PCB substrate material choiceCurrently available in lead-free process on the PCB substrate is not almost six types of materials can be selected:a gold plated plate (Electrolytic of Ni / Au)(b) OSP board (Organic Solderability Preservatives)c silver plate (Immersion 's of Ag(d) of the metal plate (Electroless Ni / Au, ENIG)(e) of tin plate (Immersion 's Tin)(f) Sn-Ag-Cu HASL (SAC HASL)(More than six kinds of sheet, tin plate and tin-silver-copper HASL process is not yet mature, market acceptance of the level, there are doubts about the case under the first not to proceed with the discussion)a gold plated plateThis is the most stable of all existing plate, the most suitable for use in a plate of lead-free process, especially in some high-priced, reliable high degree of electronic products are recommended to use this sheet as a substrate, but the cost of all plates the highest.(b) the OSP boardUse this category sheet, after a high-temperature heating, pre-covered pad on the protection film is bound destruction, while lead solder decrease, especially when the substrate after the secondary back to the welding more plus serious, therefore if the system process needed again Following a dip process, this time on the dip side will be facing welding challenges.
(c) of the silver plate"Silver" has a strong mobility, resulting in leakage happens, but today's dip silver-plated "is not in the past simple metallic silver, but with the organic silver" organic "plated" have been able to meet the future demand for lead-free process, the weldability of life than the OSP board for a long time.(d) of the metal plateThis types of substrates the biggest problem points to facilitate is "black pad" (the Black Pad) The problem, therefore lead-free process, there are many manufacturers does not agree with use of, such as HP to facilitate the provisions of all HP products are not can use this types of substrate , Dell is alsoTwo lead-free parts selection of materialsThe convenience of the most important parts is parts, temperature resistance and parts of the coating material, generally speaking parts of SMT temperature resistance requirements must be met in more than 260 ℃.
The welding equipment should Notea. SMT equipment(b) Dip equipmentc rework equipment
a. SMT equipmentGenerally speaking, the SMT lead-free system process by the use of Reflow recommendations need to use the eight heating district, if lower than the eight heating district, is not is not can be used for lead-free process, only if the furnace sub-long level is not enough, in line with use lead-free system process required for the profile, it is bound to speed decrease, so will affect capacity. In addition, as the wetting of lead-free welding consumables than 63/37 is worse, so if you want to improve then eat tin, in addition to add multi-volume active agents in the solder paste which only depend on the nitrogen future to increase the effect of eating tin. Finally the most important of convenience is cold, but District, due to lead-free melting point relatively high, in order to make cash when the case of curing the more plus close bonding, the heating of the rapid cooling, but would change the very important in order, the general decrease temperature and speed level will be the previous ° C / sec at least enhance the more appropriate to more than 2 ° C / sec. future! Nowadays we have a water cooled reflow come out.
(b) Dip equipmentWave soldering oven for 63/37 process in the past can not be used in lead-free process, mainly due to the melting point of lead-free solder bar than in the pastEnhance the 30 ~ 40 ℃, the tin bath heating power must be improved, so the thermal compensation of speed is sufficient, the various electronics factoryTest operating temperature is mostly set at 260-270 ℃ (when using tin-silver-copper composition, when using tin-copper composition of 270-280 ° C). In addition a long period of use of lead-free welding consumables, which the high example of tin composition under long-term high temperature is very easy to tin bath bi generated erosion, and therefore past use stainless steel as a tin bath of the original material will not be enough to overcome the occurrence of this phenomenon, so each equipment manufacturers have resorted to the titanium alloys are trying to prolong the life of the tin bath!
c rework equipmentNow use a soldering iron soldering station the number of watts used mostly for 30 to 40 watts, tin wire melting point lead-free process usedThan ever to improve above 30 ℃, if the continue to use this soldering station, then the temperature must be adjusted to more than 420 ~ 450 ℃Will no tin-lead wire melted, but relatively soldering iron of life also will reduce low, therefore recommended that must to be fully updated-for-lead-free dedicated welding units, tile number of at least reach more than 80 watts, the temperature the same set at 350 ~ 380 ° C, the thermal compensation speed level sufficient to shun interest intoThe behavior of tin wire welding system.
4 selection of welding materialsThe mainstream of the market lead-free welding consumables are still following the following the main:a tin-silver(b) tin-copper(c) SnAgCu
a Sn-Ag-(Sn96.5/Ag3.5 melting point of 221 ° C)This co-fund does not discuss the theory lead-free process on a number of electronic products In order, lead-free process has been proposed, in this to think that can be used to replace the original of Sn / of Pb system process, but this co-fund the surface Zhang force larger leading to its spread reduced, thereby affecting the effect of eating tin! Although some manufacturers still use this alloy, but has not been widely used in electronics industry.
(b) tin copper (Sn99.3/Cu0.7 melting point of 227 ° C)This co-fund is currently used for wave solder system process which prices most convenient should be the co-fund, is also the U.S. NEMI Association for the recommended use of co-fund, the drawback is the required operating temperature level of relatively high (270 ~ 280 ℃), and mechanical properties is not the good of the problem. To enhance the intensity of this alloy after welding, in this alloy, which add trace amounts of Ni (0.1%).
(c) SnAgCu Sn/Ag3 ~ 4% the / Cu0.5 melting point of 219 ° C)This alloy is currently on the market the most acceptable to co-fund composed of several different formulations ratio for the market, the following description of the the world organization proposed alloys range:(1) U.S. NEMI Association ---- (Sn95.5/Ag3.9/Cu0.6)(2) JEIDA Association ---- (Sn96.5/Ag3.0/Cu0.5)(3) European Union --- (Sn96.5/Ag3.8/Cu0.7)Whether the association recommended that the alloy composition together, as long as it is (Sn/Ag3 ~ 4% / Cu0.5 1) of this range are accepted by the electronics industry, the price for tin-lead complexes 2.2-2.7 times.
Process changea. SMT processDesign of steel plate: Due to the poor of the diffusion of lead-free welding consumables, lead-free solder paste diffusion rate is generally 75%, Sn63/Pb37 Paste diffusion rate is generally about 90%, so in the past used to the Sn63/Pb37 processes steel plate recessed open no longer feasible, it is recommended that the plate openings and Pad in a 1:1 ratio design, and even the length and width are coupled with long! the Profile design: part of the reference profile can be provided by the solder paste supplier! especially in the cooling zone must improve the cooling rate, otherwise there will be a tin concave or the phenomenon of tin fission hair.
Process change(b) DIP processFixture design: due to the poor mobility of lead-free welding consumables, if you want to change the mobility must be the temperature, but also to ensure that the parts can withstand, so the importance of the fixture design was much more important in order. Profile design: the reference profile can be! Provided by the solder paste supplier installation of rapid cooling, but the system but in the export of the wave soldering oven recommendations to avoid solder joint cracking of the phenomenon of tinProcess change(c) the detection process:An AOI Detection: fog symptoms due to lead-free solder joint surface of the welding consumables, therefore the original used in leaded solder set the parameters must be adjusted.ICT: Detection: If using the OSP plate, the test points on the PCB substrate solder paste, so as to avoid the probe can not reach the test point and caused a miscarriage of justice happening.6. Reliability testMay be sent by the institute or recognized unit on behalf of detection, the following tests to ensure the reliability of the product:a vibration test (Vibration, Test)(b) thermal shock (thermal cycling) test (Thermal ShockTest)(c) metallographic sliced test (Cross Section Test)(d) IC parts feet tensile test (Pull Test)(e) the resistance and capacitance thrust test (Shear, Test)not drop test (mobile phone products)
Tin-lead temperature curve
in-lead temperature curve
A preheating zone: room temperature to 150 ° C for 50 to 80 seconds (heating rate 1 C / sec)
2, the temperature zone: the maintenance of 150 ~ 170 C, about 60 to 120 seconds
Heating: heating up to 210 ~ 230 C, maximum temperature 200 C and above at 30 ~ 60sec (heating rate 1 ~ 3 C / sec.)
4, the temperature decrease: cooled down to room temperature (the temperature decrease rate of 1 ~ 3 C / sec.)
Two lead-free temperature curve
A preheating zone: room temperature to 150 ° C for 50 to 80 seconds (heating rate 1.5 ~ 3.0 C / sec)
2, the temperature zone: the maintenance of 155 ~ 185 C, about 60 to 120 seconds
Heating: heating to more than maintain the maximum temperature of 230 ~ 250 C 220C 40 ~ 70sec (heating rate 1.2 ~ 2.3 C / sec)
4, the temperature decrease: cooled down to room temperature (temperature decrease rate of 1.7 ~ 2.2 C / sec.)
