Basic requirements for lead-free solder in electronic assembly Basic processes for lead-free soldered assemblies include: a. Lead-free PCB manufacturing processes; b. 96.5Sn/3.5Ag and 95.5Sn/4.0Ag/0.5Cu eutectic and near eutectic alloy systems for solder paste applications; c. 99.3Sn/0.7Cu eutectic alloy systems for wave soldering applications; d. 99.3Sn/0.7Cu alloy systems for hand soldering applications.   Although these are feasible smt process, but the implementation of several major problems, such as raw material costs are still higher than the standard Sn / Pb process, an increase in the limitations on the degree of wetting, the requirement to maintain the wave soldering process in an inert air state (to have a sufficient amount of nitrogen), and the possibility of reflow soldering temperatures to the limit of the temperature range (between 235 ~ 245 ℃) and improve the thermal properties of the various components of requirements etc.   As far as lead-free alternatives are concerned, there is no universally recognized set of specifications, and after many discussions with many professionals in the field, we have come up with some technical and application requirements below:    Metal Price Many assembly manufacturers require that Pb-free alloys be priced no higher than 63Sn/37Pb, but unfortunately all existing Pb-free alternatives cost at least 35% more than 63Sn/37Pb. The cost of the metal is one of the *most* important factors when choosing lead-free solder rods and wires; when making solder paste, it is not yet as sensitive to the price of the metal because the cost of the technology is a relatively high percentage of the overall manufacturing cost.    Melting point Most assemblers (not all) require a solid phase temperature* as low as 150°C in order to meet the operating temperature requirements of electronic equipment,* and a high liquid phase temperature depending on the specific application.   Wave soldering rods: For successful wave soldering, the liquid phase temperature should be lower than the oven temperature of 260°C. Solder wires for manual/machine soldering: the liquid phase temperature should be below the soldering iron tip operating temperature of 345°C.    Solder Paste: The liquid phase temperature should be below the reflow temperature of 250°C. This temperature is the practical temperature limit for many existing reflow ovens. Many engineers require * high reflow temperature should be lower than 225 ~ 230 ℃, however, there is not a viable solution to meet this requirement. It is generally accepted that the alloy reflow temperature closer to 220 ℃ the better the results, can avoid the higher reflow temperature is * ideal, because it can make the degree of damage to the components to * low, * minimize the requirements for special components, but also can be the circuit board discoloration and warping to * low, and to avoid excessive oxidation of the pads and wires.    Good electrical conductivity This is a basic requirement for electronic connections. Good thermal conductivity In order to dissipate heat, the alloy must be able to transfer heat quickly. Smaller solid-liquid temperature range Non-eutectic alloys solidify at a temperature range between the liquid-phase temperature and the solid-phase temperature, and most metallurgists recommend keeping this temperature range within 10°C in order to form good solder joints with fewer defects. If the alloy solidifies over a wide range of temperatures, there is a risk of weld joint cracking and premature equipment damage.    Low-toxicity alloys and their components must be non-toxic, so this requirement will be cadmium, thallium and mercury are excluded from the scope of consideration; some also require that toxic substances can not be used by-products of refining, and therefore bismuth is excluded, because bismuth is mainly derived from the by-products of lead refining.    Good Solderability The alloy should have sufficient wetting under existing equipment and no-clean flux conditions to be used with conventional no-clean fluxes. As the cost of inerting the wave is not too high, so it is acceptable to wave soldering plus inert environment conditions of use requirements; but in terms of SMT reflow, the alloy * good to have the ability to reflow under the air, because of the high cost of inerting the reflow oven.    Good physical properties (strength, tensile, fatigue, etc.) The alloy must be able to provide the mechanical strength and reliability that can be achieved with 63Sn/37Pb without protruding fillet welds on through-hole devices (especially for alloys with a wide solid-liquid coexistence temperature range).    Production Repeatability/Melting Point Consistency The electronic assembly process is a high-volume manufacturing process that requires both repeatability and consistency to be maintained at a high level, and certain alloys cannot be given consideration if their compositions are not repeatable under high-volume con

Lead-free soldering knowledge Nowadays, in electronic assembly technology, people's environmental awareness is getting stronger and stronger, from the environmental protection, legislation, market competition and product reliability and other aspects, lead-free is imperative.    However, the current lead-free from theory to application are still immature, has not yet formed a relatively uniform normative standards.   The challenge of lead-free process on components is first of all high temperature resistance. To consider the impact of high temperature on component packaging. As the traditional surface mount components of the packaging material as long as they can withstand high temperatures of 240 ℃ to meet the soldering temperature of the leaded solder, and lead-free soldering, for complex products welding temperature as high as 260 ℃, so the component package is able to withstand high temperatures is a problem that must be considered.   In addition to consider the impact of high temperature on the internal connection of the device, IC's internal connection method has a gold ball welding, ultrasonic pressure welding, and flip-flop welding and other methods, especially BGA, CSP and the combination of composite components, modules and so on, new components, such as flip-flop BGA, CSP, the internal encapsulation of the chip bumps with the solder paste is Sn-3.5Ag welding, the melting point of 221 ℃, if such devices If such a device for lead-free soldering, then the device's internal solder joints and surface assembly of the solder joints almost at the same time and then melted, solidified once, which is very harmful to the reliability of the device. Therefore, the internal connection materials of lead-free components should also meet the requirements of lead-free soldering.   The majority of leaded components are plated with Sn/Pb, while there are many types of plating on the surface of lead-free components. Which kind of plating * good, there is no conclusion, so there is still to be lead-free components to improve the standard.   Lead-free process on the requirements of the flux, first of all, due to the chemical reaction between the flux and the alloy surface, so different alloy compositions to choose a different flux. Lead-free flux must be specially formulated. With the deepening of the lead-free process, the quality of lead-free solder paste has improved greatly due to the efforts of solder manufacturers. Current lead-free solder joints have improved in appearance from previous years.   The issue of lead-free solder reliability is of great concern to both manufacturers and users. In particular, the current is in a special stage of transition from leaded to lead-free soldering, lead-free materials, printed boards, components and other aspects of the absence of standards, and even in the reliability of the test methods are not standardized, the reliability is very worrying. The current stage of lead-free process, especially in China is still in a relatively confusing stage. Due to the mixed use of leaded and lead-free, especially when the lead-free soldering end of the components using leaded solder and leaded process, serious reliability problems will occur, these problems are not only the current transition stage of lead-free soldering to pay attention to, but also for the transition stage of leaded soldering is also a problem to pay special attention to.   Because lead is relatively soft and easily deformed, the hardness of lead-free solder joints is higher than that of Sn/Pb, the strength of lead-free solder joints is also higher than that of Sn/Pb, and the deformation of lead-free solder joints is smaller than that of Sn/Pb solder joints, but these do not equate to good reliability of lead-free. Due to the poor wettability of lead-free solder, so the hollow, displacement, monumental and other welding defects are more, in addition to the high melting point, if the activation temperature of the flux can not match the high melting point, due to the high temperature of the flux wetting area, a long time, it will make the soldering surface at high temperatures to re-oxidize and can not take place in the wetting and diffusion, and can not form a good interfacial alloy layer, and the result leads to the interface of the solder joints combined strength (tensile strength) Poor and reduce reliability.   According to the U.S. Wei-Foundation, Agilent and other companies, such as reliability tests, such as thrust test, bending test, vibration test, drop test, after hot and humid, high and low temperature cycling and other reliability test results, by and large, there is a relatively similar conclusions: most of the civilian, communications and other fields, due to the use of the environment is not too much stress, lead-free solder joints mechanical strength is even higher than the leaded ones, but in the use of But in the use of high stress places, such as high and low temperatures, low air pressure and

SMD fuses on single cell lithium batteries SMD fuses in the single-cell lithium battery applications Preface: Currently, more and more products use lithium batteries as the power supply for the system circuit. Lithium batteries have a small size, high energy density, and no memory effect, but also can be recycled for multiple use and other advantages. But lithium batteries also have accidents. 2009.1.30 Yuexiu District, Guangzhou, Donghua South Road, Lenovo computer store occurred in a cell phone lithium battery explosion, a young man in the store carotid aorta rupture, died on the spot. So the safety of lithium batteries is also increasingly attracting attention. Lithium battery protection board to choose the right chip fuse is also very important. Now back to business I. The composition of a single lithium battery Lithium batteries are mainly composed of battery cells and battery protection boards. The main function of the lithium battery protection board: (1) overcharge protection (2) Over-discharge protection (3) Over current / short circuit protection Lithium battery protection board main components: 1, protection IC   2, MOSFET  3, SMD FUSE II. Single-cell lithium battery protection board on the choice of fuse 1, package: according to the customer's circuit board to determine the trial size, generally 1206/0603 size. 2, rated voltage: 24V or more 3, rated current: generally 5A (taking into account the fuse in the lithium battery protection board is 2 times protection, so the protection IC protection delay should be taken into account). 　　(To take into account the protection IC protection delay time to determine the fuse blowing time) 4, Cold resistance: below 15mΩ. 5, safety requirements: UL certification 6, Comply with RoHS requirements and Halogen-Free requirements. Chip fuse in the single-cell lithium batteries on the application of the talk here.

SMT Printed Solder Paste Process 1. Screen printing technology Screen printing technology is the use of already made stencil, with a certain method to make the screen and printing machine direct contact, and make the solder paste in the stencil uniform flow, by the mask pattern into the mesh. When the screen off the printing plate, the solder paste in the shape of the mask pattern from the mesh off to the corresponding pad graphics on the printing plate, thus completing the printing of solder paste on the printing plate. 2. Printing solder paste process inspection Printing process is to ensure the quality of the surface assembly of one of the key processes. According to statistics in the PCB design is correct, components and board quality is guaranteed under the premise of surface assembly quality problems in 70% of the quality problems in the printing process. In order to ensure the quality of SMT assembly, must strictly control the quality of printed solder paste. Printed solder paste volume requirements are as follows: ① applied amount of solder paste uniformity, consistency. Solder paste graphics to be clear, try not to stick between adjacent graphics. Solder paste graphics and pad graphics play consistent, try not to misalignment. ② In general, the amount of solder paste per unit area on the pad should be 0.8mg / 2mm or so. For narrow-pitch components, should be 0.5mg / 2mm or so (in practice, with the template thickness and opening size to control). ③ printed on the substrate of the solder paste and hope that the weight value compared to the allowable deviation, solder paste to cover the area of each pad, should be more than 75%. ④ solder paste after printing, there should be no serious collapse, edge neatly, dislocation is not greater than 0.2mm, for narrow-pitch components pads, dislocation is not greater than 0.lmm. the substrate is not allowed to be contaminated by solder paste. Visual inspection, with a narrow pitch with 2 to 5 times magnification or 3 to 20 times microscope inspection. 3. Solder paste printing defects, causes and countermeasures Excellent printing graphics should be the vertical and horizontal direction of uniformity and bracing, full, around the clean, full of solder paste pads. With such a printed graphic placement device, after reflow soldering, will get excellent welding results. (1) Misalignment of solder paste pattern      Causes: improper alignment of the steel plate and pad offset; printing machine accuracy is not enough.      Hazard: easy to cause bridge connection.      Countermeasures: adjust the position of the steel plate; adjust the printing machine. (2) Solder paste graphics pull the tip, there are concave Causes: excessive squeegee pressure; rubber squeegee hardness is not enough; window oversized. Hazard: the amount of solder is not enough, easy to appear virtual welding, solder joint strength is not enough. Countermeasures: adjust the printing pressure; change the metal scraper; improve the template window design. (3) The amount of solder paste is too much Causes: template window size is too large; the gap between the steel plate and PCB is too large. Hazard: easy to cause bridging. Countermeasures: check the size of the template window; adjust the printing parameters, especially the gap between the PCB template. (4) uneven graphics, broken points Causes: template window wall smoothness is not good; printing plate times more than one, failed to wipe off the residual solder paste in a timely manner; paste thixotropy is not good. Hazard: easy to cause the amount of solder is insufficient, such as virtual welding defects. Countermeasures: wipe the template. (5) graphic staining   Causes: stencil printing times, failed to wipe clean in time; poor quality of solder paste; steel plate left shaking.   Hazard: easy to bridge.   Countermeasures: scrub the steel plate; change the paste; adjust the machine. In short, solder paste printing should be aware of the parameters of the paste will change at any time, such as particle size / shape, thixotropy and flux properties. In addition, the parameters of the printing machine can also cause changes, such as printing pressure/speed and ambient temperature. The quality of solder paste printing has a great impact on solder quality, so each parameter in the printing process should be treated carefully and the relevant coefficients should be observed and recorded frequently.