Component assembly on printed circuitry keeps making giant strides toward denser packaging and smaller dimensions. From a single layer to multilayer, from through holes to surface mounted components and tape applied bonds, unrelenting progress results in new, difficult problems in assembling, soldering, inspecting and controlling the manufacturing process of the new electronics. Among the major problems are the variables introduced by human operators. The small dimensions and the tight assembly tolerances are now successfully met by machines which are faster and more precise than the human hand. The same is true for soldering. But visual inspection of the solder joints is now so severely limited by the ever shrinking area accessible to the human eye that the inspector's diagnosis cannot be trusted any longer. Solutions to correcting these problems are discussed

Vanzetti, Riccardo

English

NASA Technical Reports Server

INTELLIGENT MER SOLDERING 
INSPECTION AND PROCESS CONTROL 
Riccardo Vanzetti 
Vanzetti Systems, Inc. 
Stoughton, Massachusetts 
INTRODUCTION 
Component assembly on printed circuitry keeps making giant strides toward 
denser packaging and smaller dimensions. From single layer to multilayer, 
from through-holes to surface-mounted components (SMDs) and tape-applied bonds 
(TAB), unrelenting progress results in new, difficult problems in assembling, 
soldering, inspecting and controlling the manufacturing process of the new 
electronics. 
Among the major problems are the variables introduced by human operators. 
The small dimensions and the tight assembly tolerances are now successfully 
met by machines which are much faster and precise than the human hand. The 
same is true for soldering. But visual inspection of the solder joints is now 
I 
I that the inspector's diagnosis cannot be trusted any longer. It is a slow, 
I 
so severely limited by the ever-shrinking area accessible to the human eye 
costly, unreliable and often misleading relic of the pre-automation era. As a 
solution must be found to fill this gap. 
matter of fact, it is the only operation still being performed by humans. A 
I 
I 
I NOVEL APPROACH : THERMAL FLOW 
The solution to the problem of assessing the quality of a soldered joint 
is based on monitoring how heat flows through the joint itself. Evidently, 
heat injected at one end of the joint will spread through it, reaching the 
other end faster or slower, according to the quality of the heat transfer 
path. Any obstacle along its path (voids, inclusions, discontinuities, etc.) 
will slow down the heat transfer from one end to the other. Figure 1 shows 
I how this approach is applied to a "lap-joint." 
A measured pulse of laser radiation is injected on the surface of a 
"gull-wing" wire soldered to a pad. 
temperature at the point of heat injection will "see" a temperature rise 
during the laser heating pulse and a temperature decay after it. 
signal at the detector's output is called the "infrared signature" or "thermal 
signature" of the corresponding solder joint. It contains all the information 
needed to define the quality of the joint. 
signature is affected by the following four variables affecting the joint: 
An infrared detector measuring the 
The analog 
This is because the shape of the 
i a) surface cleanliness 
I b) surface emissivity 
I c) thermal mass 
1, 
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https://ntrs.nasa.gov/search.jsp?R=19870017779 2020-03-20T09:38:58+00:00Z
d) heat sinking 
Item a) is mainly reflected in the inltial "rise" of the signature, and 
it can be caused by residual flux or any kind of deposited material or film on 
the joint's surface. 
laser beam is automatically turned off prematurely. 
It often results in mini-fires, so that sometimes the 
Item b) gives information about cold solder joints, tin depletion of the 
solder bath, presence of contaminants (gold, copper, iron, etc.) in the solder 
alloy, and excessive intermetallic formation. 
Item c) indicates either excess of solder material or insufficiency of it 
(because of voids, dewetting and the like). 
Item d) (located at the tail end of the signature) indicates whether the 
solder joint is properly connected to the heat-sinks, such as the component's 
lead and the printed wiring leading away from the pad. 
Figure 2 shows typical signatures related to some of the conditions 
listed above. 
THE LASER/INSPECT SYSTEM 
The schematic diagram of the system which developed these signatures is 
shown in Figure 3. 
laser as the heat source, together with a 0.5 milliwatt Helium-Neon laser 
(coaxial with the YAG) which illuminates with visible light the point of the 
target being heat injected. The infrared detector is an In-Sb photovoltaic 
cell cooled at 77"  Kelvin and made blind to the lasers '  wavelengths, so that 
it will only receive the blackbody (or graybody) radiation between 2.5pm and 
5 . 5 ~ 1  emitted as a function of temperature by exactly the same area heated by 
the laser. 
It is called "Laser/INSPECT" and it utilizes a 30 watt YAG 
The p.c.b. under inspection is mounted on a very fast and precise X-Y 
table controlled by the computer, which is programmed t o  bring in rapid 
succession each solder joint under the focal point of the optical head. The 
computer also opens and closes the laser shutter and processes the signature 
information arriving from the detector after analog-to-digital (A/D)  conver- 
sion. The computer's memory holds, for each joint, the standard signature 
which is used as reference against which to compare the signature of the joint 
being inspected. The results are printed out by a fast printer, so that every 
joint has its own inspection certificate on a hard copy printout. 
A TV camera extracts the visible image of the area being inspected, and 
can be used for programming the X and Y coordinates of the joints to be 
inspected and also for watching the inspection process during operation. 
Figure 4 shows the Laser/INSPECT system and its crew. From left to right 
we see the programmer, standing in front of the closed inspection compartment, 
containing the lasers, the optical head, and the X-Y table with the p.c.h. 
under inspection. The laser power supply console and the computer console are 
under the inspection compartment. In the center, the systems operator sits in 
front of the monitor with its keyboard. At far left the printouts are being 
read to look for indications of defective solder joints. 
' 86 
In Figure 5 we see how the laser beam is focused on a solder joint to be 
inspected. According to the type of joint to be inspected, there is a "best 
angle" at which the laser beam can strike it most efficiently. Accordingly, 
the optical head can be tilted in the four directions +X, -X, +Y, and -Y. 
And what about the speed o€ operation? For plated-through holes, the 
laser pulse can take between 60 and 100 milliseconds. To these we'll have to 
add another 15 miliiseconds for taking three readings during the signature 
decay, plus 50 mill€seconds for the table movement to bring the next joint at 
the focal point of the optical system. The total t i m e  adds up to 125 or 165 
milliseconds per joint, or between 8 and 6 joints/second, for plated-through 
holes. For the much smaller SMD joints, a laser pulse of 25 milliseconds is 
adequate, so that the total time to inspect one jolnt and move to the next 
position can be reduced to 90 milliseconds, which results in an operational 
speed of 11 joints/second. This is certainly faster than any visual inspec- 
tion deserving such an appellative, since this could vary between 5 
joints/second and 5 joints/minute, according to the quality requirements of 
the electronics to be inspected. 
STATISTICAL DATA 
The information supplied by the Laser/INSPECT (L/I) system can he 
elaborated and outputted by the computer in several different formats. A 
comparative evaluation program carried out by Texas Instruments is worth 
mentioning. In order to obtain permission by the U.S. Navy to use the 
Laser/INSPECT system instead of visual inspection for the electronics p.c. 
boards of the HARM (High-Velocity Antiradiation Missile) production, Texas 
Instruments (TI) ran 186,570 solder joints first through the L/I system and 
then through conventional visual inspection (accordingly to the prescribed 
MIL-SPECS). 
Figure 6 shows the result of the final comparison of the two different 
inspection approaches. For only 57% of the joints there is agreement between 
visual and the Laser/INSPECT. For the remaining 43% there is total disagree- 
ment ( 2 5 %  visual accepts, L/I rejects; 18% visual rejects, L/I accepts). Sub- 
sequent microsectioning of the "disagreed" joints proved in every instance 
that the L/I diagnosis was correct, as opposed to the visual diagnosis. 
On the basis of this evidence, the U.S. Navy authorized TI to use the L / I  
system instead of the visual inspection mandated by the MIL-SPECS. As a 
consequence, today TI delivers to the Navy better quality HARM electronics, 
while the cost of each missile has been cut in half. 
Histograms are useful in process€ng the L / I  date. They offer a quick way 
to verify whether the soldering process is within tight control or drifting 
out of it. Figure 7 is an impressive example of the large range of variations 
introduced by the human hand in the soldering operation. 
Figure 8 shows more examples of such histograms. In all these charts, 
the ordinate scale indicates in arbitrary numbers the value of the peak 
radiation of the infrared signature, while the abscissa indicates how many 
times each of those values was met by the signatures of the joints under test. 
c - &  8 7  
INSPECTING TAB ASSEMBLIES 
The Laser/INSPECT technology is applicable to most types of joints or 
bonds. Both the sizes of the laser beam and of the detector spot can be 
adjusted to meet the target dimensions. 
joints were inspected by a microscopic version of the Laser/INSPECT, in which 
the Laser spot is .022" (or 0.05mm) and the detector focal area .044" (or 
0.lOmm). Figure 10 shows some of these joints. 
Figure 9 shows a TAB assembly whose, 
It can be seen that seven of the joints were prevented from making 
electrical contact by some epoxy smear. This lack of contact is clearly 
reflected in the high peaks of their signatures, which contrast with the low 
peaks of the adjacent wires, both at left and at right in the oscilloscope 
display picture of Figure 11. 
INTELLIGENT LASER SOLDERER 
From inspection to reflow soldering the step is quite short. Just a few 
more milliseconds of exposure to the laser beam, and the solid solder becomes 
liquid. During the change of phase, the temperature won't change. Figure 12 
shows an oscilloscope display of such transition, where time runs along the 
abscissa and temperature rises along the ordinate. The two changes of phase, 
from solid to liquid (at left) and from liquid to solid (at right) are 
indicated by the two plateaux pointed by the arrows. 
inclined instead of horizontal, because in the area viewed by the detector 
there is simultaneously solid and liquld material. 
These plateaux are 
Ad hoc software processes the detector output and turns off the laser 
shortly after full liquefaction is observed. In this way every joint receives 
the right amount of heat, through a "custom-tailored" laser pulse precisely 
measured to the individual joint's needs. This is because no two joints are 
identical. They dif€er in thermal mass and in heat sinking, so that their 
heat requirements to achieve liquefaction are different. 
This is how the Laser Reflow Soldering System works, and this is why it 
is called intelligent. 
Besides intelligence, the system has speed. On SMD's soldering can 
proceed at an average speed of 4 joints/second ("average", since every joint 
will need a different dwell time, as dictated by the individual heat 
requirement). 
This 4 joints/second speed might appear slow when compared with the 1000 
joints/minute typical of the mass-soldering systems today in use. 
it is much faster BECAUSE THE JOINTS ARE ALREADY INSPECTED. Their infrared 
signature is already their inspection certificate. 
But in fact 
This means that the overall speed of the combined soldering and inspec- 
tion operation is faster for the laser approach. 
88 
NORMAL 
L A P  JOINT 
-
Lead 
Temperature "seen" by the 
Infrared Detector at the 
point of heat injection. 
L.... 
Threshold 
D E F E C T I V E  
MILLISECONDS 
Figure 1 .  Behavior of normal and de fec t ive  j o i n t s  during 
laser/thermal t e s t i n g .  
ommrn.urrm ool'.m- 
u u n t m  IO m 
w 14 n r e  2 I O  
r- .mr 
llDDCIID.0 
m. n + u  UI Ul 3 M t m  S .O 
m-r 
Figure 2.  Typical infrared signatures 
of good and defect ive  so lder  
j o i n t s  
89 
ORlGUVAt PABE IS 
OF POOR QUALrrV 
PREAMP OPTICAL 
I r HEAD I 
IR 
I 
FLEXIBLE 
OPTICAL 
FIBER 
7 
FAILSAFE 
SHUTTER 
T 
-INdECTOR/ 
MWITOR 
HEAD 
I PCB UNDER 
7 INSPECTION 
SERVO 
I I 
COMPUTER 1-1 INTERFACE I 
DEVICES 
Figure 3. Diagram of Laser/INSPECT system. 
Figure 4. Laser/INSPECT system. 
90 
ORWMl. PAGE IS 
OF POOR QUALW 
Figure 5. Laser beam exiting from tilted optical head. 
CORRELATION - 
una?.,! 
PRODUCTION PRINTED 
WIRING BOARDS 
GREEN - OC ACCEPTS/LASER REJECTS -- 
YELLOW - OC REJECTSlLASER ACCEPTS 
104 
TOTAL SOLDER JOINTS 
MICRO-SECTIONED 
BY CATEGORY 
Figure 6. Comparing visual inspection 
summary of 184,000 solder 
versus Laser /INSPE CT; 
joints 
91 
23G I 
I 
2.G I 
2:s I 
I 1 3  I 
205 It 
io0 I 
1 3  I 
MACHINE-SOLDERED 
Figure 7.  Comparing s o l d e r i n g  processes:  machine versus  manual. 
Good p r o c e s s  control L S  shown by 
n a r r o w  frequency distribution. 
Low s o l d e r  temperature or too htgh 
a c o n v e y o r  speed y i e l d s  heavy joints 
ORIGUVAL 
OF POOR 
Metalltc contarnlnation I" s o l d e r  Cold Iotntm mixed with good JOLDtS 
causes  granular or c loudy  
solder surfaces .  
=an resul t  from a variety of c a u s e s .  
Figure 8. Histograms d i s c l o s e  d r i f t i n g  
v a r i a b l e s  i n  s o l d e r i n g  process.  
92 
O R I G M  PAGE IS 
Figure 9. TAB assembly. 
Figure 10. Deta i l  of figure 9 ,  with seven unbonds. 
93 
ORIGINAL PAGE IS 
Of POOR QUALITY 
Figure 1 1 .  Seven unbonds surrounded 
by nine good bonds. 
Figure 12. Solder reflow: s o l i d  t o  l iqu id  t o  s o l i d .  
Temperature diagram. 
94 


Intelligent laser soldering inspection and process control

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