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[Watermark] [Watermark] Understand and control lead (understand lead-free)
Right, lead =JtO a{,
1. Understanding of lead. a{}W RkGA
1. 1 physical properties of lead. ，pOmH2N5u
1. 1. 1 lead is a gray heavy metal. ! sj % D * | & amp
1. 1.2 lead has good shielding performance, and X, β and other radiation rays can be shielded by it, so the materials of nuclear reactors are packed in lead boxes to prevent radiation leakage. 8iSUSk\V
1. 1.3 lead powder has strong adhesion, hard and brittle solid, and strong molecular diffusion and permeability. ! ' P*Nn '
1. 1.4 lead is toxic. When the lead concentration in blood reaches 25mg/DL, it will be life-threatening. If you are exposed to skin edema for a long time, it will lead to skin canceration in severe cases. ，4F _ _ & amp5H
1. 1.5 lead and other compounds are harmful to human body and environment; Chapter 4b ]TT
1.2 chemical substance of lead $ zw \ k
1.2. 1 lead exists in nature. El | eti
1.2.2 The chemical symbol of Lead is Pb, and the English word is lead, which belongs to the main group element, that is, ZVA group, with the atomic number of 82 and the outermost electron number of 4; z3AYnH` & lt
1.2.3 Generally speaking, the chemical properties of Pb are not very active, but under certain conditions, it can be converted into compounds with valence of "+2" and "+4", such as PBO PBO2. bk =lpX|b
Use 1.3 lead and other compounds: xe)RM # k >；;
1.3. 1 Pb is often used in soluble coatings such as paints, oils and coatings for printed circuit boards. & lt9NrAu\
1.3.2 Pb compound is widely used in various inks, batteries and gasoline. +U(！ \5
1.3.3 lead and other compounds are also used in electronic ceramic parts, optical glass and filter glass. Y63RLB\koN
1.3.4 compound is easy to be used in alloys, such as solder. O: INX[i
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Lead compound Pb -KI`Y]/)
Lead exists in the earth's crust in the form of natural metal, and most of it comes from burning crude oil, mineral deposits and artificial manufacturing. Lead is used in the production of paints and dyes, as well as in the manufacture of batteries, munitions and metal products (solder and pipes). Once lead enters the soil, it usually stays in the soil particles. Because lead can't dissipate and decompose, the lead in dust and soil should become a long-term lead pollution source. ! ih}lA~=F
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Some important lead pollution has been identified, such as lead-containing paint and lead that forms dust due to peeling, weathering and pulverization of paint; Lead floating in the air; And waste treatment are the potential lead pollution sources closest to human beings. The factors that children and fetuses are often most vulnerable to pollution attacks include: 1) The developing nervous system of fetuses and newborns is particularly sensitive to lead neurotoxicity; 2) The absorption efficiency of lead in children's gastrointestinal system is higher than that in adults; 3) Iron and calcium are elements to protect children, and the lack of them in food will promote the absorption of lead. NNi 1m5G-S
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Inorganic lead cannot be metabolized in our body, but is directly absorbed, diffused and excreted. It is mainly distributed in blood, soft tissues (kidney, bone, bone marrow, liver and brain) and heart mineralized tissues (bones and teeth). In the total lead content in human body, bones and teeth account for about 95%. The life of lead in human body is related to its residual tissues, the shortest is 25 days and the longest is more than 25 years. There are some unstable components in bones, which make lead exchange with blood and metabolic pool quickly. Lead in metabolic pool is extremely harmful, because it is a potentially important lead pollution source. . d0SRzW
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Lead can affect almost every tissue and system in our body, especially the central nervous system, especially for children. Other side effects include hindering the development of nerve and body, reducing the original synthesis of heme, improving auditory valve, lowering serum vitamin D level, and damaging kidney and reproductive system. In the case of high lead pollution, it may reduce the reaction time, cause weakness of limbs, wrists and ankles, and may also affect memory. Exposure to lead is especially dangerous for young people and unborn fetuses. Its destructive effects include premature delivery, small size, mental retardation, learning disabilities and developmental retardation. According to the standard 67/548/EEC, most lead compounds are harmful to fertility and harmful and dangerous to the environment. Standards 89/677/EEC and 91157/EEC restrict the use of lead. :9|.5 1ha I
Lead-free solder In view of the above situation, it is necessary for us to control lead. {g}wW J
1. lead-free solder h2 E@p~W is controlled in the tin furnace.
1) It is suggested to control the copper content in tin furnaces (such as SAC305 or SAC 405): 0.3 ~ 0.9%; XC|)Qci5s
2) adjust the copper content with new tin bars or 75Sn/Ag and 96 Sn/Ag; @u]%？ C@]，
3) Test metal impurities regularly; D/P; _'
4) Low density-materials that can float in Sn/Pb, such as parts and tools, may sink into tin, causing pollution; & lt_aS8p
2. The welder is damaged by 6\LRB6H/7.
1) Not every tin furnace is corroded; V5OI & indirect addressing
2) When the tin furnace covers the pot, the dissolution of iron pollutes tin; 7%ZArJDlk
3) Use corrosive stainless steel, such as type 3 16; teh6p
4) Stainless steel has anti-corrosion coating; L^xnh9|z
5) Use a lower welding temperature, such as 2600C 73K 50:cB.
3. destination pad x"T{zBGl
The tin stripper can be an additional pad or a larger pad for absorbing excessive tin; ED 5\pn
4. Use the original 63/37 tin furnace =c0! aL _Q
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1) discharge 63/37 tin; v 79.p4 & gt
2) Try to remove all tin in inaccessible places (difficulty, manpower) [M|4$d
3) cleaning and discharging with pure tin; X4l)/{) Sir
4) adding lead-free tin; nVKiJe
5. Why is lead-free more expensive? 16! & problem
1) metal price rise: $ & NWd3D, 6h
-37% of the original lead was replaced by tin or other more expensive metals; |O# '
2) Density 4m &；; {Xe
Lead-free tin has low density (about 20% lower) and high production cost; 8? A w' l
3) The production cost is 3U3
-Higher energy costs due to higher melting point. When alloying, the required temperature also increases; qQ[^gTjl]
4) The special expense is 59av.0h
-Most lead-free solders need to pay a patent fee of about 4%-8%; g:。 snx $ " & lt
6. Lead-free solder officially approved by Sony }20rE]
Alloy 305(Sn/3.0Ag/0.5Cu), Sony headquarters has approved all production departments to use SAC305Ye since April 2006, t = OD.
On this basis, the practicality and reliability of the above solder are verified twice. Through evaluation, in order to have more than one choice in the end, three recommended alloys were added for selection. As shown in table 1? liWZy
Three Kinds of Alloys Recommended by USA for Surface Mounting
Table 1 V: FCS' h
Alloy type, liquidus, solidus, scope of application, matters needing attention. =W
Sn-58Bi 1390C(*** crystal) household appliances and mobile phones can be installed at low temperature, which is * * * crystal, but it has problems of durability and heat generation. Used for surface mounting, it has better thermal fatigue resistance than tin-lead alloy. It is better than CPGA-84A and worse than CDIP-20. OE+/0
Sn-3.4ag-4.8bi2100c2050c is superior to Sn-Pb*** crystal solder in the range of 00C to 1000C. It is used for surface mounting of household appliances, portable phones, space, aviation and automobiles, and in the range of -550C to+1250. Used for fusion welding, it is separated in most cases. Sn-3.0ag-0.5cu-hu2210c (* * crystal) Crystals from 00C to 1000C mounted on the surface of household appliances, mobile phones, universes and aviation cars are the same as Sn-Pb***, but from -550C to12500c. The reliability of Sn-2.6Ag-0.8Cu-0.5Sb is not clear. W & amp[qn[f/
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From the screening results, as Sn-57Bi*** crystal alloy, due to the scarcity of Bi resources. Can't be selected as standard solder. However, solder that can be used above 2000C is very valuable. Because this solder has been used as the main material for more than 20 years, we can use it for special purposes. = l . I； t8Y$Y
In addition, some other achievements have been made: -T}Wylup.
1) Suitable objects for storing lead-free solder; ' C55hqg '
2) Controlling alloy composition can affect its price and supply. l %； [ r|
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3) Seven kinds of alloys in the database are selected and compared with Sn-Pb * * * crystal alloys by decreasing method; zTbkk'/
Table 2 8yX*MX[6
Types and precautions of lead-free alloy candidates Kc4Hz 1:
Process method, matters needing attention of alloy type, fm9NJ job evaluation.
Wave soldering of tin-silver system: * q * q? D5
Tin -3.5 silver, tin -3.5 silver -0.5 copper; M~？ -5 1w
Tin-copper system; q4= " /u
Tin -0.7 Cu-H (|fKt+
Add trace elements (silver, gold, nickel, germanium, indium, etc.). ) When the Sn-Pb alloy-plated assembly is welded to the substrate, the above alloy is easy to lift, fall off and peel off. It should be noted that if it is single-sided welding, Bi can also be added. EUAe(
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Welding high temperature series sn-agvm & amp； ! t]；
Tin -3.5 Ag -0.5 Cu-V oxide
Tin -(2-4) silver -( 1-6) bismuth; Or addIng 1-3% in these alloys with the increase of melting point temperature, the reflow soldering temperature must be controlled. Pay special attention to the applicability of bismuth or tin-lead alloy coatings.
Medium temperature series tin-zinc series; db^ DHxC
Pay attention to the applicability of Sn-9Zn and Sn-8Zn-3Bi in special corrosive environment. Pay special attention to the existence of IC. -4[ 1[]c03 |
So as to ensure the heating performance when copper is used as the electrode. It is hoped that when Ni/Au is used as coating, it should be plated to a certain thickness. \\\|0cji
XTm=X= low temperature series Sn-Bi system
Attention must be paid to Sn-Pb alloy coating in the application range of Sn- 1Ag-57Bi and {I"9.
Manual welding tin-silver system; sp？ [Ujxl
Tin -3.5 silver -0.5 copper; ? I5 dollars: 7 pounds
Sn-Cu, Sn-Bi Pay attention to the use environment and selection range of solders with different compositions 2L:KSDsdx.
The above summary lists the body, characteristics and precautions of lead-free solder. Among them, Sn-Ag-Cu solder is the best candidate as the second generation standard solder. Through the research in Europe and America, it is proved that this alloy has the most stable performance, so it is internationally recognized as a standard solder. bOQju & gte
7. Philips requires lead-free tin: solder, SAC405；; Wire -R- 15 SAC405, flux-RF 800; |4sNH
PASTE-OM3 10 SAC405, contact time-main wave 3-3.5 seconds [/L%E }[W
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Lead-free soldering: lead-free manufacturing ]R? 1 & lt； 6 inch 3 Niu
The following will describe how to start lead-free production, test the process and make necessary changes to the whole process ... u]1&;
Lead-free manufacturing HD \ r $ w
For lead-free processes, these must be prepared before starting lead-free welding. For example, in wave soldering, the welding must be replaced. . |~T 13w。 Z
Secondly, starting the lead-free process requires a good plan, and key questions must be asked, such as "What welding defects may we encounter?" ""What are the criteria for acceptance and rejection? To what extent is "He" in solder allowed to be contaminated? “N？ Q72 Six
Reflow welding equipment yh _ s {q * 5
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Flux flow management 1mU~, W.9
In lead-free reflow soldering, lead-free solder affects the process temperature, so it affects the heating temperature curve. Due to the high temperature, the chemical composition of solder paste is different, and different residues will evaporate during welding. In order to keep the machine clean and reduce maintenance downtime, a suitable flux management system is needed. The system must be installed and tested before starting the lead-free reflow soldering process. )4_`=0H
Control cooling to reach the optimal temperature curve 1? 6SkDBi
In lead-free welding, it is recommended to use a controlled cooling system, because once the furnace has proper cooling capacity, the time above liquefaction, grain structure and plate temperature can be determined. Naturally, more room temperature fans are needed. It is recommended to use advanced direct air supply, complete integration and cooling system. The system aims to provide good cooling effect with low nitrogen consumption. Cool the system by circulating a mixture of distilled water and propylene glycol. This environmentally friendly mixture does not need to be changed frequently. Treatment of AHEL# board, (PE{4 +"i
Because of the high temperature in the furnace, the steel plate is easy to warp. A support of the plate can be installed in the furnace to keep the plate flatter, and as a result, defects are reduced. c9"0@hcg
Wave soldering materials and equipment; %; ：
Preheating optimization Yys]a5/
Due to the high process temperature, lead-free welding requires a flux different from that of lead-containing solder. The type of flux will determine which preheating configuration is most suitable for the process. If water is used as flux solvent, it is recommended to use a timely heating rod to quickly heat the circuit board in the first preheating zone. In the second and third zones, the forced convection heating module can be used to evaporate the water in the flux before the PCB reaches the peak. )-IP%Fw9X/
Choose a wave soldering machine with flexible configuration. The preheating module should be easily replaced to find the best arrangement for each individual process. VW 1_cGT
Solder replacement 8; b\ #
Replacing tin/lead alloys with lead-free alloys requires more than just discharging and refilling tin cans. All lead must be completely removed from the solder bath. Any remaining lead residue is a lead-free alloy, so it is out of specification. Considering that the maximum allowable lead content does not exceed 0.2%. Be careful when changing alloys. 0"`-n0pTA
Solder replacement procedures must be strictly observed. First of all, all tin/lead solder in the tin can must be discharged cleanly. The collection box can be used to collect solder. Because most tin cans are designed with special chambers to help maintain a stable solder wave, this kind of work is quite difficult and labor-consuming. {IZs8BlQs
Once the tin can is empty, it must be filled with pure tin. Tin cans. Including all auxiliary parts and surfaces, must be thoroughly cleaned with tin. After that, the tin is discharged. Finally, lead-free solder can be melted. ] Business English
The control software of the system must also be adjusted to prevent any damage to the impeller. In particular, the starting temperature of the impeller must be increased, otherwise the impeller may rotate if the solder is not completely melted. _ ND `H0 & ampQ
Another method is to replace it with a new tin/lead tin can. One advantage of exchanging tin cans is that tin cans can be exchanged back as part of the out-of-control action plan. There are also tin pots, tin pots and non-pot-free welding products. |gXdWxV
Material compatibility 0mH! . D
What we know at present is that stainless steel 304, which is often used in tin pot parts, can no longer be used because these parts are damaged after several months of production without pot. QoPcmnn
Welding machine parts may need to use more corrosion-resistant stainless steel 3 16. This material is reliable enough for tin can parts with low welding speed. For other parts, special anti-corrosion coating can be used on SS3 16 and impeller. ei _ N ~ U； d
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Stainless steel is used instead of titanium to make parts under extreme conditions. Part of the reason is the high cost of titanium, which requires a high level of technology to manufacture these special parts. @Y？ 6jEJ
On the right is a specially coated impeller QY W T >；; T
These anticorrosive coatings are the first choice, not ceramic coatings. Because of its excellent hardness (+/-2000 Vickers hardness) and smooth surface, solder will not stick to metal, which makes any coated parts very easy to clean. “j _ IC～+0
For tin itself, steel is used because of its superior thermal conductivity. The heat-resistant coating prevents iron from melting into solder. We have noticed that the melting speed of iron in lead-free alloys determines the following factors:; 5^DnE6
Materials used; tQ'ZEajhK
Use lead-free alloy; j/ 1 1B
Welding speed; #f！ )N*E
Pollution degree; WlJF9V+
Like the traditional tin/lead welding process, a large amount of gold will be dissolved in lead-free alloy, and the dissolution rate depends on the matrix material, solder composition, solder temperature and solder flow rate. If a special metal has appeared in lead-free welding, the dissolution rate of this metal is very low. For lead-free solder, the three main pollutions are lead, copper, iron and lead. 4xu3ul & lt- C
Lead-free solder will be used in the next few years, but the surface coatings of components and circuit boards will contain lead. Therefore, any lead-free alloy containing bismuth will absorb lead from the surface coating, resulting in different melting points of solder. The melting point of the original alloy 1790C is increased to the range of 218-21kloc-0/0c of the "new" tin/lead/silver alloy. b9TPZL。 Y[
Copper pollution `np8+a (
High tin alloy absorbs copper faster than low tin alloy, and the content of copper in lead-free alloy determines the dissolution of copper. From the tin/lead process, we know that 0.2% or higher copper content will lead to the increase of tin bridge and other problems. The maximum allowable copper pollution in tin/lead is generally 0.3%, which is also the largest in some tin/silver alloys. The high tin content (96.5%) in tin/silver alloy leads to the rapid increase of copper content in the production process, especially on the circuit board with a large number of copper pads. Some processes will exceed the specifications after four or five months. In the tin/lead peak display connection, we can no longer absorb copper from tin/silver/copper alloy. Copper is stable at 1%. k]zEI & lt； +S*
Iron pollution cQ=Q~ 1d|
The dissolution rate of iron in tin/lead is very slow. Compared with lead-free alloy, the quantity is about 10 times that of lead-free alloy. For example, in a factory, the pollution of tin/silver/bismuth/antimony solder is 0.002% iron. ; . \*ApN
Generally, both lead-free wave soldering processes involve pollution. First of all, some companies that use lead-free solder will often check the chemical composition of the alloy. About a year later, they found that the pollution had stabilized more or less. If the liquid level is still within its specification range, the interval between control quantities will increase. 2 & gt& amp！ r！ Z
In contrast, other processes do worry about solder contamination. Some use tin/silver alloy for welding, which is very sensitive to the absorption of copper. Constantly exceeding the specifications will make the company look for alternatives. BJKVG\qMR
The remaining question is, what kind of pollution level is allowed? Because of the change of alloy chemical composition, the melting point will drift and the melting range will increase. Sometimes, new melting points of different alloys appear in solder, {qi} m.
Welding defect kt] 8xg _
In pot-less welding, such special defects as fillet lifting and tin whisker will appear, but other defects, if the solder joint is hollow, seem to appear more than in tin-lead process. So far, there is no international standard for defects in lead-free solder joints, which makes it more difficult to define what is acceptable. agMq_pA
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Weld leg rising 4of | U6i
Solder leg lifting refers to the separation of the solder leg from the copper pad around the PTH plated through hole during the cooling stage. The main reasons for the rise of solder feet are the mismatch between alloy compounds, pad and board thickness and the thermal expansion coefficient of materials. Bismuth-containing alloy combined with lead pollution leads to weld leg rising. However, the rise of solder feet was observed on its serpentine alloy such as tin/copper. Although it can be expected that the reliability of welded joints will decrease, in most cases, the temperature cycle test also shows that it is good. The strength of tin/silver/bismuth/antimony solder joint is very good, and the number of displayed temperature cycles is high. Therefore, coupled with the lack of lead-free electronic assembly standards, many companies accept solder feet on their products. d
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The number of pores in lead-free welding process is increasing, especially when water-based VOC-free flux is used. The diameter of the cavity ranges from 10 mm to 1 mm. Generally speaking, the gap does not affect the reliability of the solder joint. However, large cavities will reduce the crack resistance. Cavities will reduce the electrical and thermal conductivity of interconnection circuits, leading to thermal failure. `-pd+。 rJu？
There are many reasons for the formation of tooth decay. Cavities may be the result of solder shrinkage during curing. Exhaust gas from plated through holes during welding may cause voids during welding. In addition, the cavity may be the result of insufficient wetting of the solder joint. O(hFH/"
Tin whisker 3=%)Pak
The surface of pure tin is easily affected by natural crystal growth. These crystals (tin whiskers) can be 0.5 microns: the diameter of microns can be increased to several millimeters. Tin whiskers can grow after electroplating or even years later. Due to different sizes and shapes, tin whiskers may cause short circuits. f，fX0-
The growth of tin whiskers depends on temperature and humidity. The critical temperature is above 500C, and the relative humidity is 50%+Eh: $ L! ! )
In order to avoid tin whisker, the temperature stress introduced in the welding process should be as low as possible, which is another reason for adopting the linear heating reflow curve. Also, the content of tin is very important; The higher the purity of tin, the greater the chance of tin whisker formation. uvu98tdZ
Production startup) E9S"U yG
Before analyzing any data, measurements must be made. These programs are very important for machine characterization and calibration. You need to collect data to get useful analysis. Variables (measurement data with units) and features (counting data) can be distinguished. Y{ R04e2h
reflow soldering
Variables in the reflow process include machine and data recording parameters. Machine parameters include conveyor belt speed, heating zone temperature, cooling unit temperature and water temperature zone. EarqY}！ j & lt
Data recording parameters include time to reach reference temperature, time above reference temperature, average temperature, minimum slope, maximum slope, average slope, minimum temperature, maximum temperature and time to reach maximum temperature. These features refer to welding defects, such as cavities, missing welding, solder balls, tin bridges and component installation. |
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