However, lead has polluted the living environment of human beings. According to statistics, the lead content of groundwater in some areas has exceeded the standard by 30 times (allowable standard
First, the current situation of lead-free welding technology
The standardization of lead-free solder alloy composition has not been clearly defined at present. Opinions of most commercial associations such as IPC: lead content
1, lead-free solder alloy
The core and primary task of lead-free is lead-free solder. According to statistics, more than 100 kinds of lead-free solders have been developed in the world, such as solder paste, welding wire, wave soldering strip, etc., but only a few of them are truly recognized as available.
(1) is the most likely alloy material to replace Sn/Pb solder at present.
The non-toxic alloy that is most likely to replace Sn/Pb solder is tin-based alloy. Binary, ternary or multicomponent alloys are formed by adding metal elements such as tin, silver, copper, zinc, bismuth, indium and antimony. By adding metal elements, the properties of the alloy are improved, and the weldability and reliability are improved. Mainly include: Sn-Bi solder alloy, Sn-Ag * * * crystal alloy, Sn-Ag-Cu ternary alloy, Sn-Cu solder alloy, Sn-Zn solder alloy (only developed and applied in Japan), Sn-Bi solder alloy, Sn-In and Sn-Pb alloy.
(2) At present, the most widely used lead-free solder alloys Sn95.8\Ag3.5\Cu0.7 (USA) and Sn96.5\Ag3.0\Cu0.5 (Japan) are the most widely used lead-free solders for reflow soldering. Its melting point is about 2 16-220℃.
Since the lead-free solder Sn95.8\Ag3.5\Cu0.7 has been patented in the United States, the solder with 3.0 wt% Ag is not patented, which is cheaper and has better solder joint quality. IPC recommends the use of Sn-Ag-Cu solder with 3.0 wt% Ag.
Sn-0.7 Cu-Ni solder alloy is used for wave soldering. Its melting point is 227℃.
Although tin-based lead-free alloy has been widely used, compared with Sn63\Pb37*** crystal solder, lead-free alloy solder still has the following problems:
(a) The melting point is about 34℃ higher.
(b) high surface tension and poor wettability.
Price is high.
2, PCB pad surface coating material
Lead-free welding requires that the coating material on the surface of PCB pad should also be lead-free, and lead-free coating on the surface of PCB pad is easier than lead-free coating on the surface of component welding terminal. At present, lead-tin hot air leveling (HASL), electroless nickel plating and gold immersion (ENIC), OSP coating on copper surface, silver immersion (I-Ag) and tin immersion (I-Sn) are mainly replaced by lead-free metals or lead-free solder alloys.
At present, the lead-free standard is not perfect, so there are many kinds of coatings on the surface of lead-free components. There are many pure Sn and Sn/Ag/Cu coatings in the United States and Taiwan Province Province, while there are many kinds of welding end coatings in Japan, which are different from company to company. In addition to pure Sn and /Sn/Ag/Cu, there are Sn/Cu and Sn/Bi alloy coatings. Because the cost of tin plating is relatively low, there are many tin plating processes. However, due to the easy oxidation of Sn surface to form a thin oxide layer, the pressure and unevenness after electrification will push Sn out and form Sn whiskers. Sn must easily cause short circuit at QFP and other components with narrow spacing, which will affect the reliability. For low-end products and components with a service life of less than 5 years, pure Sn can be plated. For high-reliability products and components with service life exceeding 5 years, a layer with a thickness of about 1 can be plated first. M above nickel, and then electroplating 2-3? M-thick tin
3. At present, lead-free welding technology is in the transition and initial stage.
Although lead-free technology has been used in different degrees at home and abroad, it is still in the transitional and initial stage, and it is not mature from theory to application. There is no unified standard, and there is no unified understanding of solder joint reliability of lead-free welding. Therefore, regardless of the confusion of lead-free application technology at home and abroad, most enterprises still have lead at the welding end of components, although the welding materials are lead-free. What kind of lead-free solder is better? Which PCB pad coating is more favorable for lead-free soldering? Which component solder joint material is more conducive to the reliability of lead-free solder joints? What is the most reasonable temperature curve? What are the requirements of lead-free welding for printing, welding, testing and other equipment? . . . There is no clear statement. In short, there are different opinions on lead-free welding technology, each with a set of views and a set of practices. This state is very unfavorable to the reliability of lead-free welding products. Therefore, it is urgent to speed up the research of lead-free welding technology from theory to application.
Second, the characteristics and countermeasures of lead-free welding
1, lead-free soldering and main features of solder joints
Main characteristics of (1) lead-free welding
(a) The high temperature and melting point are about 34℃ higher than the traditional lead crystal solder.
(b) high surface tension and poor wettability.
(c) Small process window and difficult quality control.
(2) Characteristics of lead-free solder joints
(a) Poor wettability and expansibility.
(b) The lead-free solder joint has a rough appearance. Traditional testing standards and AOI need to be upgraded.
(c) There are many air holes in lead-free solder joints, especially when lead-free solder is mixed at the lead welding end, the lead solder on the welding end (ball) melts first, covering the bonding pad, and the flux cannot be discharged, resulting in air holes. However, the porosity does not affect the mechanical strength.
(d) Many defects-due to poor wettability, the self-positioning effect is weakened.
Lead-free solder joints have rough appearance, many pores, large wetting angle and no half moon shape. Because the appearance of lead-free solder joints is obviously different from that of lead-containing solder joints, if measured by the original inspection standard of lead, they can even be considered unqualified, but these do not affect the quality of use for general civil electronic products. So let customers believe that this is caused by poor wettability of lead-free welding. With the deepening and development of lead-free technology, due to the improvement of flux and process, the rough appearance of lead-free solder joints has been improved, and I believe there will be better progress in the future.
2. Characteristics and countermeasures of lead-free wave soldering
Lead-free wave soldering also has the characteristics of high temperature, poor wettability and small process window. Quality control is more difficult than reflow soldering.
Sn-0.7cu or Sn-0.7cu-0.05ni is usually used as solder for wave soldering, with melting point of 227℃ and soldering temperature of 250-260℃. Adding a small amount of nickel to tin-copper solder can increase fluidity and elongation. Sn/Ag/Cu solder can also be used in wave soldering, but it is generally not recommended to use Sn/Ag/Cu solder. In addition to the high cost of Sn/Ag/Cu solder, Ag will also corrode Sn pot, and the corrosion is more serious than Sn.
(2) The solder temperature of lead-free wave soldering tin pot is as high as 250-260℃, and tin will corrode the tin pot at high temperature. The higher the temperature, the more corrosive it is. Moreover, Sn content in lead-free solder accounts for 99%, which is 40% more than lead-free solder. If the traditional stainless steel pot liner is used for lead-free welding, pot leakage will occur in about three months. Therefore, the tin pot and nozzle of wave soldering equipment require high temperature resistance and corrosion resistance. At present, titanium alloy steel pot liner is widely used.
Due to the poor wettability of lead-free solder and small process window, in order to reduce the temperature difference on PCB surface during welding, the temperature of tin pot is required to be uniform.
(3) Due to the high melting point, the preheating temperature of PCB should be increased correspondingly, which is generally 100- 130℃. In order to make the temperature inside and outside the PCB uniform, the preheating zone should be lengthened. Slowly raise the temperature. The welding time is 3-4 seconds. The distance between the two waves is short.
(4) For the large-size PCB, in order to prevent the PCB from deforming, the transmission guide rail is provided with an intermediate support.
(5) Due to the high temperature, in order to prevent the solder joints from growing up due to too long cooling time, a cooling device should be added to the wave soldering equipment to cool the solder joints quickly. However, if the cooling speed is too fast, it may damage the chip components of ceramic structure and lead to cracks in the components, so it is necessary to control not to cool too fast. In addition, the tin blowing pot will affect the welding temperature, so appropriate cooling measures should be considered.
(6) Because of high temperature and poor wettability, in order to improve the activation temperature and activity of the flux, some additives can be added in the coating process.
(7) Pay close attention to the proportion of Cu in Sn-Cu solder. The composition of Cu reaches 0.2%, and the liquid temperature changes as high as 6℃. This change may lead to changes in dynamics and welding quality. When the proportion of Cu exceeds 1%, new solder must be used. Because copper increases with working time, low copper alloy is generally selected.
(8) During wave soldering, the tin plating height of the insertion hole of through-hole components may not reach 75% (75% required by traditional Sn\Pb), so the design of PCB aperture ratio, flux activity and coating amount, peak temperature, peak height and guide rail inclination angle should be considered comprehensively.
(9) Because high temperature will accelerate the oxidation of Sn, the lead-free wave soldering process still has a big disadvantage that it will produce a lot of residues, and nitrogen (N2) can reduce the formation of Sn slag. Of course, N2 can also be left unfilled, or lead-free tin slag reducing powder can be added, which will reduce a lot of residues and reuse them, but we must pay more attention to daily cleaning and maintenance than lead welding.
(10) The phenomenon of delamination after wave soldering is serious.
Thirdly, there are problems in the special stage of transition from lead welding to lead-free welding.
1, the challenge of lead-free technology to components
(1) High temperature resistance
The influence of high temperature on component packaging should be considered. Because the packaging materials of traditional surface mount components can meet the welding temperature of lead solder as long as they can withstand the high temperature of 240℃, and the welding temperature of lead-free welding of complex products is as high as 260℃, it is necessary to consider whether the component packaging can withstand the high temperature.
In addition, the influence of high temperature on the internal connection of the device should be considered. The internal connection methods of IC include gold wire ball welding, ultrasonic pressure welding, flip chip welding, etc., especially BGA, CSP, composite components, modules and other new components. The material used for its internal connection is also the same solder used for surface assembly and reflow soldering process. Therefore, the internal connection materials of lead-free components should also meet the requirements of lead-free welding.
(2) Lead-free welding end
The welding ends of lead-containing components are mostly Sn/Pb coatings, while the welding ends of lead-free components have various coatings. At present, which coating is the best is still inconclusive, so the standard of lead-free components needs to be improved.
2. The challenge of lead-free technology to PCB
Lead-free process requires PCB to have good heat resistance, high glass transition temperature Tg, low thermal expansion coefficient and low cost.
(1) Lead-free process requires a higher glass transition temperature Tg.
Tg is a unique property of polymers and a critical temperature that determines the properties of materials. In the process of SMT welding, the welding temperature is much higher than the Tg of PCB substrate, and the lead-free welding temperature is 34℃ higher than the lead-free welding temperature, which is more likely to cause PCB thermal deformation and damage components when cooling. The base PCB material with high Tg should be properly selected.
(2) Require low coefficient of thermal expansion (CTE)
When the welding temperature rises, the CTE mismatch between Z-axis and XY-axis laminates, glass fiber and Cu of multilayer PCB will cause great stress to Cu, and in severe cases, the metallized hole coating will break and fail. This is a rather complicated problem, because it depends on many variables, such as the number of PCB layers, thickness, laminated material, welding curve, distribution of Cu, geometry of vias (such as aspect ratio) and so on.
Measures to overcome the fracture of metallized holes in multilayer board;
Etching process 1- Remove the resin/glass fiber in the hole before electroplating.
So as to enhance the bonding force between the metallized hole wall and the multilayer board.
The etching depth is 13-20? m .
(3) High heat resistance
The limit temperature of FR-4 PCB is 240℃. For simple products, the peak temperature of 235-240℃ can meet the requirements, but for complex products, it may take 260℃ to weld well. Therefore, thick plates and complex products need high temperature resistant FR-5.
(4) low cost
Because the cost of FR-5 is relatively high, for general consumer products, composite CEMn can be used to replace FR-4 substrate. CEMn is a kind of rigid composite copper clad laminate with different surface and core materials, referred to as different models for short.
Fourthly, the challenge of lead-free process to flux.
(1) Requirements of lead-free process for flux
(a) Because there is a chemical reaction between flux and alloy surface, different flux should be selected for different alloy components.
(b) Due to the poor wettability of lead-free alloys, high flux activity is required.
(c) Improve the activation temperature of the flux to adapt to the lead-free high-temperature welding temperature.
(d) Less residue after welding, non-corrosive, meeting the requirements of ICT probe capability and electromigration.
(2) Flux is the key to the printability and solderability of solder paste.
After determining the lead-free alloy, the key lies in flux. For example, eight solder paste companies provide a company with lead-free solder paste with the same alloy composition for testing, and the test results are very different. There is no monument behind the solder paste with good wettability, and the resistance and capacitance of the wet paste with poor wettability shift more after welding. Therefore, the selection of solder paste should be tested to see whether the printability can meet the requirements and the quality after welding. For example, whether the rolling, filling and demoulding of solder paste are good during printing, whether the printing quality changes at intervals of 1 hour, and whether the viscosity changes at intervals of 1-8 hours. In short, choose the solder paste suitable for your own products and technology.
(3) Lead-free flux must be specially prepared. Flux in solder paste is the key material to purify the solder surface, improve wettability, prevent solder oxidation, and ensure the quality and excellent manufacturability of solder paste. At high temperature, flux can clean the oxide layer on PCB pads, component ends and pins, and activate the metal surface at the same time.
Non-cleaning tin-lead-tin paste has been used for many years and is a mature technology. The early lead-free solder paste simply mixed Sn-Pb solder, no-clean flux and lead-free alloy, and the result was very bad. The chemical reaction between flux and solder alloy in solder paste affects the rheological properties of solder paste, which is very important for printing performance.
Because of the poor wettability of lead-free alloys, it is necessary to improve the activity and activation temperature by welding. The following further analysis: No matter lead welding or lead-free welding, flux wetting area is the key area to control welding. Flux acid can't react with Cu20 at room temperature, that is, it decomposes, which will release heat and activation energy. When soldering with lead, the active reaction of flux cleans the metal surface just before the melting point of solder 183℃. When the solder melts, the metal surface gains activation energy, which can reduce the viscosity and surface tension of the melted solder, improve the wettability, and facilitate diffusion and dissolution to form an intermetallic alloy layer. But in lead-free soldering, the melting point is 2 17℃, which is 34℃ higher than that of lead. The main component of lead-free flux is rosin ester. If the traditional flux is used, the flux in the solder paste has completed the reaction before the solder melts at 183℃, and then rises from 183℃ to 2 17℃ because the flux is at a high temperature for a long time.
Therefore, lead-free flux must be specially prepared. With the deepening of lead-free process, due to the efforts of solder manufacturers, they have taken measures to improve the activity and activation temperature of flux and improve the quality of lead-free solder paste. At present, the appearance of lead-free solder joints has improved compared with previous years.
(4) Flux without VOC in wave soldering also needs special preparation. Some products also need lead-free solder paste and wave soldering water-soluble flux.
4. Discussion on the reliability of lead-free welding in transition period.
The reliability of lead-free welding is a problem that manufacturers and users are very concerned about. Especially in the special stage of the transition from lead to lead-free welding, there are no standards for lead-free materials, printed boards, components, tests, etc., and even reliability testing methods are not standardized, so the reliability is very worrying. At present, the lead-free process, especially in China, is in a chaotic stage. Due to the mixing of lead and lead, especially when lead solder and lead-free process are used for components of lead-free welding terminals, serious reliability problems will occur. These problems are not only the problems that should be paid attention to in lead-free welding in the current transitional stage, but also the problems that should be paid attention to in lead-free welding in the transitional stage.
(1) solder joint is soft and easy to deform, so the hardness of lead-free solder joint is higher than that of Sn-Pb, the strength of lead-free solder joint is higher than that of Sn-Pb, and the deformation of lead-free solder joint is smaller than that of Sn-Pb, but these do not mean that lead-free reliability is good. Due to the poor wettability of lead-free solder, there are many welding defects, such as holes, displacements and monuments. In addition, due to the high melting point, if the activation temperature of the flux can't match the high melting point, as previously analyzed, due to the long temperature and time in the wetting zone of the flux, the welding surface will be re-oxidized at high temperature, and infiltration and diffusion can't occur, and a good interface alloy layer can't be formed, resulting in poor bonding strength (tensile strength) of the welding surface and reduced reliability.
According to the reliability test results of Agilent and other companies, such as thrust test, bending test, vibration test, drop test, hot flashes, high and low temperature cycles, etc. There is a similar conclusion: in most civil and communication fields, the mechanical strength of lead-free solder joints is even higher than that of lead-free solder joints because there is not much stress in the use environment; Even in high stress places, such as military, high and low temperature, low air pressure and other harsh environments, lead-free is far less reliable than lead-free because of its large creep.
The reliability of lead-free solder joints (including test methods) is still in the preliminary research stage.
(2) Tin whisker problem
SN will grow whiskers in compressed state, which will cause short circuit in severe cases, so special attention should be paid to narrow-pitch QFP packaging components. Is the whisker diameter 1- 10? M, what is the length? M number+? The needle-like single crystal of m is easy to appear on the surface of low melting point metals such as Sn, Zn, Cd and Qg.
The fundamental reason for the growth of Sn whiskers is the stress on Sn coating, and the whisker length reaches 65438 0.5 months at room temperature. m .
Adding some impurities to Sn can avoid the growth of Sn whiskers.
(3) delamination phenomenon.
When lead-free and lead are mixed, if the lead mixed in welding exceeds the standard >: 5%, after welding, the peeling phenomenon of the common layer will be aggravated at the junction of welding occupation and welding end. In the lead-free wave soldering process of lead-containing components, peeling phenomenon is common, and even leads to PCB pads peeling together in serious cases. Therefore, SMD (solder mask definition pad) can be used in the pad design of wave soldering in the transition stage, and the solder mask is pressed around the pad, which can reduce or avoid the phenomenon of PCB pad peeling.
The mechanism of peeling (peeling and cracking) should be further studied. Whether the solder, components and PCB will not peel off after all lead-free remains to be studied.
Stripping of Sn-Pb Coating on Components
(4) Discussion on reliability when lead and lead are mixed.
① The influence of lead in lead-free solder on long-term reliability is a subject that needs further study. The preliminary study shows that different lead contents in solder joints have different effects on reliability. When the lead content is in a certain intermediate range, the impact is greatest. This is because during the final solidification and crystallization, a segregated metallographic phase is formed at the Sn weight interface, and these segregated metallographic phases begin to form cracks and continue to expand under cyclic load. For example, 2%-5% lead can determine the fatigue life of lead-free solder, but compared with Sn-Pb solder, the reliability is similar. When lead-free solder is mixed with lead solder, it is sometimes necessary to control the lead content in solder joints.
At present, it is in the transition period of lead-free and lead-free welding. Most lead-free processes are a mixture of lead-free solder and components with pins. In lead-free solder joints, the lead content may come from the solder terminals, pins or solder balls of BGA.
When lead-free solder is mixed with lead-free solder, many air holes will appear. This is because when lead-free solder is mixed with lead-free solder, the lead solder at the solder end (ball) melts first, covering the pad. When the lead-free solder alloy melts, the flux in the solder paste cannot be discharged, resulting in air holes. For wave soldering, it is necessary to monitor the lead content of solder joint because the Sn-Pb electroplating layer on the neck of the component pin is constantly melting.
(2) the mixed quality of lead welding and lead-free welding ends is the worst.
When lead solder is mixed with lead-free solder terminals, if the temperature curve of lead solder is adopted, the lead solder melts first, but the lead-free solder terminals (balls) cannot be completely melted, so that an intermetallic alloy layer cannot be formed at the interface on one side of the component, which destroys the original structures on BGA and CSP sides and leads to failure. So lead solder and lead-free solder terminals are mixed together, and the quality is the worst. BGA and CSP lead-free solder balls cannot be used in lead-containing processes.
(5) Adverse effects of high temperature on components
Ceramic resistors and special capacitors are very sensitive to the slope (temperature change rate) of the temperature curve. Because the thermal expansion coefficient CTE of ceramic body and PCB (ceramic: 3-5, PCB: about 17) is quite different, the component body and solder joint are easy to crack when the solder joint is cooled, and the component cracking phenomenon is directly proportional to the difference of CTE, temperature and component size. Small components of 020 1, 0402, 0603 rarely crack, while the large components above are more likely to crack and fail.
Aluminum electrolytic capacitors are extremely sensitive to transparency.
The failure of connectors and other plastic packaging components (such as QFP and PBGA) increases obviously at high temperature. Mainly about stratification, popcorn, deformation and so on. According to rough statistics, the reliability of humidity sensor (MSL) decreases by 1 level every time the temperature increases. The solution is to reduce the peak temperature as much as possible; The humidity sensor is dehumidified and baked.
(6) Adverse effects of high temperature on PCB
The third part analyzes the adverse effects of high temperature on PCB, such as the thermal deformation of PCB at high temperature, the decrease of strength and insulation resistance caused by aging and deterioration of resin, and the failure of metallized hole coating caused by the mismatch of CTE in Z axis and XY direction of PCB.
The solution is to reduce the peak temperature as much as possible. Generally, simple consumer products can use FR-4 substrate, while thick plates and complex products need to use high-temperature resistant FR-5 or CEMn instead of FR-4 substrate.
(7) Electrical reliability
Flux residue formed by reflow soldering, wave soldering and rework may cause electrochemical reaction between conductors under a certain voltage in humid environment, resulting in the decrease of surface insulation resistance (SIR). If there is electromigration and dendrite (tin whisker) growth, the wires will be short-circuited, thus causing the risk of electromigration (commonly known as "leakage"). In order to ensure electrical reliability, it is necessary to evaluate the performance of different non-cleaning fluxes.
(8) About lead-free repair
The repair of (1) lead-free solder is quite difficult, mainly because:
(a) The lead-free solder alloy has poor wettability.
(b) High temperature (235℃ for simple printed circuit boards and 260℃ for complex printed circuit boards).
(c) The process window is small.
② Precautions for lead-free repair:
(a) Select appropriate repair equipment and tools.
(b) Correctly repair equipment and tools.
(c) Correct selection of materials such as solder paste, flux and welding wire.
(d) Setting welding parameters correctly.
Besides adapting to the high melting point and low wettability of lead-free solder. At the same time, care must be taken in the maintenance process to minimize any potential factors that adversely affect the reliability of components and PCB.
(9) Summary of lead-free and lead-free mixing in transition period.
(a) Lead-free solder and lead-free solder terminals-the best effect.
(b) Lead-free solders and lead solder tips-they are widely used at present and can be applied, but the content of lead, copper, etc. It must be controlled, the corresponding flux should be prepared, and the process parameters such as temperature curve should be strictly controlled, otherwise it will cause reliability problems.
(c) Lead solder and lead-free solder terminals-the worst effect. BGA and CSP lead-free solder balls cannot be used in lead-free process and are not recommended.
Five, lead, lead-free mixed transition stage should pay attention to the problem.
1. Example of the problem
(1) Some SMT factories with lead technology have also encountered lead-free components. Although lead-free technology has not yet started, lead-free components, especially BGA/CSP and LLP, have also been encountered. Some component factories no longer produce lead-containing devices, so they cannot purchase lead-containing devices. It is not terrible to know that the purchased devices are lead-free, because the welding temperature can be increased to 230-235℃. Another measure is that lead-free solder and lead-free process can be adopted, because lead-free solder and lead solder terminals are widely used in the transition stage at present, and their reliability is acceptable. But worst of all, I accidentally bumped into lead-free components, which I didn't find before production. In production, lead solder and lead process are still used, and the result is very bad, because the mixing effect of lead solder and lead-free solder terminals is the worst.
(2) Pure tin hot air leveling PCB will also be encountered in the lead process.
This situation also happened unintentionally, because the welding temperature was not enough, which led to quality problems.
(3) Wave soldering problem
Wave soldering has many problems, for example, lead-free components are encountered in the current lead process; Lead-free plug-in holes and through holes are not tinned; The phenomenon of delamination is serious; There are many defects such as bridging and missing welding; There are many oxides on the surface of tin cans. . . . .
Step 2: Solution
(1) material preparation
Pay attention to whether the welding end materials of components are lead-free when preparing materials. If it is lead-free components, we must find out what coating material it is, especially BGA/CSP and newly packaged devices, such as LLP (also pay attention to lead technology).
At present, the lead-free standard is not perfect, so there are many kinds of coatings on the surface of lead-free and device-free welding terminals, such as Sn/Bi coating on Japanese component welding terminals. If the solder contains lead, when the lead content is
(2) Material management
For enterprises with lead and lead-free processes, we must pay attention to strict material management system, and never confuse lead and lead-free solder paste with components.
(3) Lead-free printing should improve printing accuracy.
Enlarge the template opening size: width-thickness ratio > 1.6, area ratio >; 0.7 1
(4) Improve the accuracy of patch.
(5) Strictly control the temperature curve to minimize the peak temperature;
The humidity sensor is dehumidified and baked.
(6) Complex and highly reliable products shall be made of high-temperature resistant PCB materials (FR5 or others).
(7) The welding quality of N2 is better than that of air, especially in wave soldering, N2 can reduce the oxidation of high-temperature solder, reduce residue and save solder. Or add lead-free tin slag reducing powder to reduce the reuse of a large number of residues, but we must pay more attention to daily cleaning and maintenance than lead welding.
6. Cost control in the process of conversion from lead to lead-free
In the process of conversion from lead to no process, cost control is mainly considered from two aspects: machine cost and process material consumption cost.
At present, quite a few enterprises have purchased the machines used in lead welding process (wave welding), which are close to the technical requirements of lead-free welding in various properties and operability. The materials and dimensions of the key parts of the machine used now can be modified accordingly and continue to be used in the processing technology of electronic products with low requirements.
Feasibility analysis of lead-free wave soldering machine replacing ordinary wave soldering machine
The difference of welding temperature between ordinary tin and lead-free tin;
The welding temperature of ordinary tin is 245℃
The soldering temperature of lead-free tin is 270℃
Difference of preheating temperature between ordinary tin and lead-free tin flux
A. The preheating temperature of soldering flux for common tin is 90℃
B the preheating temperature of lead-free tin flux is 1 10℃
Difference of metal composition between ordinary tin and lead-free tin
Metal composition Sn/Pb of common tin
B The metal composition of lead-free tin is mainly Sn/Ag/Cu or Sn/Cu.
Difference of welding equipment requirements between ordinary tin and lead-free tin
：
Requirements for common soldering equipment
No special requirements:
Requirements for lead-free soldering equipment
A requires that the part of the machine that comes into contact with tin itself cannot contain lead.
B. Lead-free tin furnaces require good corrosion resistance.
C requires the cooling speed of the machine to be faster.
According to the above requirements, the corresponding measures are as follows
1. This machine is made of titanium alloy.
2. The length of the machine preheating zone is proportional to the speed of the machine.
3. The part in contact with lead-free solder is made of lead-free material.
4. Change the cooling part of the machine into air conditioner or increase the number of cooling fans.
Effect of tin furnace transformation
Completely meets all requirements of lead-free process.
B the production speed is basically the same as before the transformation.
conclusion
It is completely feasible to transform the original ordinary wave soldering machine into lead-free wave soldering machine, and the cost is saved.
.
Material consumption
At present, the content of tin in the solder used in lead-free process is much higher than that in the original solder, and its alloy composition is also greatly improved. In the process of production and processing, the output of tin slag is also greatly improved than that of ordinary solder. If the amount of tin slag can be reduced, it will be beneficial to the cost control of material consumption.
Tin slag is mainly an oxide produced by the reaction of tin with oxygen at high temperature. Most of tin and oxygen can be separated by physical high-temperature stirring (that is, tin slag reduction), and the separated tin can be reused. Oxygen molecules in tin slag can also be replaced by chemical replacement reduction reaction and reduced to pure tin for reuse.