Introduction of sinusoidal signal across welded structure to determine reliability of integrated circuit connections is discussed. In-phase frequencies and quadrature frequency functions are used to evaluate weld reliability. Schematic diagram of test equipment and components is provided

Feinstein, L.

Hruby, R. J.

English

NASA Technical Reports Server

September 1972	 B72-10296 
NASA TECH BRIEF 
Ames Research Center
	
-liv 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Nondestructive Testing of Microtab Welds 
The problem: 
Integrated circuits are generally mounted on a 
sealed header which supports leads for attachment to 
other components. As a rule, the interconnection tabs 
of a group of integrated circuits are spot welded to 
each other to provide high reliability connections and 
to maintain low weight and small size. Present meth-
ods of assessing the reliability of spot welds involve 
the destructive testing of sample welds, but a large 
number of welds must be tested to achieve an ac-
ceptable confidence level in quality control; since the 
testing procedure involves loss of considerable time 
and material, a nondestructive method for testing 
microtab welds was desired. 
The solution: 
Introduce a sinusoidal signal across the welded 
tabs, and sweep the signal frequency through the re-
sonant frequencies of the weld. The in-phase frequen-
cies and quadrature (±900 phase shift) frequency 
functions are used to evaluate weld reliability. 
How it's done: 
Each weld specimen or type exhibits a unique 
property of the in-phase frequencies or quadrature 
frequency functions over a certain range of signal 
frequencies; thus a weld has a complex series of sig-
natures which is related to weld-parameter combina-
tions (pressure, current, and time). Once the center 
frequency of the complex series of signatures has been 
located, the range of swept-frequency has to be estab-
lished so that individual signatures corresponding to 
each point in the weld schedule can be generated. 
Often, it will be necessary to exclude part of the
total frequency response so that fine structure can be 
resolved in a small range not necessarily around 
resonance. 
Signal t___, E o( t ) Source
	
Reactive	 , Quadrature 
FunctionSensing _____ Synthesizer 
	
Element	 t
and Recorder 
___land
 
	
Variable I I I Resistive	 In-Phase I 	 Function 
	
Inductive I I I Sensing 
	 Synthesizer 
	
Reactance I I I Element 	 Recorder 
The changes in the characteristics of the individual 
signatures as a function of weld pressure and current-
pulse duration are gradual. The nondestructive in-
spection procedure depends upon the ability to dis-
cern changes in weld parameters from point to point 
in the weld schedule; in preliminary work, individual 
signatures are recorded for variations of each point 
in the weld schedule and classified as a function of 
weld integrity. During production inspection, the 
complex series of signatures for each weld are noted 
and compared against the range of individual signa-
tures established for point-to-point differences in 
(continued overleaf) 
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weld schedules. If all the individual signatures fall 
within acceptable ranges, the weld is approved. 
The nondestructive method of testing welds de-
scribed above is not sensitive to ohmic contact resis-
tance between probe leads and welded tabs. In the 
system shown in the block diagram, contact resistance 
is avoided by capacitative coupling of the signal to 
the welded junction. This approach eliminates random 
error associated with metal-to-metal contacts since a 
surface potential barrier does not form and irregular-
ities in the metallic surfaces are averaged over the 
normal surface of the probe. A negligible voltage 
drop will be developed across the contact capaci-
tances at microwave frequencies, and the contact has 
a predictable reactance at radio frequencies. A capac-
itative contact is more stable and repeatable than an 
ohmic contact since capacitance stems from an inte-
grable average over the contact surface. For greater 
precision in narrow-band applications, the variable 
inductive reactance shown in the diagram can be 
used to neutralize a contact capacitance, but it need 
not be used when broad-band signatures are obtained. 
The test-system geometry must be recorded as part of 
the signature analysis; an important parameter in the 
geometry of the test system is the separation distance 
between the capacitive contacts shown in the diagram. 
The dimensions of the weld, probe separation, and 
tab-width depths must be small compared to the sig- 
nal free-space wavelength, because it is desired to 
operate with lumped elements in order to minimize 
the skin-depth problems associated with traveling 
waves. Signal current must pass through the circum-
ference of the weld. Skin-depth effects will limit cur-
rent penetration into the weld if the weld is good, 
but a poor weld will lead to greater penetration and 
this will be readily detected by its effect on the sweep-
frequency signature. 
The testing technique is uniquely adaptable to the
inspection of micro-circuit tab welds where the in-
spection signal frequency is in the radio-frequency to 
microwave range, and the inspection test circuitry is 
formed of ultraminiature lumped elements. The center 
frequency and probe diameter which can be used in 
such tests are interrelated because the reactance of 
the capacitors formed by the probe tips and the weld 
must be small in comparison to the significant reactive 
or resistive elements within the weld; alternatively, 
the capacitance can be neutralized or kept constant 
from test to test. 
Notes: 
1. As experience with this technique of weld inspec-
tion increases, it may be possible to eliminate the 
need for correlating admittance with weld sched-
ules for each job. There should be a range of ad-
mittance which is characteristic of a good weld for 
each tab geometry in some frequency range. The 
type of test circuitry used to establish these weld 
signatures will be determined by the center signal 
frequency. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 72-10296 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94035 
Source: Lester Feinstein and Ronald J
.
 Hruby
Ames Research Center

(ARC-10176) 
B72-10296	 Category 02


Nondestructive testing of microtab welds

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