Five Steps to Successful Lead-Free Soldering: Step 4.This article is the fourth of five that describe how to convert from tin-lead reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. and wave soldering Applying liquid solder to the underside of printed circuit boards in order to bond the chips and discrete components that are placed on top of the board and whose metal leads (pins) extend through the board. processes to lead-free processes. The first article (April 2001) described materials and machine configuration parameters. The second article (May 2001) described how Taguchi experiments can be used to help develop a lead-free process. The third article (June 2001) described how to make the process more robust and less sensitive to variation. This article will describe how to begin production, monitor the process and make needed changes throughout the process. LEAD-FREE MANUFACTURING For a lead-free process, several considerations must be addressed, and certain conditions must be established. First, some equipment options and different materials can improve the process, and they must be obtained before beginning lead-free soldering. For example, in wave soldering, the solder must be exchanged. Second, beginning lead-free processing requires a good plan, and key questions must be asked, such as: "What soldering defects can we expect?" "What are the criteria for acceptance or rejection?" and "What levels of contaminants are allowed in the solder?" REFLOW SOLDERING Reflow soldering is the most common means to attach a surface mounted component to a circuit board, and typically consists of applying solder paste, positioning the devices, and reflowing the solder in a conveyorized oven. EQUIPMENT Flux flow management In lead-free reflow, lead-free solders impact process temperatures and, therefore, the heating profile. Due to the higher temperatures and different solder paste Solder paste (or solder cream) is a mix of small solder particles and flux. It is used extensively in the automated soldering processes wave soldering and reflow soldering. chemistry, different residues will evaporate during soldering. To keep the machine clean with low maintenance down time, an appropriate flux flow management system will be needed. This system must be installed and tested before beginning the lead-free reflow process (Figure 1). Controlled cooling for optimal profiling A controlled cooling system cooling system: see air conditioning; internal-combustion engine; refrigeration. cooling system Apparatus used to keep the temperature of a structure or device from exceeding limits imposed by needs of safety and efficiency. is recommended in lead-free soldering because time above liquidous, grain structure and board exit temperatures are all defined once the oven has proper cooling capability. Naturally, more than ambient air fans are needed. Instead, an advanced direct air, fully integrated, heat rejecting system is recommended. This system is designed to provide good cooling with low nitrogen consumption. This system cools by recirculating a mixture of distilled water Noun 1. distilled water - water that has been purified by distillation H2O, water - binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; and propylene glycol propylene glycol a chemical used industrially as an antifreeze, solvent stabilizer, as a preservative in liquid livestock feeds and pharmaceutically as a vehicle or solvent for medicinal preparations. . This environmentally friendly Environmentally friendly, also referred to as nature friendly, is a term used to refer to goods and services considered to inflict minimal harm on the environment.[1] mixture requires infrequent replacement. Board handling Due to higher temperatures in the oven, boards tend to warp. A board support can be installed in the oven to keep the boards flatter, resulting in fewer defects. WAVE SOLDERING MATERIALS AND EQUIPMENT Preheat optimization Lead-free soldering requires different fluxes than lead-containing solders due to higher process temperatures. The flux type will determine which preheat configuration is most suitable for that process. If water is used as the flux solvent, a quartz rod heater in the first zone is recommended to heat the board rapidly. In the second and third zones, a forced convection heating module can be used to evaporate the water from the flux before the PCB PCB: see polychlorinated biphenyl. PCB in full polychlorinated biphenyl Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. enters the wave. Select a wave soldering machine with the flexibility to change configurations quickly. The preheat modules should be easily swapped to find the best arrangement for each individual process (Figure 2). Exchange of solder Replacing tin-lead alloy with a lead-free alloy requires more than just draining and refilling the solder pot. All lead must be completely removed from the solder pot. Any lead residues remaining will contaminate con·tam·i·nate v. 1. To make impure or unclean by contact or mixture. 2. To expose to or permeate with radioactivity. con·tam·i·nant n. the lead-free alloy so that it is out of specification limits. Considering that the maximum allowed amount of lead is less than 0.2 percent, the alloys must be changed carefully. The solder changeout procedure must be followed strictly. First, all tin/lead solder in the pot must be drained. Bins can be used to collect the solder. Because most solder pots are designed with special chambers to help maintain a stable solder wave, the job is quite difficult and labor-intensive. Once the solder pot is empty, it must be refilled with pure tin. The solder pot, including all interior parts and surfaces, must be thoroughly rinsed with the tin. After that, the tin must be drained. Finally, the lead-free alloy can be melted. The system's control software must also be adjusted to prevent any damage to the impellers. Specifically, the enable temperatures of the impellers must be increased, or they may attempt to run when the solder is not fully melted. The alternative is to replace the tin-lead solder pot with a new one. One advantage of "swapping" the solder pots is that the tin-lead solder pot can always be swapped back as part of an Out of Control Action Plan. Also, the tin-lead solder pot remains available for product runs that are not lead free. Material compatibility Not much has been published regarding lead-free experiences in practice and the specific issues concerning compatibility of materials with new lead-free alloys, which are sometimes more aggressive. What we have learned so far is that, due to the high tin content of the lead-free alloys, materials like stainless steel stainless steel: see steel. stainless steel Any of a family of alloy steels usually containing 10–30% chromium. The presence of chromium, together with low carbon content, gives remarkable resistance to corrosion and heat. 304, which is often used for solder pot parts, cannot be used any more because they will be damaged after a few months in lead-free production. Soldering machine parts may have to be switched to a more corrosive-resistant stainless steel 316. This material is reliable enough for those solder pot parts where the solder velocity is low. For the other parts, a proprietary corrosive-resistant coating may be applied on the SS 316 and the impellers (Figure 3). Stainless steel, not titanium, is used for these extreme-condition parts, partly due to the high cost of titanium, and because making these specific parts out of titanium would require a very high skill level. This anti-corrosive coating is preferred, instead of a ceramic coating ceramic coating, n a thin layer of ceramic material, commonly hydroxyapatite, used to cover dental implants. This typically increases the hardness of the implant and can also make the implant bond more readily with bone. , due to its superior hardness ( plus/minus 2000 Vickers). Unlike ceramic coatings, this proprietary coating cannot be damaged by occasional careless maintenance. Another advantage is that, due to this coating's smooth surface, solder does not stick to the metal, which makes any coated parts very easy to clean. For the solder pot itself, steel is used, due to its superior heat conductivity. A heat-resistant coating prevents iron from dissolving into the solder. We noticed that the dissolution rate of iron in the lead-free alloy depends on the following factors: - the materials used - the lead-free alloy used - the solder temperature - the solder velocity. CONTAMINATION LEVELS As in traditional tin-lead soldering processes, many metals will dissolve in lead-free alloys. The dissolution rate depends on the base materials, solder composition, solder temperature and the solder flow velocity In fluid dynamics the flow velocity, or velocity field, of a fluid is a vector field which is used to mathematically describe the motion of the fluid. Definition The flow velocity of a fluid is a vector field Lead contamination During the next few years, lead-free solders will be used, but with components and boards with lead-bearing finishes. As a result, any lead-free alloy containing bismuth bismuth (bĭz`məth) [Ger. Weisse Masse=white mass], metallic chemical element; symbol Bi; at. no. 83; at. wt. 208.9804; m.p. 271.3°C;; b.p. about 1,560°C;; sp. gr. 9.75 at 20°C;; valence +3 or +5. will absorb lead from the finishes, resulting in a different melting point melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and for the solder. As another example, Figure 4 shows the results of a tin-silver alloy contaminated contaminated, v 1. made radioactive by the addition of small quantities of radioactive material. 2. made contaminated by adding infective or radiographic materials. 3. an infective surface or object. with 1.5 percent lead. Specifically, the original alloy's melting point of 179 degrees C increases to a range of 218 degrees to 221 degrees C for the "new" tin-lead-silver alloy. Copper contamination High-tin alloys pick up copper more rapidly than low-tin alloys. Also, the amount of copper in a lead-free alloy determines how much copper will be dissolved. The main question becomes what maximum level of copper is allowed. From the tin-lead processes, we know that copper contents of 0.2 percent and higher result in problems such as increased bridging. The maximum permissible copper contamination in tin-lead is usually specified as 0.3 percent. This 0.3 percent is also the maximum in some current specifications for tin-silver alloys. The high tin content (96.5 percent) in tin-silver alloys results in a rather rapid increase in the copper level during production, particularly on boards with numerous copper pads. Some processes will run out of spec after four to five months, due to this high requirement. In tin-lead wave soldering, we can separate the copper from the tin-lead and remove it. In lead-free soldering, unfortunately, we must exchange the solder. For tin-silver-copper, the story is different. First, this alloy already contains copper, so the scavenging scavenging of anesthetic. See anesthetic scavenging. of copper is slower. Second, experiments have shown that tin-silver-copper alloys containing 1 percent or more copper do not scavenge scav·enge v. scav·enged, scav·eng·ing, scav·eng·es v.tr. 1. To search through for salvageable material: scavenged the garbage cans for food scraps. 2. copper. The copper stabilizes at this 1 percent level. Iron contamination The dissolution rate of iron in tin-lead is low. With lead-free alloys, the numbers are about a factor of 10 higher. For example, in one facility, the contamination of a tin-silver-bismuth-antimony solder bath was 0.002 percent iron in one year. In general, two kinds of lead-free wave soldering processes involve contamination. First, some companies using lead-free solders will frequently check the composition of the alloy. After about one year, they find that the contamination more or less stabilizes. If the levels of the metals are still within their specification, the interval time of controlling the amounts will increase. In contrast, other companies have real concerns with solder contamination. Several are soldering with tin-silver alloy, which is very sensitive to scavenging copper. Continuously running out of specification causes these companies to look at alternatives. Still remaining is the question of what contamination levels are allowed because, as a result of the alloy's compositional change, the melting point will shift and the melting range melting range, n See range, melting. will increase. Sometimes, new melting points of the different alloys inside the solder will occur (Figure 4). SOLDERING DEFECTS In lead-free soldering, specific defects occur such as fillet fillet /fil·let/ (fil´et) 1. a loop, as of cord or tape, for making traction on the fetus. 2. in the nervous system, a long band of nerve fibers. fil·let n. 1. lifting and tin whiskers See metal whiskers. . But other defects, such as voids in the solder joints, also seem to occur more than in the tin/lead process. To date, no international standards exist for defects in lead-free solder joints, which makes defining what is acceptable more complicated. Fillet lifting Fillet lifting is a separation of solder fillets from the copper pads around plated through holes (PTH PTH abbr. parathyroid hormone Parathyroid hormone (PTH) A chemical substance produced by the parathyroid glands. This hormone is a major element in regulating calcium in the body. ) during the cool-down stage. The primary reason for fillet lifting is a coefficient of thermal expansion coefficient of thermal expansion, n See expansion, thermal coefficient. (CTE (Coefficient of Thermal Expansion) The difference between the way two materials expand when heat is applied. This is very critical when chips are mounted to printed circuit boards, because the silicon chip expands at a different rate than the plastic board. ) mismatch of the alloy composition, copper pad, board thickness and board material. Fillet lifting occurs with bismuth-bearing alloys in combination with lead contamination. But fillet lifting has also been observed with other alloys like tin-copper. Although reduced reliability of the solder joints might be expected, thermal cycle tests, in most cases, still show good numbers. Tin-silver-bismuth-antimony joints are very strong and have shown good thermal cycle data. Therefore, and due to a lack of standards for electronic assembly of lead-free solders, numerous companies accept fillet lifting on their products. Voids An increase in the number of voids occurs in the lead-free soldering process, especially when water-based VOC-free fluxes are used. The voids' diameters range from 10 micron to 1 mm. In general, porosity does not affect solder joint reliability. However, large voids may reduce fracture resistance strength. Voids can decrease electrical and thermal conductivity of the interconnection path and cause thermal failures. Many possible causes of void formation exist. Voids can result from solder shrinkage during solidification. Outgassing Outgassing (sometimes called "Offgassing," particularly when in reference to indoor air quality) is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material. in the plated through holes during soldering may produce holes in the solder. Alternately, voids can result from poor wetting of the solder joint. Tin whiskers Pure tin surfaces are vulnerable to spontaneous crystal growth. These crystals (tin whiskers) can have diameters of 0.1 to 5 micron and can grow to several millimeters in length. Tin whiskers can start growing after plating or even after a few years. Due to their dimensions and different shapes, they may cause circuit shortages. Whisker growth depends on temperature and humidity. Critical temperatures are about 50 degrees C, with a relative humidity relative humidity n. The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. of 50 percent. To avoid tin whiskers, the thermal stresses introduced in the soldering processes should be as low as possible; another reason for a straight ramp reflow profile. Also, the tin content is important; the higher the pure tin level, the greater the opportunity for tin whiskers to form. PRODUCTION START-UP Before any data can be analyzed, it must be measured. These procedures are important for machine characterization and calibration. Collecting good data is needed to obtain useful analysis. One may differentiate variables (measure data in units) and attributes (data that has been counted). Reflow soldering Variables in the reflow process include machine and datalog parameters. Machine parameters include conveyor speed, temperatures of heating zones, temperatures of the cooling unit and water temperature zones. Datalog parameters include time to reference, time above reference, mean temperature, minimum slope, maximum slope, average slope, peak difference, minimum temperature, maximum temperature, and time to reach maximum temperature. Attributes are soldering defects such as voids, skips, solder balls, bridges and tombstones tombstones a cellular phenomenon in pemphigus vulgaris; rows of basal cells of the epidermis remain attached to the basal membrane, reminiscent of rows of tombstones. . Increased yield and decreased machine downtime are goals that must be achieved after the introduction of lead-free soldering. After starting the lead-free process, try to establish a repeatable measuring process, a method for properly calculating process capability (Cp) values and a method to use this data to calibrate To adjust or bring into balance. Scanners, CRTs and similar peripherals may require periodic adjustment. Unlike digital devices, the electronic components within these analog devices may change from their original specification. See color calibration and tweak. the machine. For most companies, 6-Sigma is a "magic" word. This 6-Sigma is equal to a Cp value of 2; a defect number of 0.002 parts per million parts per million mg/kg or ml/l; see ppm. (ppm). Statistically, a process is capable if the Cp=1. However, this Cp value corresponds to a defect level of 2,700 ppm. Because this number is rather high, other companies consider Cp=1.33 as the target. This number is equal to a defect level of 64 ppm. Reflow machine calibration A reliable calibration procedure requires that an appropriate PCB is measured. For example, FR-4 boards are not preferred for calibrating measurements because the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). (Tg) values will drop after a number of runs, and these boards will warp because the temperatures in lead free are much higher. Also, delamination delamination /de·lam·i·na·tion/ (de-lam?i-na´shun) separation into layers, as of the blastoderm. de·lam·i·na·tion n. 1. A splitting or separation into layers. 2. can occur after multiple reflow cycles. As a result, thermocouples mounted to FR-4 calibration boards may loosen and measure gas temperature instead of the temperature of the board material. Profiling sensor devices that run through the process tunnel deliver more reliable results and better data. Remember, all tools used for calibrating the process must themselves be calibrated cal·i·brate tr.v. cal·i·brat·ed, cal·i·brat·ing, cal·i·brates 1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument): periodically. Thermocouples Do not use high-temperature tape for connecting thermocouples. Thermocouples connected with kapton tape have an accuracy of plus/minus 5 degrees C and are very dependent on the operator's skill level. High-temperature solder or epoxy delivers much more consistent results and has an accuracy of plus/minus 1uC. Wave soldering In wave soldering, an important distinction exists between machine and datalog parameters. Machine parameters include conveyor speed, preheater temperature, and solder temperature. Datalog parameters include temperature slope (maximum and minimum), average slope, maximum temperatures, deltaT at the wave, dwell time The time cargo remains in a terminal's in-transit storage area while awaiting shipment by clearance transportation. See also storage. (for chip and main wave), contact length, parallelism to the wave and immersion depth in the solder wave. For verifying the flux quantity, a procedure must be defined. Attributes include soldering defects such as voids, skips, hole filling, fillet lifting, solder balls, bridges, tombstoning and solder excesses. Wave machine calibration The same statistical approach used in reflow soldering can be used in wave soldering. Calibration must be done with equipment designed for wave soldering. Commercial tools are available that can measure contact times and preheat profiles. Also, flux performance can be checked with water-sensitive paper or a glass plate. Some manufacturers prefer a more precise verification method and measure the amount of dry flux applied on a very accurate balance. BIBLIOGRAPHY 1. Cieslinski, M. 2001. Data analysis for machine characterization & calibration. Proceedings of APEX 2001. San Diego, CA. Northbrook, II: IPC (1) (InterProcess Communication) The exchange of data between one program and another either within the same computer or over a network. It implies a protocol that guarantees a response to a request. . 2. Wassink, R. 1994. Soldering in Electronics. 2nd edition. 3. Gickler, A., et al. Contamination of Lead-Free Solders with Copper and Lead. Johnson Manufacturing and Northwestern University, Evanston. 4. NASA NASA: see National Aeronautics and Space Administration. NASA in full National Aeronautics and Space Administration Independent U.S. . Tin Whiskers Information Page. NASA Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately 6.5 miles northeast of Washington, D.C. . http://nepp. nasa.gov/whisker. 5. Hartono, H., et al. Assembly Process Characteristics for Pb-Free Through-Hole Soldering. Binghampton, NY: UIC UIC University of Illinois at Chicago UIC Underground Injection Control UIC Union of Islamic Courts UIC United Industrial Corporation UIC Union Internationale des Chemins de Fer (International Union of Railways) . Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV in The Netherlands; e-mail: gdiepstraten@nl.vitronics-soltec.com. http://www.circuitsassembly.com Copyright [copyright] 2001 CMP CMP (cytidine monophosphate): see cytosine. (1) (CMP Media LLC, Manhasset, NY, www.cmp.com) Part of United Business Media, CMP is a leading integrated media company that offers a wide variety of publications and services in the information Media LLC (Logical Link Control) See "LANs" under data link protocol. LLC - Logical Link Control |
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