Solderability Testing of Kovar with 60Sn-40Pb Solder and Organic Fluxes Flux type, cleaning procedure and solder bath temperature were evaluated for their effects on solderability BY P. T. VIANCO, F. M. HOSKING AND J. A. REJENT ABSTRACT. The solderability of 60Sn- the component is most often secured to pability of the solder. 40Pb solder on Kovar was examined as a the printed circuit board by soldering the The calculation of 6Q is based upon the function of surface cleaning procedure, Kovar leads to the conductive landing. measurement of two properties of the flux and solder bath temperature. Organic Therefore, the wetting behavior of solder solder meniscus formed on the vertical acid fluxes were more effective than a alloys on a Kovar substrate is of particular coupon (Fig. 1) (Refs. 2, 3): the height to mildly activated, rosin-based (RMA) flux interest in the microelectronics industry. which it has risen, H; and the weight of the on chemically etched Kovar with contact The goal of this investigation was to solder contained in the meniscus, W. An angles as low as 29 ± 5 deg as compared quantify and evaluate the wetting behav­ analytic expression for W is given by the to 61 ± 11 deg, respectively. Varying the ior of 60Sn-40Pb (wt-%) solder on Kovar following equation: solder temperature through the range of as a function of flux, solder temperature 215° to 288°C (419° to 550° F) caused an and the procedure used to clean the Ko­ PH I 4TLF W = pg- (2) insignificant change to the contact angle var surface prior to tinning. These results w for the RMA flux and a decrease of the were then compared with wetting exper­ where p is the solder density; g is the ac­ contact angle for a candidate water-based, iments performed on nickel-plated and celeration due to gravity; and P is the pe­ organic acid flux. The dilution strength of gold-nickel-plated Kovar. In addition, rimeter of the coupon. The values of p, g the flux and the elapsed cleaning time sig­ T-peel strength measurements were made and P are known. Values of H and W are nificantly influenced the solder-flux inter­ of Kovar oxygen-free, high-conductivity measured experimentally by the menis- facial tension, Y . T-peel strengths of Ko­ LF (OFHC) copper joints formed by the sol­ cometer and wetting balance, respec­ var 60Sn-40Pb oxygen-free, high-conduc­ der alloy to determine whether this data tively. Therefore, Equation 2 is solved for tivity (OFHC) copper joints had a low would correlate with the wettability of the the value of 7LF, which is then introduced correlation with the contact angle derived Kovar. into the following expression that calcu­ from the solderability experiments. The The wetting behavior or solderability is lates the contact angle: results of the solderability tests, T-peel quantified by the contact angle, dc, formed mechanical tests and subsequent mi­ between the liquid solder and the sub­ ( PgH2\ croanalysis of the as-soldered and T-peel strate as defined in Fig. 1 (Ref. 1). The 0c = arcsin\j-— J (3) samples revealed that with the use of an contact angle is dependent upon the RMA flux, the best results were achieved equilibrium balance of the interfacial ten­ Two other parameters originating from with electropolishing and a 240° to 260°C sions as expressed by Young's equation: the experimental analysis are the rate at (464° to 500°F) solder temperture. A rel­ which the solder wets the substrate, W, atively low contact angle of 31 ± 2 deg 7SF - 7SL = 7LFCOS0C (1) and the time required for the meniscus to was observed with no evidence of crack­ reach its maximum weight, tw- ing or thick film intermetallic formation where 7SF is the solid (substrate)-flux in­ terfacial tension; 7SL is the solid-liquid (sol­ at the Kovar-solder interface. Further­ Experimental more, T-peel strengths were nominally der) interfacial tension, and 7LF is the 9.4 ± 0.5 X 106 dyne/cm. liquid-flux interfacial tension. The smaller Materials the value of 8Q, the better the wetting ca- The Kovar (29Ni-17Co-0.2Mn-bal Fe, Introduction nominal wt-%) coupons measured 0.028 X 2.54 X 2.54cm(0.01 X 1.0 X 1.0 The unique thermal expansion proper­ KEY WORDS in.) and were exposed to the following ties of Kovar have allowed this material to heat treatment prior to testing in order to have many engineering applications, from Solderability Testing simulate an industrial glassing process:1 1) glass-metal seals to conductor leads in mi­ Kovar Solderability chemical etch, 2) high-temperature oxida­ croelectronic packages. In the latter case, 60Sn-40Pb Solder tion, and 3) second chemical etch. A sec­ 60Sn-40Pb for Kovar ond set of samples was electroplated with Organic Acid Fluxes 2.4 to 6.0 /xm of nickel. A third group con- P. T. VIANCO, F. M. HOSKING AND j. A. Surface Cleaning RE/ENT are with Sandia National Laboratories, Rosin (RMA) Flux Interfacial Tension Albuquerque. N. Mex. 1 T-Peel Strength Aside from ensuring uniform surface condi­ Paper presented at the 19th International A WS Electropolishing tions on all of the samples, this process mod­ Brazing and Soldering Conference, held April eled the engineering application for which this 19-21, 1988, In New Orleans, La. study was conducted. 230-s | JUNE 1990 SOLID (sample coupon) COUPON FLUX LIQUID (solder alloy) SOLDER CO YSF-YSL=\F ^C z—" —x (a) (b) Fig. 1—A — Testing geometry for solderability evaluation; B —equilibrium balance for interfacial tensions, described by Young's Equation. sisted of nickel-plated specimens that were The nickel- and gold-nickel-plated sam­ tom. To determine the correct value of subsequently electroplated with 0.43 to ples were cleaned by an agitated rinse of W, the buoyancy force acting on the 0.57 p,m of gold. trichloroethylene followed by a similar coupon must be calculated. Referring to The solder used in the wettability ex­ rinse in isopropyl alcohol to remove or­ Fig. 3, which shows the geometry of the periments was a 60Sn-40Pb alloy. ganic contaminants from the specimen risen meniscus, the total immersion depth The five candidate fluxes examined surfaces. is [(H - Vo) + (ID)] where (ID) is the depth here are:1) Flux A-RMA, 2) Flux B-OA- to which the coupon extends below the CHOH, 3) Flux C-OA-CHOH, 4) Flux Solderability Testing solder surface and (H-V0) is the additional D-OA-H20 and 5) Flux E-OA-H20. submersion due to the risen meniscus. Flux A is a rosin-based, mildly activated The meniscus height, H, was evaluated Total buoyancy force is then given by: (RMA) flux while the other four are halide- on the meniscometer, which is a traveling BF = pgwt[(H - V ) + (ID)] free, organic acids (OA). The solvent is al­ stage microscope capable of measuring 0 (4) cohol (CHOH) for fluxes B and C and the vertical movement of the meniscus. Six sample coupons were evaluated per water (H20) for fluxes D and E. Unless Each sample was allowed to preheat for test series. The maximum meniscus weight otherwise stated, all fluxes were used in a 30 s at a distance of 1 cm (0.4 in.) above was taken as the average value between dilution of one-to-one (1:1) by volume the solder before immersion into the pot 10 and 20 s of testing. With the average with isopropyl alcohol. at a rate of 1.3 cm/s to a depth of 0.375 meniscus height determined from the me­ cm (0.15 in.). The meniscus height was re­ niscometer data and the average menis­ Cleaning Procedures corded after 20 s. Four samples were cus weight compiled from the wetting evaluated per test series. balance tests, the solder-flux interfacial A brief description of the cleaning pro­ The meniscus weight was measured by tension and contact angle were calculated cedures is presented below. The details means of a microbalance. A schematic di­ for each test. The absolute error on yLf are found in the appendix. A set of plain agram illustrating the meniscus weight as a and dc was the maximum and minimum Kovar samples was etched for 2 min in 300 function of time is shown in Fig. 2. The values based upon the range of values of mL of a commercially available chemical physical behavior of the meniscus respon­ the meniscus height and weight estab­ etchant composed of a mixture of nitric, sible for certain features of the wetting lished by plus-or-minus one standard de­ hydrochloric and phosphoric acids that curve has been included at the figure bot­ viation about the sample mean. was held at 80° to 85°C (176° to 185°F). The remaining cleaning steps detailed in the appendix were included to rinse away the corrosive agents of the etchant. The I Fig. 2 - Typical samples were coated with flux immedi­ , wetting curve ately after cleaning to protect the freshly describing meniscus etched surface. /C weight as a function of time. An electropolishing process was per­ formed on some of the plain Kovar cou­ W MAX MUM pons using a 250 mL solution of 9 parts MENISCUS MENISCUS WEIGHT.W glacial acetic acid to 1 part perchloric acid WEIGHT (60%) (Ref. 4) held at 30°C (86°F). The (dynes) coupon was the cathode, while the anode BUOYANCY TIME was made of AISI Type 347 stainless steel \ J FORCE, (ID) (sec) screen. The polishing time was 5 min and the current density was 0.124 A/cm2 (un­ der current control).