Parallel-gap resistance welding of silicon solar cells with copper interconnects results in complex microstructural variations that depend on the welding variables. At relatively low heat input solid-state welds are produced. At medium heat the Ag-Cu eutectic forms resulting in a braze joint. High heat produces a fusion weld with complete melting of the silver layer on the silicon solar cell. If the silicon is also melted, cracking occurs in the silicon cell below the weld nugget. These determinations were made using light microscopy, microprobe, and scanning electron microscopy analyses

Baraona, C. R.

Moore, T. J.

Watson, G. K.

English

NASA Technical Reports Server

MICKOSTKUCTURAL ANALYSIS OF SOLAR CELL WELDS 
T.J. Moore, G.K. Watson, and  C.K. Baraona 
NASA Lewis  Resea rch  C e n t e r  
Cleveland,  Ohio 
SUMMARY 
Parallel-gap resistance welding of silicon solar cells with copper inter- 
connects results in complex microstructural variations that depend on the weld- 
ing variables. At relatively low heat input solid-state welds are produced. At 
medium heat the Ag-Cu eutectic forms resulting in a braze joint. 
produces a fusion weld with complete melting of the silver layer on the silicon- 
solar cell. If the silicon is also melted, cracking occurs in the silicon cell 
below the weld nugget. These determinations were made using light microscopy, 
microprobe, and scanning electron microscopy analyses. 
High heat 
INTRODUCTION 
Para1 lel-gap resistance welding (PGRW) has been used extensively in the 
production welding of interconnects to solar cells for space-power applica- 
tions. Despite the fact that thousands of welds have been made, little informa- 
tion has been published on the microstructure of solar-cell welds. This inves- 
tigation, though limited in scope, is an attempt to characterize the various 
microstructures that can be obtained when welding 39-urn (1.5-mil) thick copper 
interconnects to 200-um (8-mil) thick silicon solar cells by PGRW. Solar-cell 
welds, made using three different weld schedules, were examined by optical and 
scanning electron microscopies. Compositional traces were obtained with an 
electron probe microanalyzer. 
American Welding Society (AWS) terms and definitions are used throughout 
this report to make the technical discussions as clear as possible. 
Joint Method Diagram (f-ig. 1) is based on the physical state of the materials at 
the weld interface during coalescence (ref. 1). Thus, three metallurgical 
classifications of welding processes are defined: 
Fusion weldinq for liquid/liquid reaction 
Solid-state welding for solid/solid reaction 
Brazing and soldering for liquid/solid reaction 
The AWS 
The authors are grateful to Frank M. Terepka, who performed the electron probe 
analysis. 
MATER I ALS 
Cross 
shown in f 
sectional sketches 
gure 2. These cel 
of the silicon solar cell used in this study are 
s had 2-ohm cm, P-type base resistivity, shallow 
237 
https://ntrs.nasa.gov/search.jsp?R=19830007562 2020-03-22T00:05:13+00:00Z
junctions and wraparound insulator type contacts. At the P contact areas, four 
layers of material were deposited on the silicon (Si) cell: 
(1) Aluminum (Al) about 2000 4 thick 
(2) Titanium (Ti) about 1000 1 thick 
(3) Palladium (Pd) about 250 a thick 
(4) Silver (Ag), 9 um thick (7 to 13  um by metallography) 
The outer layer of silver was applied for electrical conductivity. The 
wraparound N-type contacts had the same silver, palladium, and titanium metal 
ldyers as the P contacts. However, instead of aluminum, the first layer depos- 
ited on the silicon cell was silicon dioxide (SiO2) dielectric about 1 um 
thick. This Si02 layer provided electrical insulation between the N-type 
contacts and the P-type silicon base layer. 
The copper interconnect tab was cold-rolled stock with an average thickness 
of 39 um. Metallographic examination showed a thickness variation of 37 to 46 
m. 
PROCEDURE 
Welding Equipment 
Parallel-gap resistance welding is a variation of series resistance welding 
in which the electrodes are placed very close together. 'This electrode place- 
ment tends to produce relatively high heating apd formation o f  a fusion weld 
nugget between the electrode. 
variables, it is possible to produce individual solid-state welds under each 
electrode. 
However, by controlled adjustment of the welding 
In the welding head, an assembly with two molybdenum electrodes is used. 
Each side is electrically insulated from the other. Both electrodes move down- 
ward together on activation o f  their common supporting arm. Each electrode is 
permitted a controlled degree of flexing, which compensates for minor irregular- 
ities in flatness or thickness. This is important because good mechanical fit 
insures proper electrical contact between the electrodes and the work. 
A constant-voltage power supply is used to produce a dynamically controlled 
single pulse of welding current. Welding is accomplished at a constant, preset 
voltage. 
force is applied to the work by the electrodes. 
Initiation of the welding power is automatic when a predetermined 
Welding Schedules 
Three weld schedules were applied in welding the copper interconnects to 
the silicon cells. 
proportionality with welding current. Heat is generated in all portions of the 
circuit according to the formula: 
Weld voltage is a primary variable because of the direct 
H = 12Rt 
where H is the heat in joules, I is the current in amperes, R is the resis- 
tance in ohms, and t is the time of current flow in seconds. Welding time was 
238 
the second variable in this program (table I). 
interfacial resistance, and the gap between the electrodes were held constant. 
Thus, the three weld schedules can be described as follows: 
Electrode force, which controls 
Schedule A: Low heat for a long time 
Schedule B: Moderate heat for an intermediate time 
Schedule C: High heat for a short time 
Welds were made with the same schedules for both the P and N contact areas 
in the solar cells. 
Weld Examination 
The solar-cell welds were mounted in clear epoxy for sectioning and metal- 
lographic polishing. Cross sections of the welds were examined at various 
locations along the electrode footprint denoted by section A-A in figure 3 .  
Specific examples shown in this report were at positions 30 percent and 40 
percent through the joints. A flat was polished on the side of the cylindrical 
metallographic mounts so that the weld footprint could be clearly seen. 
specific location of the weld cross section after metallographic polishing could 
then be measured optically. 
The 
All of the weld cross sections were polished using standard metallographic 
techniques. Although 
the welds were examined in both the unetched and etched conditions, all of the 
photomicrographs shown are in the unetched condition. 
The cross sections were examined optically up to 1OOOx. 
Selected samples were examined using a scanning electron microscope (SEM). 
Various areas of the samples were qualitatively analyzed using energy dispersive 
spectroscopy (EDS). 
obtain quantitative elemental distribution along continuous-line traverses 
through the weld regions. 
In addition, an electron probe microanalyzer was used to 
RESULTS AND DISCUSSION 
In the results to follow, an overview showing the electrode positions and 
Higher magnification light photomicrographs 
the kinds of welds that were obtained with the various weld schedules will first 
be shown at lOOx magnification. 
with corresponding microprobe chemical analyses of the various weld joints will 
then be presented and discussed. The SEM was used to further examine braze 
areas, the solid-state weld interface and the Si02 layer at N contact area of 
the cells. 
Schedule A Welds 
The N contact weld shown in figure 4 (40 percent through the joint, table 
I) is a combination of solid-state welding and brazing. The SEM photomicrograph 
in figure 5 shows that dendrites in the braze metal are oriented normal to the 
braze interface, which is parallel to the expected direction of solidification. 
This shows that Cu interconnects can be joined to the Ag layer on the cell by 
resistance brazing rather than by conventional fusion resistance welding. 
Ag-Cu solid-state welded areas (fig. 4) were present towards the outsides 
o f  the electrode footprints beyond the brazed regions. 
239 
Solid-state welding is achieved instantaneously when the resistance heated 
Ag and Cu surfaces are brought into intimate contact. Welding occurs at about 
0.8 Tm of Ag (710" C), where Tm is the absolute melting point of silver. 
Since the metallic bonds are believed to form on contact, since diffusion is not 
the cause of adhesion (ref. Z ) ,  and since the welding time is short (100 ms or 
less), the term solid-state resistance welding is a more appropriate tern than 
isis diffusion welding (ref. 1). Solid-state welding was judged to be achieved 
i f  microexamination of unetched joints up to lOOOx revealed no unwelded areas. 
Subsequent microprobe examination showed that in the solid-state weld areas, 
only a small amount of interdiffusion had occurred between Ag and Cu. 
An instant after solid-state welding occurred under the electrodes, a Ag - 
40-atomic-percent-Cu eutectic composition formed at, or very near, the solid- 
state weld interface and became a liquid phase when the joint was heated to 
779" C (fig. 6 and ref. 3). When the eutectic formed, it flowed by capillary 
action and brazed unwelded areas of the joint. The brazing action can continue 
with Ag-Cu alloys of varying composition as resistance heating increases the 
temperature of the melt well above the eutectic temperature. The complexity of 
the metallurgical situation is considerable. For example, when initially solid- 
state welded areas are heated above the Ag-Cu eutectic temperature, they become 
molten and thus part of the brazed area of the joint. 
there are unwelded areas between the electrodes and towards the outside of the 
electrode footprints. The former area is believed to contain trapped air which, 
as will be shown later, can result in voids in braze metal or in a weld nugget. 
At the N contact weld just discussed, less heating effect was observed than 
at a P contact weld where brazing occurred between the electrodes (not shown). 
However, insufficient data are available to demonstrate whether this effect 
correlates with the contact area welded or with other welding variables. 
Note in figure 4 that 
Schedule B Welds 
The two weld cross sections in figure 7 show a considerable difference in 
microstructure for the same weld schedule. This variation is at least partly 
due to the fact that maximum heating occurs midway through the joint. 
contact weld cross section (fig. 7(a)), was taken 30 percent through the joint. 
It shows a braze joint in the electrode gap and under the inside of the elec- 
trode footprints. A line of small pores is present in the Ag-Cu braze metal. 
Solid state welding took place beyond the braze. 
The P 
More heating is evident at the N contact cross section of figure 7(b), 40 
percent through the joint. 
between the electrodes to form a fusion weld nugget. 
alloy. A brazed region borders the nugget, and solid-state welding took place 
beyond the limits of the braze. 
the fusion weld of figure 7(b) are believed to be air bubbles formed in the 
molten braze or weld metal (discussed previously regarding fig. 4). 
ized trapped air effect is also believed to have produced the bulge in the 
copper interconnect between the electrodes (fig. 7( b) ) .  
In this case all of the silver has been melted 
This nugget is a Ag-Cu 
The small pores in the braze joint of figure 7(a) and the large pores in 
The local- 
2 40 
Schedule C Welds 
In figure 8 two fusion welds are shown with brazes and solid-state welds 
towards the outside of the electrode footprints. The weld nugget at the N 
contact (30 percent through the joint) i s  a Ag-Cu alloy (fig. 8(a)). 
Si02 layer on the cell acted as a refractory material to contain the Ag-Cu 
melt and thus prevent the nugget from alloying with the silicon. 
layer also inhibited solid-state diffusion and possible formation of the 
atomic percent silicon eutectic which melts at 830" C (ref. 3 and fig. 9). 
Although no cell cracking is evident in figure 8(a), fine cracks in the silicon 
cell were observed under another similar Ag-Cu weld nugget. 
At the P contact weld (40 percent through the joint) (fig. 8(b)) no protec- 
tive Si02 layer was present on the silicon cell. This weld is similar to the 
N contact weld except that the weld nugget here is a Ag-Cu-Si-Pd alloy. The 
lens-shaped crack, observed under the weld nugget in fig. 8(b) is typical for 
all cases where the silicon cell was melted. Several factors contribute to the 
cracking. First o f  all the silicon cell has essentially no ductility at room 
temperature. Thus, local thermal shock alone could produce cracking. Second, 
solidification shrinkage of the pancake-shaped weld nugget must produce some 
deformation, that i s ,  dishing under the weld nugget. Third, residual stresses 
are produced when the weld nugget contracts on cooling from the solidification 
temperature to room temperature, with the maximum residual stress being limited 
to the yield strength of the Ag-Cu-Si-Pd alloy (refs. 4 and 5). 
The l-pm 
The "i% 9-15.4 
Electron Probe Microanalysis 
Electron-probe, continuous-line traverses were run on all three metallurgi- 
cal classes of welds between the copper interconnect and the silicon cell: 
(a) Solid-state weld between Ag and Cu 
(b) Ag-Cu braze joint 
(c) Two fusion weld nuggets; one Ag-Cu, the other Ag-Cu-Si-Pd 
For each weldment examined, microprobe traces confirmed the presence of 
thin layers of titanium and palladium, which had been intentionally deposited on 
the contact surface of the Si cells during manufacturing. 
palladium micrbprobe traces are not shown in the figures for purposes of clar- 
ity. No attempt was made to confirm the presence of aluminum at the P contacts. 
These titanium and 
The solid-state weld joint of figure 10 shows a small amount of Ag-Cu 
interdiffusion. Interdiffusion between silver and silicon was nil. The dark 
band (about 1 urn wide) between the silicon and silver and the increase in 
microprobe specimen current in this vicinity gave evidence of the Si02 layer 
on the cell at this and other N contact areas. 
This Ag-Cu braze joint (fig. 11) is only about 2.5 pm wide. Because of 
equipment limitations, it could not be established that the braze metal was o f  
the eutectic composition. But the microprobe trace does show significant 
alloying near the Ag-Cu interface. Interdiffusion between silver and silicon 
was nil. Since this is a P contact area, no Si02 layer was present. 
A microprobe trace of a Ag-Cu fusion weld nugget containing large trappe- 
dair voids is shown in figure 12. In this case the silver coating was com- 
241 
pletely melted and formed an alloy of about 50 Ag - 50 Cu (weight percent). 
1-pm Si02 refractory layer in the N contact area protected the cell. Thus, 
the molten weld metal nugget was prevented from alloying with the silicon solar 
cell. 
The 
The fusion weld nugget (fig. 11) is an alloy of Ag-Cu-Si and palladium. 
Although the quantity of palladium was very slight, the microprobe traverse 
showed alloying of palladium in the weld nugget. The melting and alloying of 
the silicon cell is evident by the wavy nuggetlcell weld interface and by the 
microprobe trace. Since this was a P contact, there was no protective Si02 
coating on the cell. Two cracks are shown in figure 13 in the silicon below the 
weld nugget. Lens-shaped cracking of this type occurred below the nugget in all 
cases where the silicon was melted, and thus became part of the weld. 
CONCLUSIONS 
1. Three metallurgical categories of resistance welds were identified under 
(a) Low heat for a long time: 
various conditions of parallel-gap resistance welding: 
under each electrode with an unwelded area between the electrodes. 
Combination solid-state welding and brazing 
(b) Moderate heat, intermediate time: A Ag-Cu braze joint between the 
electrodes with solid-state welding at the periphery. 
(c) High heat for a short time: A conventSona1 fusion weld nugget between 
the electrodes with brazing and solid-state welding at the periphery. 
o f  weld is always accompanied by cracking in the Si cell if melting of the Si 
occurs. 
This kind 
2. The 1-pm layer of Si02 at the N contact areas of the cell proved to be 
a refractory barrier that contained the molten weld nugget and prevented 
interaction with the Si cell. 
RECOMMENDATIONS 
1. Programs to develop solar array welding technology should start with 
well-defined metallurgical goals. 
2. The weld joint metallurgy required to give optimum solar-cell relia- 
bility should be identified. 
3. The welding process, associated control systems, and NDE procedures 
should be tailored to give assurance that the metallurgical goals are achieved. 
242 
REFERENCES 
1. Welding Terms and Definitions. American Welding Society, A3.0-80, 1980. 
2. Ludemann, W. D.: A Fundamental Study of the Pressure Welding of Dissimilar 
3. Hansen, M.: Constitution of Binary Alloys. McGraw-Hill Book Co., Inc., 1958. 
Metals Through Oxide Layers. UCRL-50744, 1969. 
4. Linnert, G. E.: Welding Metallurgy. Vol. 2. American Welding Society, 1967. 
5. Lancaster, J. F.: The Metallurgy of Welding, Brazing and Soldering. George 
Allen and Unwin Ltd., 1965. 
TABLE 1. - PARALLEL-GAP RESISTANCE 
WELDING SCHEDULES 
[Electrode force, 8.9 N (2.0 lb); elec- 
trode pressure, 6 . 1 ~ 1 0 ~  N/m2 (885 
psi); electrode gap, 0.30 mm.] 
Schedule We1 d We1 d 
voltage, time, 
V ms 
(a> 
A 0.58 100 
B .63 80 
C .68 50 
aApproximate 
243 
Welding 
Joining process 
method class 
Operation 
F i g u r e  1. - J o i n i n g  method d i a g r a m .  
- Back 
Front c-- 
I I I  I 1: 0.64;. 58k- 0.304 
F i g u r e  2 .  - S e c t i o n i n g  p r o c e d u r e .  
u 
P contact 
u 
N contact 
F i g u r e  3. - Cell c o n t a c t s .  
244 
N contacf 40 percent through joint (see table 1) 
Figure 4. - Schedule A combination solid-state weld and braze joint. Air is 
believed to be in the unwelded area between the electrodes. 
Figure 5. - Dendritic structure of an Ag-Cu braze metal in a schedule A weld 
at an N contact. 
245 
As Atomic percent copper cu 
I I 1 I I I I I 1 1 1  
0 10 20 30 40 50 60 70 Bo 90 100 
Weight percent copper 
Figure 6 .  - Silver-copper equilibrium diagram (ref. 3). 
-- 
Solid 
state 
weld 
P contact, 30 percent through joint 
N contact, 40 percent through joint 
(a) Braze between electrodes with small voids in braze metal. Solid-state 
(b) Porous Ag-Cu fusion weld nugget producing bulge with brazing and solid- 
welds are outside of braze region. 
state welding outside of the nugget. 
Figure 7. - Microstructural variations in schedule 5 welds. 
246 
N contact, 30 percent thmugh joint 
P contact, 40 percent thmugh joint 
(a) Porous Ag-Cu fusion weld nugget with brazing and solid state welding 
(b) Porous Ag-Cu-Si-Pd fusion weld with brazing and solid state welding beyond 
the nugget. 
fusion weld. 
Figure 8. - Microstructural variations in schedule C welds. 
outside nugget. 
A lens-shaped crack in the silicon cell i s  present under t h e  
I I 1 I 1  I I I l l  
0 5 10 15 20 30 40 60 80 100 
Weight percent silicon 
Figure 9. - Silver-silicon equilibrium diagram (ref. 3 ) .  
247 
Figure 10. - Solid state weld at 
between silver and copper. 
N contact area showing slight interdiffusion 
Schedule C; weld under electrode. 
Figure 11. - Silver-copper braze joint, approximately 2.5 pm wide, with small 
voids in braze metal. 
this P contact joint. 
Significant Ag-Cu alloying is shown at braze in 
248 
Figure 12. - Fusion weld nugget of Ag-Cu alloy at N contact with complete 
melting o f  silver layer. Schedule C; weld between the electrodes. 
Figure 13. - Fusion weld nugget o f  Ag-Cu-Si-Pd alloy at P contact showing 
cracking in the silicon celllayer. 
electrodes. 
Schedule C; weld between the 
249 


Microstructural analysis of solar cell welds

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