Joint Tolerances in Capillary Piping Joints

Excessive and non-uniform joint clearance is one of the major causes of poor flow

BY Ft. B. EDWARDS

ABSTRACT. Solderless regions re­ short length at the end of a thin wall shown these solder joints to have garded as defects can be formed in copper tube. The is accom­ broad success in meeting the require­ solder joints in copper piping during plished by capillary filling of the ments for piping systems. fabrication. This study examines fluxed, assembled, and heated joint In 1960, NIBCO began further ex­ typical joints removed from field in­ opening with a molten solder sup­ tensive study of the capillary flow stallations. Principles of capillary plied to the joint edge by hand feed­ solder joint. This continuing research flow are presented to show how sol­ ing. Fittings and tube are made to has contributed to further under­ der flow is related to joint clearance. special tolerances to provide the standing of the joint itself and to the Recommendations are given to pro­ capillary joint clearance suitable for soldering processes associated with mote better filling of joints than is this method of soldering. The tech­ it. shown by the samples examined. nology for these joints in piping was standardized on the basis of inves­ Actual Joints Introduction tigations carried out at the National What is the quality of piping joints Solder joints widely used in copper Bureau of Standards and published piping applications consist of a close by A. Maupin and W. Swanger in 1 MR. EDWARDS is director of research for fitting short sleeve soldered over a 1940. Subsequent experience has NIBCO Inc., Elkhart. Indiana.

Fig. 1—Radiograph of ¥4-in. wrought copper elbow, with tracing to show location of regions void of solder in the joint. Defect types are identified by roman numerals I, II, III, and IV

WELDING RESEARCH SUPPLEMENT! 321-s as they exist in actual installations? spection technique for these defects, joint soldering tests made by many Long experience has shown that but is, of course, destructive. different investigators, for example field failures are not a common oc­ This miscellaneous collection of Keyes.2 Studies of soldering process currence. But does this mean that all fittings is intended to: (a) represent conditions have revealed many cor­ joints are perfect? Speciment joints some actual practice, (b) to relations with joint defects. It is im­ (812 joints in 452 fittings) were ob­ indicate at least some of the range of portant to note that all of these de­ tained from scrap yards in Cleveland, joints actually accepted for use and fects are formed by peculiarities of Ohio, Islip, L.I., N.Y., a few from (c) to see what kind of defects might solder flow, or the lack of it, oc­ Osterville, Mass., and a few from be present and common. No attempt curring during the soldering opera­ various other areas of the United was made to select the fittings on tion. Various types of causes for de­ States. Most of the fittings were Vz or the basis of quality, brand, appear­ fects may be described, such as bad % inch size and were reportedly ance, age, original source, size, or cleaning, bad , and bad solder, from housing torn down for new specific use. No attempt was made but they all act by influencing the sol­ building or roadway construction. All to trace or date the fittings. They cer­ der flow behavior within the joint to joints were soldered with lead-tin tainly do not represent either all generate a region void of solder. The solder. good or all bad joint making practice. effects of joint dimensional toler­ As far as could be determined, all The sampling of actual solder joining ances are discussed here as causes the fittings had been giving satisfac­ practice represented by the collec­ of defects. tory service prior to their removal. tion is recognized to be limited and The presence of regions void of sol­ suitable only as an indication of prac­ Joint Tolerances der is illustrated in Fig. 1 by a radio­ tice with smaller piping sizes. Joint clearance between the tube graph of a wrought copper ell. A and the sleeve is specified by current tracing showing the void regions is Experimental Joints standards B16.18-1963 for cast given to clarify the x-ray interpre­ The types of defects found in the fittings and B16.22-1963 for tation. All 452 fittings were x-rayed collection have also been found to wrought fittings. Diametral clear­ and the relative filling of joint areas occur frequently in experimental ance is supposed to be no less than was evaluated by viewing the x-ray film. Essentially four kinds of regions unfilled with solder were found. TABLE 1—Types of Defects Present in Sample of Used Joints and Their Distribution by Number and Percent These defects, according to their radiographic appearance, are iden­ Type of defect Number of joints Percentage of sample tified here as: (I) small closed circular I ALL (812) 100% spot, (II) irregular shaped isolated II 328 40.3 closed area, (III) irregular area adjoin­ III 194 23.9 ing a joint edge or another defect re­ IV 232 28.5 gion, (IV) large or extensive area only 183 22.4 usually adjoining an edge of the II only 227 34.1 joint. The types are identified in Fig. III only 87 10.7 1 by their respective numbers. Fre­ IV only 129 15.8 quencies for these types of defects I, II III 33 4.1 I, II IV 29 3.5 are listed in Table 1. I. II III, IV 39 4.8 It is apparent that imperfect joints I, III, IV 35 4.3 are common. In joints that were cut and peeled apart, illustrated by Fig. 2, the closed solderless areas fre­ quently contained solid or semi-solid flux residues and the extensive void areas were frequently untinned. Clearly, all of these defects were formed during soldering of the joint, i.e., none are due to subsequent action on the joint such as corrosion or erosion. There were 550 cast bronze fitting joints and 262 wrought copper fitting joints in the collection. No significant differences with respect to solder fill were attributed to the difference between bronze and copper. X-radiography was used as the pri­ mary means for assessing the solder joint fill. It is a good nondestructive inspection method for this purpose. Ultasonic techniques are capable of detecting unfilled solder areas, but have been limited by manual operation of the test probe to slow examination rates. Cutting and peel­ ing of the joints, with visual or micro­ scopic examination of the opened surfaces, is the most reliable in­ Fig. 2—Cut and peeled joints showing some typical defects

322-s I JUNE 1972 0.002 in. and not more than 0.006 TABLE 2—Diametral Clearances from Standards B16.18-1963 and B16.22-1963 in. for VA in. tubes and varies up to and the Half Maximum Clearance between faying surfaces (Vi Maximum) 0.011 in. for the 8 in. size piping. Standard watertube Diametral clearances, in The minimum and maximum diamet­ size, in. Minimum Maximum Vi Maximum ral clearances for each piping size Vs 0.002 0.006 0.003 are given in Table 2. If the joint parts 1 were geometrically perfect cylindri­ /4 " 0.006 0.003 cal shapes and were assembled cen­ 3/8 " 0.006 0.003 Vi " 0.006 0.003 tered and aligned, joint clearance Ve " 0.006 0.003 would be a concentric uniform gap % " 0.006 0.003 between tube and sleeve. Clearance 1 " 0.007 0.0035 between faying surfaces would thus 114 " 0.007 0.0035 be one-half the diametral clearance Vh " 0.0085 0.00425 value. The values for half the maxi­ n 2 0.0085 0.00425 mum diametral clearances are also 0.0085 0.00425 TA " listed in Table 2. Real joint openings 3 0.0085 0.00425 " usually vary from zero to the maxi­ 31/2 " 0.009 0.0045 4 " 0.009 0.0045 mum diametral clearance because of 5 " 0.009 0.0045 imperfect shape and assembly. 6 " 0.009 0.0045 8 " 0.011 0.0055 Solder Flow Properly cleaned, fluxed, assembled, and heated joints are normally sol­ dered by melting Vs-in. wire solder against the entrance edge of the sleeve member. The solder melts to TABLE 3 Maximum Static Heights for Molten Solder in Capillary Gaps* form a puddle which bridges the cap­ illary gap between the tube and the Parallel plate gap thickness, Maximum molten solder height sleeve. The behavior of the initial d, in. h, in. liquid solder bridge in the capillary 0.001 13.5 space can be visualized as being sim­ .002 6.75 ilar to the behavior of India ink in a .003 4.5 .004 3.37 drafting pen. The liquid drawing pow­ .005 2.7 er of a drafting pen is a function of .006 2.25 the adjusted space between the pen .007 1.93 nibs. If the nibs are adjusted too far .008 1.68 apart, both of them will become wet .009 1.5 with ink, but a liquid bridge will not .010 1.35 form. The beginning of capillary flow .011 1.23 can be observed at the time this .012 1.13 bridge is formed if the pen nibs are .013 1.04 .014 0.97 adjusted to a reasonable spacing. From this liquid bridge an exten­ "Computed using a value of 378 dynes/cm for the interfacial tension of 50-50 sive solder meniscus can develop as solder in contact with flux, 8.89 g/cc for the density of solder, and zero contact more solder is fed and melted. It is angle. this meniscus as it wets the capillary gap walls that pulls liquid solder into the joint. This meniscus is the source of the capillary force that fills the joint. The liquid solder meniscus is a TABLE 4—Capillary Liquid Solder Heights for Maximum Clearance Gaps free liquid surface and exhibits Standard water tube Diametral clearance, Liquid capillary surface tension. Because the liquid size, in. max., in. solder height, in. solder has cohesive and adhesive Vs 0.006 2.25 forces, it wets the capillary walls, 0.006 2.25 V4 spreads and pulls solder liquid with it Va 0.006 2.25 (this wetting is assumed to be ideal V2 0.006 2.25 here — as if the joint were pre- Va 0.006 2.25 tinned). The limiting force tending to 3 0.006 2.25 /4 draw solder into the joint is the 1 0.007 1.93 0.007 1.93 meniscus force or pressure differ­ VA ence. A free liquid surface becomes Vh 0.0085 1.59 2 0.0085 1.59 curved when a pressure difference Vh 0.0085 1.59 exists across it. The general descrip­ 3 0.0085 1.59 tion of the pressure difference with 31/2 0.009 1.5 respect to curvature for surface ten­ 4 0.009 1.5 sion phenomena is ascribed to 5 0.009 1.5 Laplace: 6 0.009 1.5 8 0.011 1.23 Ap = o \ R, R2 )

WELDING RESEARCH SUPPLEMENT! 323-s Where: AP = pressure difference 12- a = surface tension R, = radius in one principal direction 10-

R2 = radius in the other principal direction The radii of curvature in two prin­ cipal directions describe the net cur­ co vature of a meniscus over the area of LiJ X interest. The solder joint clearance o approximates the gap between par­ 2 allel flat plates, in that curvature of a meniscus within the gap will have a low value in one principal direction. 4- This is approximatelyAp~,(r(2/d), where d is the distance across the gap. When the walls are wetted to some degree, but the gap is not full of solder, the internal pressure in the I i I I i I I i I I I i T" i i solder liquid is reduced and a net .002 .004 006 .008 .010 .012 014 .016 force Ap x A, where A is the gap area, is exerted by the meniscus d INCHES climbing the walls trying to pull li­ Fig. 3—Maximum head height (h) versus capillary gap dimension (d) for 50-50 solder quid after it through the action of ten­ with flux present sile forces. The maximum force in one direction is achieved when the contact angle is zero (complete wet­ interesting comparison, particularly the minimum standard joint clear­ ting) and the radius of curvature is for the larger joint sizes. It is clear ance by tight control of size toler­ minimized. The meniscus supports, tnat capillary filling alone cannot be ances offers two major aids toward or fails to support, the pressure dif­ depended on for these standard joints freedom from joint defects: (a) the ference. in the larger sizes. gap has less variation in thickness The maximum static height to As noted earlier, the joint space is and therefore the flow can advance which a meniscus will hold a liquid almost always of nonuniform thick­ more evenly (the smaller clearance in a parallel plate capillary of a given ness due to geometrical imperfec­ limits both the maximum gap size size is given by: tions of the tube and sleeve and due and the misalignment angle) and (b) to misaligned assembly. The pulling the smaller gap gives higher capil­ h = 2g cos 8 force exerted by the meniscus is a lary drive forces to fill the joint. function of gap thickness and will Conclusions d p g therefore be nonuniform as well. Sol­ der viscosity effect on flow is rel­ Common defects of solder joints Where: atively small until the gap size be­ are generated by nonuniform solder h = capillary head height comes appreciably less than 0.001 flow when the joints are made. An a = surface tension in. This means that flow of liquid sol­ important cause for nonuniform flow can be nonuniform joint clearance. 0 = contact angle der will be nonuniform. In fact, non- Good geometry and good sizing of d = capillary gap dimension uniform flow is a general character­ istic for capillary piping joints as evi­ tube and sleeve help this problem. p = density of solder denced by x-ray motion picture ob­ Other causes of nonuniform solder g = gravity acceleration servations. This is supported by evi­ flow, such as insufficient joint clean­ Some values for the maximum dence from many other types of ex­ ing, dirty or inactive flux, poor solder static parallel plate capillary height perimental soldering situations. Of feeding and uneven heating are very for liquid 50-50 tin-lead solder are course, nonuniform flow may also be important but will certainly not com­ given in Table 3 and illustrated in Fig­ due to a number of things other than pensate for uneven joint space. ure 3. These are computed using a uneven capillary gap, but uneven gap Portions of this and related work value of 378 dynes/cm. for the is a major direct cause. have been reported earlier in a surface tension of the solder in con­ paper4 which contains further refer­ When the liquid solder meniscus tact with flux in the capillary gap.3 ences on solder flow behavior. advances unevenly through the joint The values represent the heights it may engulf flux, entrap bubbles, or References molten solder can climb when it is it may advance rapidly to the narrow 1. Maupin. A. R. and Swanger, W. H., "Strength of not limited by wetting or spreading Soft-Soldered Joints in Copper Tubing," National Bu­ gap region of the tube-stop and seal or other related problems. These reau of Standards Report BMSS8. 1940. off the joint entirely from any further 2. Keyes, J. M., "Factors Affecting Quality of Soft Sol­ values also represent pressure head inflow of solder at all. These mechan­ dered Joints in Copper Water Tube," ASTM Special forces available to cause solder flow Technical Publication No. 319. 1963, Papers on Solder­ isms of engulfment, entrapment, and ing, 1962 pp. 39-82. into the joint space. seal off are the mechanisms by 3. Bailey, G. L. J. and Watkins, H. C, "The Flow of Li­ quid Metals on Solid Metal Surfaces and Its Relation to Table 4 now combines information which defects — solderless areas or Soldering, , and Hot-Dip Coating," Journal of from Tables 2 and 3 to show liquid voids — are commonly formed in the Institute tor Metals, Vol. 80, 1951, p. 57. 4. Jayne, T. D. and Martin, L., "Improving Control of solder heights possible for standard smaller sized joints. Soft Soldering in Copper Piping Joints," ASME Publica­ maximum joint clearances. This is an Thus it may be seen that keeping tion No. 70-PVP-21. Sept., 1970. ,A.

324-s I JUNE 1972