Temperature Selection for Wave Soldering with Lead-Free Alloys -- Wetting balance testing established the best wave soldering temperatures for several lead-free alloys with a typical low-solids, no-clean flux.Wetting balance has long been a useful laboratory test for evaluating solder wetting properties as a pre-screen for what can be expected in a printed circuit assembly process on the production floor. Three materials are required to conduct a wetting balance measurement: a substrate, flux and solder. Accordingly, three principal applications exist for wetting balance testing. The substrate can be an area of metallization Met`al`li`za´tion n. 1. The act or process of metallizing. on a printed circuit board surface, an electronic component lead or its termination. Wetting balance testing is used to assess the solderability of the metallic surface. The test procedures are detailed in 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. J-Standards -002 and -003. The wetting balance test can also be used as a screening tool to evaluate the wetting efficiencies of alternative soldering flux Noun 1. soldering flux - flux applied to surfaces that are to be joined by soldering; flux cleans the surfaces and results in a better bond flux - a substance added to molten metals to bond with impurities that can then be readily removed compositions. Recently, wetting balance was used to evaluate the wetting properties of several alternative solder alloys and, in particular, lead-free solders. This testing was part of a comprehensive study of lead-free 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. , whose purpose was to evaluate materials compatibility, solderability and solder joint quality. The study included a broad mix of alloys, solder fluxes, solder pastes, board finishes, surface-mount and through-hole components and a specially designed test board. The wetting balance instrument was chosen to determine the appropriate solder pot temperatures for the various lead-free alloys to be used in the wave soldering phase of the study. Wetting Balance Test Methodology Five lead-free solder alloys were evaluated, including binary alloys of tin with silver and copper, a ternary (programming) ternary - A description of an operator taking three arguments. The only common example is C's ?: operator which is used in the form "CONDITION ? EXP1 : EXP2" and returns EXP1 if CONDITION is true else EXP2. alloy of tin/silver/copper and quaternary quaternary /qua·ter·nary/ (kwah´ter-nar?e) 1. fourth in order. 2. containing four elements or groups. qua·ter·nar·y adj. 1. Consisting of four; in fours. alloys of tin/silver/copper with 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. and antimony antimony (ăn`tĭmō'nē) [Lat. antimoneum], semimetallic chemical element; symbol Sb [Lat. stibium,=a mark]; at. no. 51; at. wt. 121.75; m.p. 630.74°C;; b.p. 1,750°C;; sp. gr. (metallic form) 6. . Eutectic tin/lead solder was included in the study for comparison. The specific alloys evaluated and their melting ranges are shown in Table 1. Test substrate The test substrates were copper coupons with dimensions of 1.0 in. 3 0.5 in. and 0.005-in. thickness; the coupons complied with ISO (1) See ISO speed. (2) (International Organization for Standardization, Geneva, Switzerland, www.iso.ch) An organization that sets international standards, founded in 1946. The U.S. member body is ANSI. 1634-CU-ETP condition HA, per IPC-TM-650. The coupons were precleaned as follows: - degrease de·grease tr.v. de·greased, de·greas·ing, de·greas·es To remove grease from: degrease machinery. de·greas in boiling isopropyl alcohol isopropyl alcohol: see isopropanol. - deoxidize de·ox·i·dize tr.v. de·ox·i·dized, de·ox·i·diz·ing, de·ox·i·diz·es To remove oxygen from (a compound); reduce. de·ox with copper surface conditioner - rinse in deionized water Deionized water (DI water or de-ionized water; also spelled deionised water, see spelling differences) is water that lacks ions, such as cations from sodium, calcium, iron, copper and anions such as chloride and bromide. - final rinse in isopropyl alcohol. The wetting balance testing was conducted with freshly cleaned coupons and clean coupons that were oxidized oxidized having been modified by the process of oxidation. oxidized cellulose see absorbable cellulose. for one hour at 100 degrees C. Test Parameters Solder wetting measurements were conducted using a wetting balance tester. The instrument's solder pot was filled consecutively with each alloy to be tested. Coupons were dipped in a low-solids no-clean test flux to a depth of 0.1 in. The coupons were next preheated by suspension 0.1 in. above the solder pot for five seconds. They were then immersed in the solder pot to a depth of 0.1 in. for five seconds. Both the immersion and withdrawal rates of the coupons from the flux and solder were 1 in./sec. Both oxidized and non-oxidized copper coupons were used with the test flux. Testing was conducted using a range of solder pot temperatures. Fifteen test coupons were used for each combination of solder alloy, solder pot temperature and coupon surface condition. Test Results Two measurements were recorded for each wetting balance test: wetting time and wetting force. The wetting time recorded is the time required to cross the zero wetting force axis in seconds. A low wetting time is desired. The wetting force recorded is the final wetting force in microN/mm. A high wetting force is desired. The wetting balance results obtained from this study are presented in Figures 1 to 4. Each data point represented the average of 15 readings. Standard deviations in test data were low for each test condition. Figures 1 and 2 represent the wetting time and wetting force results for each test alloy and the test flux, using clean (non-oxidized) copper. Figures 3 and 4 represent the wetting time and wetting force results for each test alloy and the test flux, using oxidized copper. The results indicated that 271 degrees C appeared to be an appropriate solder pot temperature for the lead-free alloys when a low-solids, no-clean rosin rosin or colophony, hard, brittle, translucent resin, obtained as a solid residue from crude turpentine. Usually pale yellow or amber, its color may vary from brownish-black to transparent depending on the nature of the source of the crude type flux was used. With clean copper and low solids flux, the wetting time generally decreased with the lead-free alloys until 271 degrees C was reached, and then the wetting time leveled off or increased slightly with higher temperature. Similarly, wetting force generally increased to 271 degrees C and then leveled off or decreased slightly with higher temperature. The binary and If two conditions are combined by and, they must both be true for the compound condition to be true as well. Likewise, two bits may be combined with and: x y x AND y 0 0 0 0 1 0 1 0 0 1 1 1 I.e. ternary tin, silver and copper alloys Copper alloys are alloys with Copper as their principial component. They have high resistance to corrosion. Due to its high electric conductivity, pure electrolytic copper is used mostly for making of electrical cables. produced faster wetting times and higher wetting forces than the quaternary alloys and the control tin/lead alloy. The tin/copper alloy required a minimum temperature of 260 degrees C to attain positive wetting force. The quaternary alloys required a minimum temperature of 249 degrees C to attain positive wetting. The tin/silver and tin/silver/copper alloys attained positive wetting at the lowest test temperature of 232 degrees C, as did the tin/lead control solder. With oxidized copper and low solids flux, the results with the different alloys were more closely bunched together. With oxidized copper substrates, a solder pot temperature of 271 degrees C generally gave better results than at lower temperatures. However, some alloys gave marginally better results at the maximum test temperature of 277 degrees C. The binary and ternary tin, silver and copper alloys yielded slightly faster wetting times and slightly higher wetting forces with clean copper than with oxidized copper. This result was expected; oxidized copper was more difficult to wet in a metallurgical sense with solder. However, the trend was just the opposite with the quaternary alloys and the tin/lead control. They actually yielded slightly better wetting balance results with oxidized copper than with clean copper. Conclusions Wetting balance testing indicated that 271 degrees C was an appropriate solder pot temperature for the lead-free alloys evaluated when a low-solids, no-clean rosin type flux was used. The tin/silver, tin/copper and tin/silver/copper alloys generally produced the fastest wetting times and maximum wetting forces. The suitability of the individual lead-free alloys and solder pot temperatures must be verified in an actual wave soldering process evaluation using printed circuit boards with both through-hole and surface-mount electronic components. Bibliography Bastecki, C. (1997). A Benchmark Process For The Lead-Free Assembly Of Mixed Technology PCB's, Alpha Metals publication. IPC. (Nov. 2000). IPC/ANSI J-STD-006A, Proposal, Appendix A Solder Alloys, Table A-1 Composition, and Temperature Characteristics of Lead-free Solder Alloys. IPC. (Oct. 1998). IPC/EIA J-STD-002A, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires. IPC. (Apr. 1992). IPC/EIA J-STD-003, Solderability Tests for Printed Boards. IPC. IPC-TM-650, Test Methods Manual. Number 2.4.14.2. Smelik, G. Internal communications. Cookson Performance Solutions. Zarrow, P. (1999). Lead-free: Don't fight a fact, deal with it! Circuits Assembly, pp. 18-20. --- Al Schneider is director of pastes and chemicals R&D, Sanju Arora is manager of chemical R&D, and Bin Mo is a development chemist, all with the Central Research Center of Alpha-Fry Technologies, Jersey City, NJ; e-mail: aschneider@alpha-metals.cookson.com. http://www.circuitsassembly.com Copyright [copyright] 2001 Miller Freeman LLC (Logical Link Control) See "LANs" under data link protocol. LLC - Logical Link Control |
|
||||||||||||||||||||
Printer friendly
Cite/link
Email
Feedback
Reader Opinion