No hassle lead-free HASL: it's not a drop-in replacement for HASL. But moving to lead-free HASL can be less painful than you think.As the July 2006 Restrictions on Hazardous Substances (RoHS) deadline looms, 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. fabricators and even designers continue to search for information on lead-free processes. Many PCB shops are preparing to convert to alternative surface finishes (or at least add the capability). But despite all the ink that's been spilled--and hot air expounded--on the subject of alternative surface finishes, there seem to be more questions than answers. There are numerous alternative surface finishes available, and all have their supporters and detractors. One of these, lead-flee hot-air solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i. leveling (LFH LFH low-fragmentation heap LFH Local Failure Handler ) has shown success in lead-flee manufacturing. This Q&A session should help provide some clarity on LFH--what it is, what it isn't and how it works. Q. What are the operating costs operating costs npl → gastos mpl operacionales of LFH? A. Cost for the LFH surface finish includes several elements: the cost of metals, density of metals and coating thickness. Since the LF alloys are almost pure tin or tin with 3 to 4% silver, their metal cost is significantly higher than eutectic tin-lead (63/37) solder. LF alloys are approximately 15 to 16% lighter in density, which means that 84 kilograms of a LF alloy will equal the volume of 100 kilograms of the higher density tin-lead alloy. There are indications that the average thickness of LF solder is about 50% less than that of tin-lead solder, which means less solder is consumed. This observation has been confirmed by comparing consumption data of LF solder with that of tin-lead solder. The reduced consumption offsets the higher cost of the LF alloy. Q. What are the laminate laminate, n a thin slice of porcelain or plastic fabricated in a dental lab, which is cemented to the front of the teeth to cover gaps, whiten stained teeth, or reshape chipped or broken teeth. requirements for LFH? A. The most commonly used laminates for circuit boards using LFH as a surface finish are standard FR-4 with a Tg from 130 to 165[degrees]C, which works well with either black or alternative oxide. Work has also been done with higher performance, multifunctional laminates. Single-sided boards made of CEM-1 material have been successfully run with no discoloration dis·col·or·a·tion n. 1. a. The act of discoloring. b. The condition of being discolored. 2. A discolored spot, smudge, or area; a stain. Noun 1. or staining of the unprotected backside of the board. 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. of multilayer boards has not been an issue, despite the higher operating temperature of the solder pot and assembly with multiple 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. cycles at higher temperatures. Q. What 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. is most commonly used with LFH? A. SAC alloys, such as 305 (Sn3.0Ag0.5Cu), are the solder pastes most commonly used at this time. Q. Have any incidents of whisker formation been reported from deposits of LFH? A. The authors are not aware of any reports of the formation of tin whiskers See metal whiskers. . Q. What applications will be suitable for LFH? A. LFH can be used for those applications whose requirements are currently being met by tin-lead HASL (language) HASL - SASL plus conditional unification. ["A Prological Definition of HASL, A Purely Functional Language with Unification Based Conditional Binding Expressions", H. Abramson in Logic Programming: Functions, Relations and Equations, D. DeGroot et al eds, P-H 1986]. . Uniformity of solder thickness is better with LFH than tin-lead, but deposits are not co-planar. Q. How does LFH compare to tin-lead HASL (TLH TLH The Lutheran Hymnal TLH Tallahassee, FL, USA (Airport Code) TLH Total Listening Hours (Internet Radio) TLH Top-Level Hierarchy (Microsoft Exchange Server) ) for uniformity of solder thickness or co-planarity? A. LFH has considerably better solder thickness uniformity than TLH. This is more easily observed with the horizontal process than the vertical. Of course, it is also dependent upon the set-up conditions for the machine and the process, but under optimum conditions the results will be better. This can be demonstrated with the following graphs for QFP (Quad FlatPack) A square, surface mount chip package that has leads on all four sides and comes in several varieties. PQFP (Plastic QFP) may refer to all of the following QFP types. All quad flatpacks use gull-wing leads, except for the CQFP, which stick straight out. and BGA (Ball Grid Array) A popular surface mount chip package that uses a grid of solder balls as its connectors. Available in plastic and ceramic varieties, BGA is noted for its compact size, high lead count and low inductance, which allows lower voltages to be used. pads (FIGURES 1 and 2). [FIGURES 1-2 OMITTED] Q. Why should I spend engineering time and money now on LFH when I don't have orders for it yet? A. Not much study is needed to be ready for the process conversion. Remove tin-lead solder from the machine and add back pure tin to remove lead impurities. Repeating a second time will usually result in the lead concentration being <0.1%. No extra equipment investment is required, unless heaters or heating control are not adequate. The time to reach acceptable quality deposits and optimize the machine on a preliminary basis is typically about four hours. Q. What process changes are required for implementation of LFH? A. Since the melting points 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 of lead-free solders--whether tin-copper-nickel or tin-silver-copper--are higher than tin-lead, the solder pot temperature is higher. The operating window comparison is shown in TABLE 1. With tin-lead solder, the copper concentration must be maintained below 0.3%, while for tin-copper-nickel, the process control range is from 0.7-0.9%. With the tin-copper-nickel alloy, nickel must be controlled between 0.025-0.05%. It is necessary to increase the preheat pre·heat tr.v. pre·heat·ed, pre·heat·ing, pre·heats To heat (an oven, for example) beforehand. pre·heat  er n. in horizontal application horizontal application - An application program common to different business processes, e.g. office automation.Compare vertical application. to aid in clearing holes. The additional heat required depends on the same variables as for tin-lead solder: board thickness, the number of inner layers and the amount of copper sandwiched within the layers. With tin-lead solder, it is possible to achieve acceptable results without the use of an etching cleaner. With LFH, a micro-etch is required for cleaning. When compared to tin-lead solder, lower pressures on the air knives can be used with LFH to clear the holes, resulting in a more uniform solder thickness. Q. What is the shelf life of boards using lead-free solder vs. that of the HASL process? A. Data show the shelf life of lead-free solder to be more than 1 year under proper storage conditions, which is the same as for tin-lead solder. Q. How many PCB shops are running LFH in their machines at this time? A. As of June 2005, approximately 63 shops were running LFH globally (FIGURE 3). [FIGURE 3 OMITTED] Q. Can tin-copper be used to replace tin-copper-nickel in order to reduce the cost? A. Deposits of fin-copper suffer from a grainy grain·y adj. grain·i·er, grain·i·est 1. Made of or resembling grain; granular. 2. Resembling the grain of wood. 3. Having a granular appearance due to the clumping of particles in the emulsion. surface and show increased growth of intermetallic compound (IMC (Internet Mail Consortium, Santa Cruz, CA, www.imc.org) An industry trade association founded in 1996 by Paul Hoffman and Dave Crocker that promotes Internet e-mail standards and features. ), compared to tin-copper-nickel. It has been found that the presence of nickel in the lead-free alloy leads to a finer grain structure and has a definite role in controlling the growth of IMC. Q. Can LFH using Sn-Cu-Ni be reworked? A. In some instances, improved sol der uniformity can be achieved by running panels a second time through the machine. There is evidence that a second immersion in a vertical solder pot will remove approximately 0.7 [mu] of copper and a second dip will show a total copper removal of 1.0 [mu], which is well within acceptable limits for tinlead solder. Q. How much difference is there in the thermal profiles for reflowing lead-free solder paste on a board with the LFH surface finish? A. A thermal profile for lead-free solder paste was recommended by Ravi Sharma of Microchip Technology  Microchip Technology (NASDAQ: MCHP) is a manufacturer of microcontroller, memory and analog semiconductors, founded in 1989 when in Application Note 233. The lower
control limits for tin-lead solder and lead-free solder were similar,
but the upper control limits were slightly higher for lead-free solder
(FIGURE 4). The comparison was made with the assumption that matte tin
and tin-lead was the finish on the components. Factors that influence
thermal profiles are circuit board thickness, interconnect density,
surface finish and compatibility of the assembly process.
[FIGURE 4 OMITTED] Q. Is cracking on the surface of lead-free solder joints a new problem, one not experienced with tin-lead solder? A. Surface cracks on lead-free solder joints are the result of the rate of cooling of the solder from the wave solder process and are influenced by the difference of the alloy from its eutectic composition. The cracks are believed to be superficial, with no long-term deleterious deleterious adj. harmful. effects. Q. What is the maximum permissible lead concentration permitted in LFH? A. To meet lead-free requirements, the lead concentration in the electronic component must be <0.1%. A thorough job of removing tin-lead solder from the solder pot will include two cycles of purging with pure tin, prior to adding the lead-free solder, and result in a lead concentration of <0.04%. The bigger risk of lead contamination will come from reworking tin-lead panels through lead-free solder. Q. What is the difference between a flux for TLH and one for LFH? A. Flux for LFH is more thermally stable, with lower activity, and it is less corrosive to the solder pump and solder tank. One study has shown that an ideal LFH flux can promote good solder coverage and still be rated non-corrosive and non-flammable and be shipped and stored as a non-hazardous material. Q. How are the copper and nickel concentrations maintained in balance, when using tin-copper-nickel alloy in the LFH process? A. Samples of solder must be taken from the solder pot daily or weekly, depending on the surface area of copper run through the machine, and then analyzed for copper and nickel. Both elements are already being analyzed in the tin-lead solder analytical scheme, but adjustments must be made to analyze them at higher concentrations. Bath dilutions are made with tin, to reduce the copper concentration, and then nickel is added to increase its concentration. Q. There are reports that nickel in the IMC can cause weak, brittle IMC and cracks in the solder joints. Does the same thing happen with LFH, when tin-copper-nickel is being used? A. There are reports in literature that document the effect of high concentrations of nickel on the reliability of solder joints. In all cases where this behavior was reported, the source of nickel was an electroless nickel barrier layer to protect copper from migrating and penetrating the thin gold top layer. This is not the case in LFH where the source of nickel is the solder itself. There have been no reports of weak, brittle solder joints from 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. with tin-copper-nickel. Q. Does a special soldermask have to be used for LFH, or will standard products work? A. Standard soldermasks will work for most applications. In areas where soldermask is used for hole plugging, testing should be done to be sure the adhesion of soldermask in plugged holes will remain and not peel off at the higher LFH temperature. Q. Is it possible to skim lead-free solder by reducing the temperature as with tin-lead solder? A. Work has been done to optimize the skimming Skimming An electronic method of capturing a victim's personal information used by identity thieves. The skimmer is a small device that scans a credit card and stores the information contained in the magnetic strip. process and tin-copper needles can be removed from lead-free solder by holding the temperature in the 235-240[degrees]C range, just slightly above the melting point. When the lead-free solder is Sn-Cu-Ni, nickel is also removed with the skimmed skim v. skimmed, skim·ming, skims v.tr. 1. a. To remove floating matter from (a liquid). b. To remove (floating matter) from a liquid. c. solder, so its concentration must be replenished. The skimming process can reduce the cost of lead-free solder by 50 to 80%. See FIGURE 5. [FIGURE 5 OMITTED] JACK FELLMAN is a senior engineer with Rohm and Haas Rohm and Haas Company (NYSE: ROH), a Philadelphia, Pennsylvania based company, manufactures miscellaneous materials. A Fortune 500 Company, Rohm and Haas employs more than 17,000 people in 27 countries. The annual sales revenue of Rohm and Haas stands at about USD 8.2 billion. Electronic Materials; jfellman@rohmhaas.com. SIMON Simon, in the Bible. 1 One of the Maccabees. 2 or Simon Peter: see Peter, Saint. 3 See Simon, Saint. 4 Kinsman of Jesus. 5 Leper of Bethany in whose house a woman anointed Jesus' feet. LEE is imaging business manager with Rohm and Haas Electronic Materials; tslee@rohmhaas.com.
TABLE 1. Comparison of process operating windows
for HASL with different alloys.
ALLOY MELTING OPERATING PROCESS
POINT TEMPERATURE WINDOW
([degrees]C) ([degrees]C) ([degrees]C)
Tin-Lead 183 250-260 67-77
Tin-Silver-Copper 217 265-270 48-53
Tin-Copper-Nickel 227 265-270 38-43
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