Scanning laser acoustic microscopy was used to nondestructively evaluate solar cell interconnect bonds made by resistance welding. Both copper-silver and silver-silver welds were analyzed. The bonds were produced either by a conventional parallel-gap welding technique using rectangular electrodes or new annular gap design with a circular electrode cross section. With the scanning laser acoustic microscope, it was possible to produce a real time television image which reveales the weld configuration as it relates to electrode geometry. The effect of electrode misalinement with the surface of the cell was also determined. A preliminary metallographic analysis was performed on selected welds to establish the relationship between actual size and shape of the weld area and the information available from acoustic micrographs

Baraona, C. R.

Frey, W. E.

Klima, S. J.

English

NASA Technical Reports Server

EVALUATION OF SOLAR CELL WELDS BY SCANNING ACOUSTIC MICROSCOPY 
S.J. Klima, W.E. Frey,  a n d  C.R. Baraona 
NASA Lewis  Kesea rch  C e n t e r  
Cleveland,  Ohio  
SUMMARY 
Scanning l a s e r  acoustic microscopy was used t o  nondestructively evaluate 
so la r  c e l l  interconnect bonds made by resis tance welding. B o t h  copper-silver 
and s i lver -s i lver  welds were analyzed. The bonds were produced e i t h e r  by a 
conventional parallel-gap welding technique using rectangular electrodes or 
new annular gap design with a c i r c u l a r  e lectrode cross  section. 
scanning laser  acoustic microscope, i t  was possible t o  produce a real  time 
te lev is ion  image which reveales the weld configuration as  i t  r e l a t e s  t o  elec- 
t rode geometry. The e f f e c t  of electrode misalinement w i t h  the surface of the 
c e l l  was a l so  determined. A preliminary metallographic analysis  was performed 
on selected welds t o  e s t a b l i s h  the relat ionship between actual s i z e  and shape 
o f  t h e  Weld area and  the  information avai lable  from acoustic micrographs. 
W i t h  the  
INTRODUCTION 
Welding i s  becoming an increasingly a t t r a c t i v e  a l t e r n a t i v e  t o  soldering 
The as a means of joining interconnect tabs t o  solar  c e l l  contact surfaces. 
potent ia l  advantages include reduced weight, a b i l i t y  t o  withstand higher peak 
operating temperatures, increased thermal fa t igue  resis tance,  and improved 
automation capabi l i ty  f o r  fabr ica t ing  large arrays. 
welded so lar  c e l l  interconnects i s  not new ( r e f .  l ) ,  acceptance of the tech- 
nology remains limited due t o  a lack of experience in space f l i g h t  applica- 
t i o n s  and the need f o r  a r e l i a b l e  technique f o r  nondestructively evaluating 
bond qual i ty .  Whereas visual examination i s  generally s u f f i c i e n t  f o r  eva’lu- 
ating solder j o i n t s ,  welded j o i n t s  require more sophisticated procedures be- 
cause the bond area i s  hidden from view. 
connect j o i n t s  can be nondestructively evaluated only by ind i rec t  methods. 
The scanning l a s e r  acoustic microscope, which combines l a s e r  technology with 
ul t rasonic  technology, appears t o  be a good choice f o r  this  application. The 
instrument i s  capable of operating a t  high ul t rasonic  frequencies with resu l t -  
ant h i g h  resolut ion and, a t  100 MHz, can produce a real-time image of an area 
about 2.3 by 3 mm in s i z e  a t  a magnification of 75. 
s o l a r  c e l l  interconnect j o i n t s  can be produced with the  scanning laser  acous- 
t i c  microscope. However, substant ia t ing evidence r e l a t i n g  weld geometry t o  
the  acoustic information was n o t  presented. 
t iga t ion ,  therefore ,  t o  show t h a t  a re la t ionship does e x i s t  between actual 
weld area and the r e s u l t s  obtained w i t h  the  acoustic microscope. T h i s  was 
accomplished by generating acoustic micrographs o f  welds having various geo- 
metric configurations and using the acoustic images t o  map out areas t o  be 
sectioned f o r  metallographic analysis.  
and bond area were t h u s  obtained. 
Although the idea of 
Therefore, welded s o l a r  c e l l  inter-  
I t  has previously been shown ( r e f .  2 )  t h a t  acoustic images of weldea 
I t  was t h e  purpose of t h i s  inves- 
Correlations between acoustic r e s u l t s  
251 
https://ntrs.nasa.gov/search.jsp?R=19830007563 2020-03-22T00:05:24+00:00Z
MATERIALS, APPARATUS, AND PROCEDURE 
Welding 
Two types of electric resistance weld electrodes (fig. 1) were used to 
make the welds that were analyzed in this program. Figure l(a) shows a pair 
of conventional parallel-gap electrodes made of molybdenum. The tips were 
rectangular in cross section measuring 0.65 by 1.25 mm on a side, with a gap 
of 0.25 mm. Figure l(b) shows a new annular-gap electrode design, also made 
of molybdenum, which consists of a center electrode surrounaed by dn annular 
electrode. 
by C. R. Baraona and A. F. Furestieri). The outside diameter of tne annulus 
was 2.0 mm and the inside diameter was 1.5 mm. 
(The annular gap design was developed at the Lewis Research Center 
The gap width was 0.13 mm. 
The interconnect materials were either copper or silver foil, 0.05 mm 
thick. Solar cells were 2 by 4 centimeters and had wraparound contacts. The 
cells were not encapsulated in or attached to a solar array substrate or blan- 
ket material such as Kapton or aluminum honeycomb. 
made on glass covered cells, 400 pm thick, while the annular-gap welds were 
made on uncovered cells, 225 pm thick. The interconnects were welded to both 
positive and negative contact surfaces. 
1.0 V, applied for 50 to 100 ms. 
36 N on the tab for the duration of the welding operation. 
The parallel-gap welds were 
Voltage settings ranged from 0.6 to 
The electrode tips maintained a load of 9 to 
Nondestructive Evaluation 
The scanning laser acoustic microscope and its principles of operation 
are described in detail in reference 3. 
the application of the acoustic microscope for evaluation of solar cell welds. 
The solar cell is placed on the sample stage with the interconnect tab on top. 
A coverslip consisting of a clear plastic material and a film of gold a few 
angstroms thick on one surface is placed on top of the tab. The purpose of 
the coverslip is to provide a reflective surface for the laser beam in instan- 
ces where the surface of the material being evaluated is not sufficiently re- 
flective. The ultrasonic transducer, located at the bottom of a shallow well 
in the top surface of the sample stage, produces continuous waves at a frequen- 
cy of 100 MHz and an incident angle of loo to the cell surface. Tne ultrasonic 
waves are transmitted through the water couplant and into the solar cell. 
The energy is further transmitted into the interconnect tab wherever bonding 
exists. 
interconnect tab sets up a dynamic ripple which is then transmitted to the 
cover slip by a water coupling medium. A laser beam constantly scans the area 
in raster fashion. The intensity of the reflected laser light is modulated by 
the dynamic ripple and is proportional to the amplitude of sound waves at the 
reflective surface of the coverslip. These variations in light intensity are 
seen by the photodetector as an AC signal which is processed and used to pro- 
duce a real-time TV image in black and white. 
screen represent areas o f  highest ultrasonic amplitude and black regions are 
indicative of little or no ultrasonic transmission. 
an area 2.3 by 3 mm on a side at a magnification of 75. 
at a magnification of 32 were made by photographing the TV screen using Polar- 
oid film. 
Figure 2 shows a sketch illustrating 
The interaction of sound waves at an angle to the top surface of the 
The brightest regions on the 
The real-time image covers 
Acoustic micrographs 
252 
Metallography 
To determine the ability of the acoustic microscope to provide visual evi- 
dence of the integrity of interconnect-cell bonds, it was necessary to perform 
a destructive analysis on selected welds. 
welded joints that produced unique or interesting patterns on the acoustic mi- 
crographs and sectioning the cell to permit a cross sectional view o f  the bond 
line between the interconnect tab and the solar cell contact. Sectioning was 
done with a dicing saw. The weld specimens thus removed were mounted on edge 
in an epoxy encapsulation that served as a specimen holder. The encapsulated 
specimens could then be ground in specified increments to obtain views of 
planes at predetermined locations within the weld joint. 
bond or nonbond in each plane was determined metallographically. Standard 
techniques were used to polish the metallographic specimens. 
microstructural study of the weld joint was not required, photo-micrographs 
were taken of the surfaces in the unetched condition only. 
sidered to be bonded when no interfacial features indicative of disbond could 
be detected at a magnification of 750. 
This was accomplished Dy selecting 
The extent o f  tne 
Since a complete 
Surtaces were con- 
RESULTS AND DISCUSSION 
Results obtained with the scanning laser acoustic microscope and the relat- 
ed metallographic analyses are presented in figures 3 to 5. Each figure shows 
a photograph of the surface o f  the interconnect tab after welding, an acoustic 
micrograph of the region of the Weld, and a photomicrograph of a crossection 
through tile middle of the weld. The ability of the acoustic microscope to de- 
scribe the size and shape of the bonded area is discussed. 
Figure 3 shows the results obtained from an analysis of a joint made by 
welding a copper interconnect tab to a silver solar cell contact using the 
parallel-gap resistance welding technique. A partial imprint of the electrode 
pair was embossed in the surface of the relatively soft copper (fig. 3(a)). 
The partial imprint indicates that the electrodes applied nonuniform pressure 
to the tab, probably because of a minor misalinement relative to the solar cell 
surface. A complete imprint would consist of two parallel rectangles which 
resemble footprints. Figure 3(b) is an acoustic micrograph of the weld and 
surrounding area. On first observation it was not clear whether the welu zone 
was represented by the white region of the acoustic micrograph only or by the 
white area plus the smaller gray area immediately to the left. Thus, the ac- 
tual size and shape of the weld had to be accurately determined by other means. 
This was accomplished in two ways. First, it was found that the unbonded part 
of the copper tab could be removed by folding it back over the bonded part and 
carefully tearing it at the perimeter of the weld nugget with tweezers, leaving 
the welded part of the tab in place. The result of this operation is shown in 
figure 3(c). The welded copper material is intact in the center of the pic- 
ture, surrounded by the silver solar cell contact surface. The black residue 
on the contact surface is a material of unknown origin which was present be- 
neath the unbonded part of the tab. Note the partial footprint of the right 
hand electrode which covers almost half of the welded copper tab. It is ob- 
vious that the weld nugget was not centered between the electrodes nor was it 
confined to the region beneath the electrodes. It is also apparent that, al- 
though the electrode footprints provide some information regarding the general 
location, they say little about either the size or the shape of the weld. 
ures 3 ( b )  and (c) show that the weld is represented only by the white area on 
Fig- 
2 5 3  
the acoustic micrograph, which closely approximates the size and shape of the 
weld nugget. Although not apparent in this series of photographs, it was fur- 
ther observed that the acoustic microscope can also reveal the location of the 
weld relative to a feature such as the edge of the tab, which is visible in 
the top of figure 3(a). 
tile nature of the bond was obtained through a metallographic analysis. 
3(b) shows two dark spots within the white region of the acoustic micrograph, 
indicating the presence of defects which blocked ultrasonic transmission. 
cross section through the larger of these defects is shown in figure 3(d). 
The photomicrograph reveals a bonded section measuring 0.78 mm long with a 
nonbonded portion in the middle. The nonbond, which corresponds to the dark 
spot on the acoustic micrograph, is 225 urn long and only 4 urn wide. Other 
bondline pores less than 30 urn in diameter are visible in the photomicrograph 
but were not resolved individually in the acoustic micrograph. 
Additional information about the dimensions of the weld zone as well as 
Figure 
A 
Figure 4 presents the results of an analysis of a joint made by welding a 
silver interconnect tab to a silver solar cell contact using the annular-gap 
electrodes. 
4(a). The circular feature was made by the annular electrode, and the spot in 
the middle is the result of pressure applied by the center electrode. 
imprint on the tab is closely approximated in size and shape in the acoustic 
micrograph (fig. 4(b)), indicating that welding occurred beneath a major 
portion of the circumference of the annular electrode and the middle part of 
the center electrode. Supporting evidence is shown in the photomicrograph of 
a crossection through the middle of the weld (fig. Li(c)). 
sections, one in the middle of the photomicrograph and one on the right side, 
correspond to white areas of ultrasonic transmission in the acoustic 
micrograph. Little or no bonding is evident on the left side of the 
photomicrograph, which agrees with the information in the acoustic micrograph. 
The electrode imprint on the tab surface is shown in figure 
The 
Two bonded 
Figure 5 shows the results obtained from another silver-silver weld made 
with annular-gap electrodes. The imprint embossed in the tab surface (fig. 
5(a)) looks similar to the one in figure 4(a). However, the acoustic micro- 
graph (fig. 5(b)) indicates that the weld geometry is quite different from 
that in figure 4. Bonding appears to have taken place primarily under one 
side of the annular-gap electrode, completely spanning the gap. 
ing appears to have taken place under the other half. The photomicrograph 
(fig. 5(c)) of the cross section through the middle of the weld confirms the 
information obtained from the acoustic micrograph. With the exception of some 
bondline porosity, the bond between the tab and the metallized contact is 
continuous under the right hand side of the electrodes. On the left, little 
or no bonding exists, again confirming the evidence presented in the acoustic 
micrograph. 
Little bond- 
CONCLUDING REMARKS 
The scanning laser acoustic microscope appears to be a viable tool for non- 
destructive evaluation of solar cell interconnect bonds made by welding. 
Metallographic analyses of selected welds showed that acoustic microscopy can 
accurately determine the size and shape of welded areas in both uncovered 
cells and cells with a protective glass cover adhesively bonded to the front 
sJrface. Although the glass cover and adhesive increase scatter and decrease 
254 
resolution, the adverse effects appear to be small, and the final result is 
acceptaole for cells in the thickness range evaluated in this investigation. 
I n  addition to generally descriaing the size and shape of weld areas, the 
acoustic microscope demonstrated a capability for detecting small nonbonaed 
?reas of the order of 200 pm diameter within an otherwise continuous weld zone. 
Conversely, it is expected that weldeo areas 200 pm diameter and larger could 
be detected against a background of unweldea material. 
The results of this investigation suggest that application o f  scanning 
laser acoustic microscopy can be extended from individual solar cells to solar 
cell arrays, providing the cells are not encapsulated or attachea to a sub- 
strate or blanket material in such a way as to block the transmission of acous- 
tic energy. Any air gap between the substrate and the cell in the region to 
be nondestructively evaluated would preclude evaluation by ultrasonic trans- 
mission. It is therefore recommended that array manufacturers consider the 
requirements for successful nondestructive evaluation in the design stage to 
insure that accommodations for adequate inspection are made. 
REFERENCES 
1. Keinhartz, K.; and Capart, J.: Status of helaed Solar Cell Module 
Technology at ESRO, Proceedings of the Eighth IEEE Photovoltaic 
Specialists Conference, 1970. 
Cells and Solar Cell Welds. Presented at the Fifteenth IEEE Photovoltaic 
Specialists Conference, May 12-15, 15181. 
2. Stella, P. M.; Vorres, C. L.; and Yuhas, D. E.; SLAM Examination of Solar 
3. Kessler, L. W.; ana Yuhas, 0 .  E.: Acoustic Microscopy - 1979. Proceedings 
of the IEEE, Vol. 67, No. 4, April 1979. 
255 
(a )  Conventional parallel-gap electrodes. 
(b) Annul  ar-gap electrodes. 
Figure 1. - Resistance weld electrodes used fo r  joining solar ce l l  
interconnects. 
r C L E A R  COVERSLIP DYNAMIC RIPPLE-\ 
REFLECTIVE FILM-., 
TRANSDUCER 
SAMPLE STAGE 
Figure 2. - Application o f  the scanning laser  acoustic microscope for  
nondestructive evaluation pf so lar  interconnect welds. 
256 
7 
I \-- WELDED UNWELDED WELDED 
( a )  Pa r t i a l  imprint on t a b  surface. 
(b )  Acoustic migrograph o f  weld zone. 
( c )  Nugget s i z e  revealed by removal of unwelded portion of tab. 
( d )  Photomigrograph of c ross  sec t ion  A-A through middle o f  weld zone. 
Figure 3.  - Results of des t ruc t ive  and nondestructive ana lys i s  of Cu-Ag 
para l le l -gap  weld. 
257  
(a) Complete electrode imprint of tab surface. 
( b )  Acoustic micrograph on nearly complete weld. 
(c) Photomicrogrdph of cross section A-A through middle of weld zone. 
Figure 4. - Results of destructive and nondestructive analysis of Ag-Ag annular 
-gap weld zone. 
1 A 
(a) Complete electrode imprint of tab surface. 
( b )  Acoustic micrograph on nearly complete weld. 
(c) Photomicrograph of cross section A-A through middle of weld zone. 
Figure 5. - Results of destructive and nondestructive analysis of Ag-Ag annular 
-gap weld zone. 


Evaluation of solar cell welds by scanning acoustic microscopy

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