of high- temperature plasma, a specimen density close to theoret ical is reached at 1600~ i.e., at a t e m p e r a - ture not less than 400~ lower than the sintering tempera tures of powders of part icle size more than 1 pro. Sintering lowers the amounts of carbon and oxygen in Mo and W by more than half compared with the s t a r t - ing condition.

L I T E R A T U R E CITED

1~

2. 3. 4.

5.

6.

7.

8.

A. N. Zelikman, Molybdenum [in Russian], Metal lurgiya, Moscow (1970). A. N. 7elikman and L. S. Nikitina, Tungsten [in Russian], Metal lurgiya, Moscow (1978). V. G. Syrkin, Carbonyl Metals [in Russian], Metal lurgiya, Moscow (1978). V. N. Troitskii and T. V. Rezchikova, "Prepara t ion of ultrafine molybdenum powder in a h igh- t empera - ture plasma s t ream," in: Fine Crystall ine Powders in Mater ia ls Science [in Russian], Kiev (1980), pp. 9-13. A. N. Zelikman, Metal lurgy of Rare Metals [in Russian], Metal lurgiya, Moscow (1980). V. I. Torbov, V. N. Troitski i , and A. Z. Rakhmatullina, "Hot press ing and vacuum sintering of fine t i - tanium nitride powder," Poroshk. Metall. , No. 12, 27-32 (1979). R. A. Andrievskii , "A simple method for determining the pycnometric densities of powdered m a t e r i a l s " in: Problems in Powder Metal lurgy and Strength of Mater ia ls [in Russian], Kiev (1958), pp. 29-31. A. A. Ivan'ko, Hardness (Handbook) [in Russian], Naukova Dumka, Kiev (1968).

C A P I L L A R Y I N F I L T R A T I O N OF H I G H - P O R O S I T Y N I C K E L

S T R I P S BY L E A D - TIN A L L O Y S

N. M. K h o k h l a c h e v a , V. N. P a d e r n o , M. A. T o l s t a y a , V. P . S m i r n o v , a n d M. E . S h i l o v s k a y a

UDC 621.762:620.187.2

Some aspects of the vertical capillary infiltration of high-porosity nickel strips by lead have already been investigated by the authors [1, 2]. In this article the results are presented of an investigation into the capillary infiltration of rolled porous strips from nickel (carbonyl and electrolytic) powders by lead-tin al- loys containing from 1 to 61g (POS-61) of tin. The contact phenomena occurring at the interface between solid nickel and lead-tin alloys have been the subject of a number of investigations [3-6]. In [7] a study was made of the infiltration of high-porosity nickel strips with lead-tin solder alloys by a method based on the spread- ing of a drop on a specimen surface; however, nothing appears to have been published as yet on the vertical capillary infiltration of these materials with lead-tin alloys.

The objects investigated in the present work were specimens of high-porosity nickel strips produced from fine electrolytic and carbonyl nicke ! powders by rolling and sintering in hydrogen. Some characteristics of the strips, which were about 140-pro thick, are given below, and their structure is shown in Fig. 1.

Carbonyl Ni Electrolytic Ni powder (strip I) powder (strip 2)

Mean pore size,~p m 1,5--2,0 7.0--I0.0 Total porosity. ~/o 35 32

The method of ver t ical capi l lary infiltration of specimens is descr ibed in [1]. After infiltration each rec tangular , 30-ram-high, 10-ram-wide specimen was cut into sections. The section (5-mm-high) which had been in di rect contact with molten metal was removed (after the extent to which it had been attacked by the melt was assessed) . The next section, A, was cut off at a distance of 10 mm from the spec imen/mel t bound- ary. Another section, B, was cut off at a height of 20 mm. For quantitative determinations of amounts of the components of an infil trating alloy in specimens, which consisted in ca r ry ing out chemical and polarographic analyses and weighing, sections A were mainly employed. Scatter of resul ts of the analyses was much grea ter

Institute of Materials Science, Academy of Sciences of the Ukrainian SSR. Translated from Poroshkov- aya Metallurgiya, No. 1(229), pp. 51-56, January, 1982. Original article submitted, after revision, June 9, 1981.

0038-5735/82/2101- 0045507.50 �9 1982 Plenum Publishing Corporat ion 45

Fig. 1. Structures of specimens of nickel s tr ips produced from earbonyl 1 (a) and electrolyt ic 2 (b) powders in s tar t ing condi- tion. Magnification: a) x1900; b) x1800.

20

} b b z0 ~5 Sn in melt, %

Fig. 2. Variat ion of intensity of capi l lary infiltration of nickel s t r ips 1 (1) and 2 (2 and 3) in portions A with tin concentrat ion in melt at various t empera tures : 1) 460; 2)380; 3) 360~

than in the study of the infiltration of high-porosi ty s tr ips by lead [1, 2], and consequently data were averaged over 10 specimens.

The intensity of ver t ical capi l lary infiltration in the portions A of specimens of high-porosi ty nickel s t r ips 1 and 2 was expressed as a percentage fraction of the theoret ical ly possible amount of lead, filling all pores and pore channels, N, %, as given by the formula

N = g/(6.S-11.p). 100 %,

where 6 and S a re the thickness and surface a rea of the nickel s tr ip section investigated, respect ively; II, po- ros i ty of the specimen, expressed as a fraction of the specimen volume; p, density of the infiltrating alloy component under considerat ion; and g, amount of this component in %he given specimen section.

As a resu l t of experiments it was established that at a tempera ture of 380~ specimens of both mate - r ia ls (strips f rom powders 1 and 2) experienced only capi l lary infiltration, but at a tempera ture of 460~ the surfaces of specimens made from nickel powder 2 were wetted by the l e a d - t i n alloy, which rose ver t ica l ly over their surfaces above their lines of contact with the melt. This metal usually covered the sections A, which made it impossible to determine the distribution of the alloy components in nickel s t r ips 2 at this t em- pera ture by analyt ical methods.

The intensities of lead infiltration into specimens of nickel s tr ips 1 and 2 in the portions A at a t empe r - a ture of 380~ and into specimens of nickel str ip 1 at 460~ are shown in Fig. 2. (Because of the scale of the graph, the very smal l amounts of tin found in these specimens, which varied in the range from 0.1 to 0.5~ a l - most r ega rd le s s of the composit ion of the l e a d - t i n alloy, could not be plotted in this figure. It was only af ter infiltration by a pure tin melt that the amount of this metal in the portions A of specimens of nickel s t r ips 1 and 2 rose to 1.5-2.0~.)

When a nickel str ip 1 was infiltrated with POS-61 l e a d - t i n alloy (infiltration was per formed for 30 min in a horizontal direction to simulate the action of a solder alloy in solder ing at 290-460~ a part ia l or total

46

Fig. 3. Charac ter i s t ic s t ruc ture types of specimens of nickel str ip made from carbonyl powder and infil trated at 460~ by l e a d - tin alloy containing 5g of tin: a) with full infiltration, • 1650; b) with infiltration of some parts of s t ruc ture , • 650; c) with a r r e s t of mel t migrat ion front, • 920.

~, m m

pot, 2 ~ 6 ~ 5n,~

Fig. 4. Variation of height of r ise of in- f i l t rat ing alloy in niekel s t r ips 1 and 2 with tin concentrat ion in mel t at various t empera tures t: a, e) 460; b) 380~

destruction of specimens invariably occur red . X- ray s t ruc tura l analysis revealed that the powder formed as a resul t of the disintegration contained the a-Ni and a - P b phase and the compound Ni3Sn 4.

The intensity of infiltration of specimens of nickel str ip 1 in the sections A by lead increased in the Dresence of 1~.. of tin in the melt . This effect manifested it_self par t icular ly s t rongly at a t empera ture of 460~ As the amount of tin in the melt was ra i sed from 1 to 10~, the intensity of lead infiltration decreased , and at a tin content of about 30~ the intensity of infiltration was less than that observed in the infiltration of porous nickel by pure Iead. The amount of lead in the sections A of specimens af ter capi l lary infiltration was always less with nickeI s t r ips 2 than with niekel s t r ips 1. The same observat ion had been made in our ea r l i e r inves- tigation into the infiltration of s imilar specimens by pure lead [1].

~fhe cha rac te r of l e a d - t i n solder alloy distribution in the s t ructure of specimens was studied on s u r - faces obtained by sectioning them at 5-ram intervals over their heights. The height to which the infiltrating melt had r isen was determined by noting the appearance of a charac te r i s t i c melt network in the s t ruc ture of the s tar t ing sintered mater ia l (Fig. 3a). Using the resul ts obtained, hypothetical melt r i se height h curves were constructed for carbonyl (1) and electrolyt ic (2) nickel s t r ips infiltrated at various t empera tu res by

47

N J" I~

Pb

L . . , . _ .

! I I l

~, ~)b Pb

I i I b l I I - i C I I

N,

V/

I i i i l I !

d

Fig. 5. D is t r ibut ion of components of l e a d - ] ~ t in a l loy in s t ructure of s p e c i m e n of c a r b o n y l n i c k e l s t r i p i n f i l t r a t e d a t 380~ as funct ion of d i s t a n c e f rom l ine of c o n t a c t with m e l t : a) 1; b) 3; c) 10; d) 2 0 m m .

l e a d - t i n a l l o y s con t a in ing v a r i o u s a m o u n t s of t in (F ig . 4). The m e l t r i s e he igh t was found to depend on the n a t u r e of the n i c k e l p o w d e r , t e m p e r a t u r e , and p e r c e n t a g e a m o u n t of t in in the m e l t .

A s tudy was m a d e of the d i s t r i b u t i o n of the c o m p o n e n t s of the l e a d - t i n s o l d e r a l l o y s o v e r the he igh t s of c a p i l l a r y - i n f i l t r a t e d p o r o u s n i c k e l s t r i p s . This involved both e x a m i n i n g p a r t s of i n t e r n a l s e c t i o n s u r f a c e s and d e t e r m i n i n g the c o m p o s i t i o n s of v e r t i c a l m e t a l l a y e r s f o r m e d on the s u r f a c e s of s p e c i m e n s of n i c k e l s t r i p s f rom e l e c t r o l y t i c powder 2 d u r i n g t h e i r i n f i l t r a t i o n a t a t e m p e r a t u r e of 460~ The s tudy d e m o n s t r a t e d tha t t h e s e e x t e r n a l l a y e r s could be d iv ided a c c o r d i n g to the c o m p o s i t i o n into s e v e r a l z o n e s . In t h e i r l ower p a r t s the a m o u n t s of l ead and t in c o r r e s p o n d e d a p p r o x i m a t e l y to the c o m p o s i t i o n of the m e l t , s u b s e q u e n t zones w e r e i n c r e a s i n g l y d e p l e t e d of t in , and the u p p e r p a r t s of the l a y e r s con ta ined on ly l ead . A s i m i l a r s i t ua t i on was o b s e r v e d in [7].

In a l l the c a s e s of v e r t i c a l c a p i l l a r y i n f i l t r a t i o n e x a m i n e d , t i n w a s d e t e c t e d on ly a t d i s t a n c e s of not m o r e than 1 .5 -2 .0 m m f r o m the l ines of c o n t a c t wi th m o l t e n m e t a l , w h e r e a s l ead r e a c h e d , depend ing on s p e c i f i c c o n d i t i o n s , he igh ts r a n g i n g f r o m a l m o s t z e r o to the ful l s p e c i m e n he igh t (30 mm) .

A s the tin con ten t was r a i s e d f r o m 1 to 10~ , the he igh t of l ead r i s e by the c a p i l l a r y m e c h a n i s m g e n e r - a l l y d e c r e a s e d . The o b s e r v e d t e n d e n c y of t in to c o n c e n t r a t e in the l o w e r p a r t of a s p e c i m e n is i l l u s t r a t e d by the r e s u l t s of e l e c t r o n p r o b e m i c r o a n a l y s e s shown in F ig . 5. E x a m i n a t i o n o f the da ta fo r two a d j a c e n t s e c - t i o n s , a t d i s t a n c e s of 1 and 3 m m f r o m the l ine of c o n t a c t with the m e l t , shows tha t t in was found on ly in the f o r m e r (F ig . 5a). The a m o u n t of l ead fe l l wi th i n c r e a s i n g d i s t a n c e f rom the c o n t a c t l ine (F ig . 5b); a t a he igh t of 10 m m on ly t r a c e s of l ead w e r e d e t e c t e d (F ig . 5c), and a t a d i s t a n c e of 20 m m the l ines of the c o m p o n e n t s of the i n f i l t r a t i n g m e l t w e r e a l t o g e t h e r a b s e n t {Fig. 5d).

I t was a l so e s t a b l i s h e d tha t v e r t i c a l c a p i l l a r y i n f i l t r a t i o n of p o r o u s n i c k e l s p e c i m e n s by l e a d - t i n m e l t s is not a l w a y s un i fo rm o v e r the s p e c i m e n he igh t and a c r o s s the s p e c i m e n width. On s e c t i o n s s t r u c t u r a l f i e lds w e r e o c c a s i o n a l l y o b s e r v e d in which s o m e v o l u m e s w e r e i n f i l t r a t e d and o t h e r , a d j a c e n t v o l u m e s u n i n f i l t r a t e d by m o l t e n m e t a l {Fig. 3b). In s o m e c a s e s the i n f i l t r a t i o n f r o n t was found to have s topped a t a d i s t a n c e of 1 - 1.5 m m f r o m a s ide edge of a s p e c i m e n (F ig . 3c).

F r o m l i t e r a t u r e da ta [3-6] i t is known tha t the we t t i ng of so l id n i c k e l by lead is i n t e n s i f i e d by the a d d i - t ion of t in to the m o l t e n m e t a l , but one canno t conc lude tha t th is m u s t n e c e s s a r i l y a p p l y a l s o to h i g h - p o r o s i t y m a t e r i a l s . A p a r t f r o m t h i s , i t i s a l so n e c e s s a r y to t ake into a c c o u n t the c h a r a c t e r of the n i c k e l - l e a d and n i c k e l - t i n cons t i t u t i on d i a g r a m s [8]. The i n c r e a s e d s o l u b i l i t y of h i g h - p o r o s i t y n i c k e l m a t e r i a l s in a l e a d - t i n m e l t m e a n s tha t a c a p i l l a r y s y s t e m is in f ac t f o r m e d by a t h r e e - c o m p o n e n t a l l o y c on t a in ing l ead , t in , and n i cke l . Under t h e s e c o n d i t i o n s , the f o r m a t i o n is p o s s i b l e of l i m i t e d so l id so lu t i ons both on the n i c k e l - l e a d and on the l e a d - n i c k e l s ide (as has been d e m o n s t r a t e d in [2]), e u t e c t i c s , and c h e m i c a l compounds of n i c k e l and t in . In [7], e . g . , i n t e r m e t a l l i c compounds of v a r i o u s c o m p o s i t i o n s w e r e found to fo rm d u r i n g the s p r e a d - ing of l e a d - t i n a l l o y s on the s u r f a c e of a h i g h - p o r o s i t y n i c k e l s t r i p .

4 8

The observed increase in the intensity of ver t ica l capi l lary infiltration may be attributed to the en t ryo f smal l amounts of the tin present in the melt into pore channels. From the point of view of react ive diffusion theory the presence of even a minute quantity of tin must have a specific catalytic action which dec reases the energy of activation for the dissolution of solid nickel in a melt through the formation and destruct ion of "pseudocomplexes" in the diffusion layer at the interface [6]. In the s t ructure of a hlgh-porosi ty nickel str ip the role of "pseudocomplexes" may be played by metastable compounds of nickel and tin atoms. The poss ibi l - ity of simultaneous formation of solid solutions and chemical compounds increases , according to [3, 5], the wettability of pore walls and channels, and this facil i tates capi l lary motion.

The spreading of a l e a d - t i n alloy on the surface of an electrolyt ic nickel str ip specimen infiltrated a t a tempera ture of 460~ and the absence of this phenomenon in the ver t ical capi l lary infiltration of a carbonyl nickel str ip under identical conditions are apparent ly attributable to differences in the s t ruc tures of pores and pore channels in these mater ia l s . A nickel str ip produced from a carbonyl powder has an equiaxial s t ruc - ture with a multitude of more or less equal -s ized pore channels (Fig. la); a nickel str ip made from an e lec - trolytic powder, which has a dendrit ic s t ruc ture , is charac te r ized by a network of ext remely i r r egn la r - shaped capi l lar ies of varying sizes (Fig. lb), as a resu l t of which blocking of pore channel mouths by intermetal l ic compound phases is more intense than in a s tr ip f rom a carbenyl powder. In this case the driving force of the process of molten metal spreading over a specimen surface covered with a network of open capi l lar ies is grea ter than the driving force of ver t ical capi l lary infiltration.

Capil lary infiltration of high-porosi ty nickel s tr ips by l e a d - t i n alloys in h igh- tempera ture dispersion soldering p rocesses [9] takes place in a horizontal direction under p ressure . Clearly, in this case a high- porosi ty nickel str ip whose thickness does not exceed 140 pm is fully infiltrated not only by lead but also by tin, since in ver t ical capi l lary infiltration a concentrat ion of tin is observed in a 1.5- to 2 .0-mm thick zone above the melt contact boundary. Under these conditions intermetal l ic compound phases will inevitably form in the capi l lar ies of the high-porosi ty str ip, and this may prove to have a deleterious effect in soldering with an alloy of high tin content. The authors have found that good wettability with the formation of only small amounts of intermetal l ic compound phases in soldered joints is exhibited by infiltrating l e a d - t i n alloys con- raining 1-5~ of tin.

LITERATURE CITED

1.

2.

3.

4.

5.

6.

7.

8. 9.

M. A. Tolstaya, S. P. Chizhik, N. M. Khokhlacheva, M. E. Shilovskaya, and L. K. Gr igor 'eva , "Capil lary infiltration of high-porosi ty nickel s t r ips by lead," Poroshk. Metall. , No. 4, 90-94 (1980). N. M. Khokhlacheva, V. N. Paderno, Mo E~ Shilovskaya, and P. A. Verkhovodov, "Interact ion of molten lead and porous nickel," Poroshk. Metall . , No. 8, 80-84 (1980). A. Bondi, "The spreading of liquid metals on solid surfaces . Surface chemis t ry of high-energy sub- s tances," Chem. Rev., 53, No. 2, 417-458 (1953). G. L. Bailey and H. C. Watkins, "The flow of liquid metals on solid metal surfaces and its relat ion to soldering, brazing, and hot-dip coating," J. Inst. Met., 80, No. 2, 57-76 {1951-1952). J. W. Taylor , "The significance of wetting in r eac to r technology," J. Nucl. Energy, 2, No. 1, 15-30

(1955). A~ N. Parfenov, "An investigation into the kinetics of the react ions of copper, nickel, s i lver , gold, pa l - ladium, and rhodium with t i n - l e a d solder al loys," Author ' s Abs t rac t of Candidate 's Disser ta t ion, Mos - cow (1972). G. A. Yakovlev, "Reactions of lead-base alloys with porous nickel during composite soldering," Elektron. Tekho, Mater . , N o . l l , 10-18 (1971). M. Hansen and Ko Po Anderko, Constitution of Binary Alloys, McGraw-Hil l , New York (1957}. I. D~ Morokhov, S. P~ Chizhik, Kh. B. Khokonov, A. V. Pushkov, V. N. Lapovok, and L. K. Gr igor ' eva , "Fal l in the magnitude of the thermodynamic pa ramete r s determining the kinetics of p rocesses in a s y s - tem containing finely divided metall ic phases ," in: Physics of Interracial Phenomena [in Russian],

Nalchik (1977), pp. 73-75.

49