sistance of their conducting particles are counterbalanced by those of their gaps. Theo- retical relationships between the CTE of the binder and conducting components are established which are necessary for the formulation of thermally compensated systems.

.

LITERATURE CITED

B. S. Gal'perin, Nonwire Resistors [in Russian], Energiya, Moscow-Leningrad (1968).

CAPILLARY INFILTRATION OF HIGH-POROSITY NICKEL STRIPS BY LEAD

M. A. Tolstaya, S. P. Chizhik, N. M. Khokhlacheva, M. E. Shilovskaya, and L. K. Grigor'eva

UDC 621.762

A new method of Joining parts made of different materials, involving the use of sintered high-porosity nickel strip interlayers infiltrated with low-melting-point soldering metals, is finding increasing application for the assembly of semiconductor devices. The method, de- veloped by S. P. Chizhik and others, is known as low-temperature dispersion soldering.

An infiltration is one of the key operations in dispersion soldering. It is of interest to study the interaction between low-melting-point soldering metals (in particular lead) and porous nickel strips produced from fine nickel powders. Data have already been published on the kinetics of infiltration of porous nickel specimens made from powders of ~50-~m particle size by molten lead [i]. Now such a system differs sharply in mean pore size from nickel strip made from a fine nickel powder, which is used in dispersion soldering processes. Apart from this, no attempt has been made as yet to study the distribution of lead in porous nickel.

In the work described below an investigation was carried out into the capillary infiltra- tion, by molten lead, of high-porosity strips from fine nickel powders prepared by two dif- ferent techniques and an evaluation was made of the suitability of such strips for the dis- persion soldering of nickel-plated parts. Porous strips produced, by rolling and sintering in a hydrogen atmosphere, from fine carbonyl (No. i) and electrolytic (No. 2) nickel powders were chosen for investigation. The strip thickness was of the order of 140Bm. The carbonyl powder strips had a mean pore size in the range 1.5-2 ~m, and their total porosity was 35%. The distribution of pores in the electrolytic nickel powder strips showed large scatter, and the mean pore size lay in the range 7-10 ~m at a total porosity of 32%.

The rectangular, 35-mm-highx10-mm-wide specimens were placed, after being degreased in alcohol and dried in air, in a vertical position in a steel tank. They were inserted into slots cut in the tank cover and additionally fastened by means of molybdenum wire passing through holes in their upper parts above tank cover. Granules of "analytical" grade lead were poured into the tank in an amount ensuring that the level of metal after melting was 5 nun above the bottom edges of the specimens. A six-specimen assembly was placed in a tube furnace through which argon was continually passed. Infiltration was performed for 60-180 min at temperatures in the range 340-5500C.

After infiltration each specimen was cut into sections. The portion of the specimen which had been in contact with molten lead was removed, and two sections were obtained by cutting the remaining part of the specimen at heights of i0 mm (section A) and 20 mm (sec- tion B) above the boundary of contact between the specimen and molten nickel. The distribu- tion of lead in infiltrated specimens was determined by chemical and polarographic methods and by weighing. Data averaged over six specimens showed quite satisfactory reproducibility. Error amounted to 5-8% at capillary infiltration times of not less than 60 min, but at shorter process times (not less than 60 min) the reproducibility of data was poor because the amount of lead transferred into a specimen under these conditions was extremely small, of the order of 8.10 -~ g/cm z (0.96%). X-ray structural examinations of specimens were carried out, using copper radiation, in an RKU-II4 camera.

Moscow Aeronautical Technological Institute. Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 90-94, April, 1980. Original article submitted July 6, 1979.

286 0038-5735/80/1904-0286507.50 �9 1980 Plenum Publishing Corporation

Fig. i. Microstructures of specimens after in- filtration, • a) carbonyl powder strip in- filtrated at 460~ b) the same strip infil- trated at 5500C; c) electrolytic powder strip infiltrated at 460=C; d) the same strip infil- trated at 550=C.

After the capillary infiltration of specimens sections A and B were subjected to chemi- cal analyses (Table I). As can be seen from the table, the amounts of lead in sections A were higher for specimens made from the carbonyl powder, while the distribution of lead be- tween sections A and B was more even for porous nickel strip specimens produced from the electrolytic powder. In addition, with electrolytic nickel powder specimens, too, the varia- tion of the intensity of vertical penetration of lead with temperature was greater.

Determinations were made of the character of molten lead distribution in porous nickel strips at various temperatures (Fig. 1). At a temperature of 460=C porous strips made from the carbonyl powder were fairly evenly infiltrated by molten lead, which filled both their pores and pore channels (Fig. la). When the melt temperature was raised to 550~ some parts of the structure were found to be quite severely attacked (Fig. ib). In the infiltration of porous strip specimens made from the electrolytic nickel powder attack on pore channel walls and the formation of "microcaverns" occurred already at 460=C (Fig. ic) and became very marked at 550~ (Fig. Id).

TABLE i

] A mrs. of lead (%) in Infiltration t~mperaturelTime, min Specimen

earbonyl electrolytic section specimens from

I powder ] powd er

340 60 A 2,2 1,2 B 0,9 0,8

370 60 A 4,4 2,8 B 2,4 2,2

370 120 A 5,5 4,4 B 3,7 4,3

430 60 A 12,6 6,7 B 5,3 5,2

460 60 A 16,1 14,8 B 8,1 13,3

550 60 A 26,4 20,8 B 19,4 18,3

287

tO

0 300 ~oo 500 ~, T

A Fig. 2. Variation of percentage of lead with temperature: i) car- bonyl powder specimens; 2) electrolytic powder specimens; 3) electro- lytic powder specimens , infiltration from vapor phase.

An x-ray structural study of a specimen produced from the carbonyl nickel powder and infiltrated at 550"C showed that the crystal lattice parameter of the nickel in the infil- trated ffpecimen (a = 3.528• ~) had changed slightly compared with pure nickel (a = 3,5238 A), pointing to a small solubility of lead in nickel. Raising the temperature is known to increase the mutual solubility of these metals [2].

The infiltration process depends on nickel being wetted sufficiently well by molten lead. It has been established that, under conditions similar to those which obtained in this work, the angle of contact of molten lead on nickel is approximately 60-20*3' [i, 3-6].

To determine the dependence of the degree of infiltration on some characteristics of nickel strips and process temperature, a comparison was made of the mean percentages of lead in sections A of various specimens. Curves illustrating this dependence are shown in Fig. 2. The infiltration of specimens made from the carbonyl powder (curve i) was more intense than that of specimens produced from the electrolytic powder (curve 2). Figure 2 gives also data obtained for electrolytic powder specimens impregnated in lead vapor (curve 3). In this case specimens were arranged horizontally at a distance of i0 mm above the melt surface. It will be seen that in this process, too, impregnation of specimens with lead took place, but its extent was much less than that in the case of capillary infiltration.

The intensity of the capillary infiltration process under consideration is strongly affected both by the total porosity of the material, which is linked with the porous strip production process, and by the shape and size of the particles of the starting nickel pow- ders, which determine the shape and size of the pore channels in the material (Table 2). A carbonyl nickel powder is composed of comparatively equiaxed particles, which leads to the formation of smaller pore channels and to a more ordered pore channel arrangement in the ma- terial compared with nickel strips made from an electrolytic powder, which is characterized by dendritic particle structure. To a first approximation, however, it may be taken that both these materials have pore channels of round cross sections.

The driving force during the capillary infiltration of a porous nickel strip by molten lead is, when the capillaries are wetted by the melt, the capillary pressure AP. In a cap- illary of round cross section the capillary pressure is proportional to the surface tension of the melt Ol-g and the cosine of the contact angle O and inversely proportional to the capillary radius r,

TABLE 2

Exptl. [Nickel, I Lead point I pores, um [

I 1 Melt 2 10 Melt 3 10 Vapor

288

AP ,--, m'g .cose [ 3 . 7 - - 9] . r

It would appear that the more intense impregnation with lead, under identical infiltra- tion conditions, of sections A of specimens of porous nickel strips made from the carbonyl powder was due to the much smaller radius of the pore channels in this material. With rise in temperature, the angle of contact of lead on nickel decreased and AP grew, as a conse- quence of which the degree and rate of infiltration of~porous nickel strips with molten lead increased.

Experiments were also carried out in which materials were joined together by dispersion soldering with the aid of porous nickel strips. A porous strip made from a fine nickel pow- der was introduced into a gap between two nickel-plated parts together with a soldering plate (alternatively, a preinfiltrated strip was used). The resultant system was heated at tem- peratures of 350-3800C with the application of an external pressure of i-2 kgf/mm a. The op- eration was performed in a hydrogen or argon atmosphere; the former atmosphere had an addi- tional purifying effect on the parts in contact.

The system resembled that formed by the specimens in the vertical capillary infiltration experiments because the real arrangement of the pore channels in the porous nickel strip specimens was not ordered in any particular direction, and the infiltration process was fa- cilitated by the fact that the lead melt had to cover but a very short distance during its spontaneous travel in the capillaries, since the strip thickness was only about 140 ~m. The application of external pressure promoted an even filling of the pore channels with molten lead.

In the light of theories concerning the mechanism of liquid-phase sinterlng [i0] it can be concluded that in such a system, an excess of nickel dissolved in the lead melt migrates toward metallic contacts at the boundary between porous and solid nickel, which is borne out by results of metallographic examinations. This leads to the formation of bridges between the surfaces being welded together and thus enables a strong joint to be obtained.

LITERATURE CITED

I. V. N. Ermnenko and N. D. Lesnik, "Kinetics of infiltration of porous solids by liquid metals," in: Surface Phenomena in Metals and Alloys and Their Role in Powder Metallurgy [in Russian], Izd. Akad. Nauk Ukr. SSR, Kiev (1961), pp. 155-177.

2. M. Hanson and K. P. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York (1957). 3. Yu. V. Naidlch, Contact Phenomena in Metallic Melts [in Russian], Naukova Dumka, Kiev

(1972). 4. A. Bondi, "The spreading of liquid metals on solid surfaces. Surface chemistry of high-

energy substances," Chem. Roy., 52, No. 2, 417-458 (1953). 5. G. L. Bailey and H. C. Watkins, "The flow of liquid metals on solid metal surfaces and

its relation to soldering, brazing, and hot-dip coating," J. Inst. Met., 80, No. 2, 57-76 (1951-1952).

6. J. W. Taylor, "The significance of wetting in reactor technology," J. Nucl. Energy, 2, No. i, 15-30 (1955).

7. V. K. Semenchenko, Surface Phenomena in Metals and Alloys [in Russian], Gostekhizdat, Moscow (1957).

8. B. D. Summ and Yu. V. Goryunov, Physicochemical Principles of Wetting and Spreading [in Russian], Khimiya, Moscow (1976).

9. A. D. Zimon, Adhesion of Liquids and Wetting [in Russian], Khimiya, Moscow (1974). i0. V. N. Eremenko, Yu. V. Naidlch, and I. A. Lavrinenko, Sintering in the Presence of a

Liquid Metallic Phase [in Russian], Naukova Dumka, Kiev (1968).

289