in practical production set temperature area: because temperature area, heat preservation zone, the rapid increase in area, reflow area. For Sn63Pb37 solder paste type, its melting point solder paste for 183 ℃, with a certain type of reflow soldering welding stove, each printed circuit board design appropriate components shall be welding parameters, do a printed circuit board a temperature curve.

   This is a 9 temperature regions reflow soldering furnace, the actual temperature test has three test points, including figure 5 is the actual temperature curve. The temperature parameters are set to meet the following requirements: 1) heating area from room temperature to 100 ℃, the heating rate does not exceed 2 ℃ / s; 2) thermal insulation area: 100 ℃ ~ 150 ℃ from time to maintain 70 ~ 120 s; 3) the rapid increase in area: 150 ℃ ~ 183 ℃ from keep time don't more than 30 s, the rate of temperature should be in 2 ~ 3 ℃ / s: 4) backflow area: the highest temperature of 205 ℃ ~ 230 ℃, in liquid phase line above time 40 to 60 s; 5) cooling area: the cooling speed for 2 ~ 4 ℃ / s. The figure 4 and figure 5 theory and practical, PCB temperature curve contrast, actual reflow temperature area in the standard temperature range, and concluded that the list of welding stick PCB device that accords with a requirement, PCB surface mount device of electrical properties. Special attention should: reflow oven must weekly test once, will test the temperature curve and standard temperature curve is compared, and determine whether both perfectly. Check the main parameters are: warming district heating rate, heat preservation zone keep time, the rapid increase in the area and the backflow rate of temperature, peak temperature, the time above liquidus, cooling area, and cooling rate curve whether there are abnormal fluctuations.

   Surface mount technology penetration in every field, can directly affect the electronic products, electronic products and welding level of performance and quality. Narrative surface mount technology the whole process, this paper expounds the reflow soldering welding process of principle and temperature curve. Contrast the actual production process in a printed circuit board standard reflow soldering temperature curve and the actual reflow soldering temperature curve, as long as meet the practical reflow temperature area in the standard temperature range, can meet the performance index of components to assembly.