CA1159546A - Inspection of solder joints by acoustic impedance - Google Patents
Inspection of solder joints by acoustic impedanceInfo
- Publication number
- CA1159546A CA1159546A CA000376912A CA376912A CA1159546A CA 1159546 A CA1159546 A CA 1159546A CA 000376912 A CA000376912 A CA 000376912A CA 376912 A CA376912 A CA 376912A CA 1159546 A CA1159546 A CA 1159546A
- Authority
- CA
- Canada
- Prior art keywords
- solder
- solder joint
- joint
- acoustic
- frequencies
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Abstract
INSPECTION OF SOLDER JOINTS BY
ACOUSTIC IMPEDANCE
ABSTRACT:
A method for detection of unacceptable solder joints, especially solder pads on printed circuit boards (PCBs), by means of the application of acoustic vibrations over a range of frequencies to the solder joint and observation of the acoustic impedance of the joint as reflected back through the driver to the acoustic generator or through an acoustic detector near the generator to electrical detecting means.
ACOUSTIC IMPEDANCE
ABSTRACT:
A method for detection of unacceptable solder joints, especially solder pads on printed circuit boards (PCBs), by means of the application of acoustic vibrations over a range of frequencies to the solder joint and observation of the acoustic impedance of the joint as reflected back through the driver to the acoustic generator or through an acoustic detector near the generator to electrical detecting means.
Description
I~ESCP~[PTION~
INæECTICN OF SOIIDER JOINTS BY
ACOUSTIC IMPEDANCE
Background of the Invention In my U.S. patent, 4,21~,922, I described two methods for detecting flaws or unacceptable conditions in solder joints wherein an acoustic signal, with or without an electrical current, was applied on one side of a solder joint/ passed through the joint, and received on the other side. The input was m~dulated by the joint and a ccmparison between the m~dulated output and the input was used to identify bad solder joints.
It occurs m~ny times that a discrete element or integrated circuit is soldered by its lead wires to a conductor pad on a printed circuit board wherein the "other side" of the solder joint is inaccessible. For example, this is the case where the leadi is soldered to a conductor padi which passes through the substrate to connect elements on the other sidie. Therefore, it would be kenefi-cial to have a non-destruictive testing methodiwhereby the test could be run on the solder pad itself Common ultrasonic flaw detectors which apply an acoustic wave to the joint and receive back an echo fron the flaw are good for some flaws but not for othiers which are more subtle.
Summ~y of the Inventlon It is an object of the invention to provide a non-diestruct-ive, qualitative method of detecting unsatisfactory soldbr joints in electrical devi oe s.
.~ , .
. . . ~
It is also an object of the invention to provide a method which is quick and reliable and which can be easily automated.
It is also an object to provide such a 5 method which can be used with a wide variety of flaws and unsatisfactory bonding.
It is an object to provide an inspection method which can be applied to but one lead of the solder joint or merely to the joint itself.
In accordance with the objectives, the invention is an acoustic method for flaw detection in solder joints. ~he method utilizes the characteristic acoustic impedance of the solder joint which modulates an applied acoustic wave and 15 is observed by a receiving transducer. The electrical impedance of the receiver as affected by the acoustic impedance of the solder joint is monitored over a range of applied frequencies giving a description of the impedance as a function of 20 frequency. This impedance/frequency description is also known herein as the spectral response and is a graphic representation of the condition of the solder joint.
The method for inspection of defects in 25 a solder joint comprises applying an acoustic vibration to the solder joint, sweeping over a frequency range which preferably falls within the limits of 20 hz to 1 Mhz, or more preferably sweeping over a range of 150 khz-650 khz. The 30 frequency range should include at least one, and preferably more than one, natural resonant frequency of the solder joint. The acoustic vibrations pass through or are reflected back from the solder joint and are modulated thereby. The 35 vibrations are then received by a receiving ~ 5~3~6 transducer which is capable of converting the vibrations into electrical signals. The electrical impedance of the receiver (which is affected by the acoustic impedance of the solder joint) is then 5 observed as a function of applied frequency (the spectral response) and compared to standard spectral responses of known unacceptable or acceptable solder joints whereby the latter are used to characterize the tested solder joint as 10 unacceptable or acceptable.
The acoustic driver applying the acoustic energy and the receiver may be the same device in which case the reflected impedance will be observed.
The receiver may alternatively be a second 15 transducer located either near the driver or on the other side of the solder joint. Preferably the receiver and the driver are separate elements but are both contacted directly with the solder joint as far as possible apart during inspection.
20 Detailed Description of the Invention In Figure 1, an insulating substrate 1 is shown with a conductive feed-through 4 connecting metal conductor layers 9 and 10 on opposite sides of the substrate. Wires 2 and 3 are respectively 25 soldered to the metal conductor layers 9 and 10 by solder joints 6 and 5.
Acoustic transducer or driver 7 is used to apply an acoustic vibra~ion to the solder joint 6 to test whether joint 6 is acceptable. The 30 transducer is connected to a conventional acoustic generator ~not shown). The transducer may be piezoelectric, magnetostrictive, or electromagnetic.
These are preferably used for inspection at high, intermediate, and low frequencies, respectively.
~ 55~5~6 The invention may be used over a wide range of frequencies which is dependent mainly on the type and sensitivity of the solder joint and wire leads.
Miniaturized, sensitive joints and leads 5 may dictate the use of frequencies in the 1 r~hz range, whereas ordinary, heavier-wire joints may dictate the use of from about 20 hz-200 khz.
Frequencies swept over a range of 150-650 khz were found most useful in the course of experimenting 10 with the invention.
Returning to Fi~ure 1, acoustic driver 7 is shown at the toe end of the lead wire 2 and acoustic receiving means 8 is shown near position A
at the heel end thereof. This is the preferred 15 location of the driver and receiver in order to make more reliable comparisons of one joint wi~h others.
Coupling of the receiver to the solder joint constitutes an acoustic impedance and should therefore be kept constant from sample to sample in order 20 that it can be ignored in the comparison. Constant loading force should be applied to the receiver at the location at A which is the most stable on the joint. Other positions for the receiver, such as B
or even C and D, may be used if necessary, ~S however, they are not preferred. Positions C and D
would also involve interference of the impedance of solder joint 5 into the independent observation of the impedance of solder joint 6.
The coupling of the receiver to the joint 30 should be under modest pressure. ~f the driver has a high impedance compared to the joint, defects will be difficult to detect in a tightly coupled system.
On the contrary, if coupling is too light, only a small amount of energy can be transferred to the 35 joint and sensitivity will be low.
~ .
~595~;
The receiver 8 may be a conventional transducer pickup which is responsive to acoustic impedance and broad-band input. A high-resolution spectrum analyzer may be used to display or record 5 impedance data for manual inspection or to feed data to processing means for automated operation. The receiver should be responsive to input frequencies over the 20 hz-l ~lhz range~
Description of the Preferred Embodiments .
Printed circuit boards, each comprising a substrate and 20, 14-lead flat packs (integrated circuit elements) were prepared incorporating a variety of known defects such as inclusions, cold solder, voids, dewetting, poor filleting, excessive 15 solder, insufficient solder, gold entrapment, bent lead, cracks, etc. Good solder joints were also prepared as standards. A setup such as shown in Figure 1 was used to apply vibrations to the individual solder joints over a sweep of frequencies 20 in the range of 150 khz-650 khz. The receiver and driver were located as shown in Figure 1. The spectral responses obtained over the 150 khz-650 khz sweep (see Figures 2-4), show that major resonant frequencies in the transmitter were present at about 25 200 khz, 270 khz, and 520 khz~
The equipment used for testing were:
(1) A Wavetek 114 Sweep frequency generator to drive the acoustic transducer through a range of frequencies;
INæECTICN OF SOIIDER JOINTS BY
ACOUSTIC IMPEDANCE
Background of the Invention In my U.S. patent, 4,21~,922, I described two methods for detecting flaws or unacceptable conditions in solder joints wherein an acoustic signal, with or without an electrical current, was applied on one side of a solder joint/ passed through the joint, and received on the other side. The input was m~dulated by the joint and a ccmparison between the m~dulated output and the input was used to identify bad solder joints.
It occurs m~ny times that a discrete element or integrated circuit is soldered by its lead wires to a conductor pad on a printed circuit board wherein the "other side" of the solder joint is inaccessible. For example, this is the case where the leadi is soldered to a conductor padi which passes through the substrate to connect elements on the other sidie. Therefore, it would be kenefi-cial to have a non-destruictive testing methodiwhereby the test could be run on the solder pad itself Common ultrasonic flaw detectors which apply an acoustic wave to the joint and receive back an echo fron the flaw are good for some flaws but not for othiers which are more subtle.
Summ~y of the Inventlon It is an object of the invention to provide a non-diestruct-ive, qualitative method of detecting unsatisfactory soldbr joints in electrical devi oe s.
.~ , .
. . . ~
It is also an object of the invention to provide a method which is quick and reliable and which can be easily automated.
It is also an object to provide such a 5 method which can be used with a wide variety of flaws and unsatisfactory bonding.
It is an object to provide an inspection method which can be applied to but one lead of the solder joint or merely to the joint itself.
In accordance with the objectives, the invention is an acoustic method for flaw detection in solder joints. ~he method utilizes the characteristic acoustic impedance of the solder joint which modulates an applied acoustic wave and 15 is observed by a receiving transducer. The electrical impedance of the receiver as affected by the acoustic impedance of the solder joint is monitored over a range of applied frequencies giving a description of the impedance as a function of 20 frequency. This impedance/frequency description is also known herein as the spectral response and is a graphic representation of the condition of the solder joint.
The method for inspection of defects in 25 a solder joint comprises applying an acoustic vibration to the solder joint, sweeping over a frequency range which preferably falls within the limits of 20 hz to 1 Mhz, or more preferably sweeping over a range of 150 khz-650 khz. The 30 frequency range should include at least one, and preferably more than one, natural resonant frequency of the solder joint. The acoustic vibrations pass through or are reflected back from the solder joint and are modulated thereby. The 35 vibrations are then received by a receiving ~ 5~3~6 transducer which is capable of converting the vibrations into electrical signals. The electrical impedance of the receiver (which is affected by the acoustic impedance of the solder joint) is then 5 observed as a function of applied frequency (the spectral response) and compared to standard spectral responses of known unacceptable or acceptable solder joints whereby the latter are used to characterize the tested solder joint as 10 unacceptable or acceptable.
The acoustic driver applying the acoustic energy and the receiver may be the same device in which case the reflected impedance will be observed.
The receiver may alternatively be a second 15 transducer located either near the driver or on the other side of the solder joint. Preferably the receiver and the driver are separate elements but are both contacted directly with the solder joint as far as possible apart during inspection.
20 Detailed Description of the Invention In Figure 1, an insulating substrate 1 is shown with a conductive feed-through 4 connecting metal conductor layers 9 and 10 on opposite sides of the substrate. Wires 2 and 3 are respectively 25 soldered to the metal conductor layers 9 and 10 by solder joints 6 and 5.
Acoustic transducer or driver 7 is used to apply an acoustic vibra~ion to the solder joint 6 to test whether joint 6 is acceptable. The 30 transducer is connected to a conventional acoustic generator ~not shown). The transducer may be piezoelectric, magnetostrictive, or electromagnetic.
These are preferably used for inspection at high, intermediate, and low frequencies, respectively.
~ 55~5~6 The invention may be used over a wide range of frequencies which is dependent mainly on the type and sensitivity of the solder joint and wire leads.
Miniaturized, sensitive joints and leads 5 may dictate the use of frequencies in the 1 r~hz range, whereas ordinary, heavier-wire joints may dictate the use of from about 20 hz-200 khz.
Frequencies swept over a range of 150-650 khz were found most useful in the course of experimenting 10 with the invention.
Returning to Fi~ure 1, acoustic driver 7 is shown at the toe end of the lead wire 2 and acoustic receiving means 8 is shown near position A
at the heel end thereof. This is the preferred 15 location of the driver and receiver in order to make more reliable comparisons of one joint wi~h others.
Coupling of the receiver to the solder joint constitutes an acoustic impedance and should therefore be kept constant from sample to sample in order 20 that it can be ignored in the comparison. Constant loading force should be applied to the receiver at the location at A which is the most stable on the joint. Other positions for the receiver, such as B
or even C and D, may be used if necessary, ~S however, they are not preferred. Positions C and D
would also involve interference of the impedance of solder joint 5 into the independent observation of the impedance of solder joint 6.
The coupling of the receiver to the joint 30 should be under modest pressure. ~f the driver has a high impedance compared to the joint, defects will be difficult to detect in a tightly coupled system.
On the contrary, if coupling is too light, only a small amount of energy can be transferred to the 35 joint and sensitivity will be low.
~ .
~595~;
The receiver 8 may be a conventional transducer pickup which is responsive to acoustic impedance and broad-band input. A high-resolution spectrum analyzer may be used to display or record 5 impedance data for manual inspection or to feed data to processing means for automated operation. The receiver should be responsive to input frequencies over the 20 hz-l ~lhz range~
Description of the Preferred Embodiments .
Printed circuit boards, each comprising a substrate and 20, 14-lead flat packs (integrated circuit elements) were prepared incorporating a variety of known defects such as inclusions, cold solder, voids, dewetting, poor filleting, excessive 15 solder, insufficient solder, gold entrapment, bent lead, cracks, etc. Good solder joints were also prepared as standards. A setup such as shown in Figure 1 was used to apply vibrations to the individual solder joints over a sweep of frequencies 20 in the range of 150 khz-650 khz. The receiver and driver were located as shown in Figure 1. The spectral responses obtained over the 150 khz-650 khz sweep (see Figures 2-4), show that major resonant frequencies in the transmitter were present at about 25 200 khz, 270 khz, and 520 khz~
The equipment used for testing were:
(1) A Wavetek 114 Sweep frequency generator to drive the acoustic transducer through a range of frequencies;
(2) A Tektronix 434 Storage oscilloscope for monitoring the voltage to the driver.
Voltage was about 1 volt peak to-peak to keep the stress on the solder to less than 1 psi;
` ' : ''' ~
: .
~5~
Voltage was about 1 volt peak to-peak to keep the stress on the solder to less than 1 psi;
` ' : ''' ~
: .
~5~
(3) A Tektronix 7L5 Spectrum analyzer (associated with the receiver 8) and a Tektronix 7613 oscilloscope for monitoring the response of the system.
The principle on whlch the invention relies is that each solder joint presents the inspection apparatus with electrical properties (particularly impedance) characterizing the physical condit~on of the joint. Observation and comparison of the 10 impedance at various frequencies with that of known solder joints enables the characterization of the sample solder joint consistent with previously known similar joints.
For exampl~, one mechanism of response of 15 the lead wire and solder joint (as shown in Figure 1) to the vibrations may be related to the behavior of an elastic rod constrained along its length. If the rod is long and slender and constrained to eliminate flexural modes, it will act like a mechanical filter 20 to pass only those signals at frequencies at or near its longitudinal resonances. Hence, as the frequency of the applied wave sweeps through at least one resonant frequency of the joint, a peak will occur in the spectral response (impedance 25 versus frequency) curve. The shapes and heights of the resonance peaks will be a function of the conditions of the lead and the joint. Losses at grain boundaries and interfaces between solder and inclusions or voids, will affect 30 the shape. Defects which affect the stiffness of the joint will also affect amplitude o the spectral curve. Generally the broadened peaks indicate unsatisfactory conditions in the solder joint. Depressed peaks at the lower 35 frequencies and magnified peaks at the higher .
., : .
~ , ' frequencies also seem to indicate unsatisfactory conditions in the joint.
Figure 2 shows a typical spectral response of the output impedance (as measured by the voltage 5 drop) of the receiver in the inspection of so~ne good solder joints as the acoustic transducer applies vibrations to the solder joint in a sweep over the range of 150 khz-650 khz. The two peaks on the left are at about 200 khz and 270 khz. Several 10 small peaks appear in the 520 khz area.
The spectral responses of some defective solder joints are shown in Figures 3 and 4. The curves selected represent rather extreme cases.
Other marginally defective joints may produce 15 spectral responses that are not always significantly different from the good responses ma]cing it appear that the test is very subjective. However, we believe that this is occasioned only somewhat by the apparatus limitations and the test itself but 20 probably more by the limitations in manually preparing the "good" and "bad" samples. In other words, some of the attempts at making good and bad samples actually did not produce the desired good or bad joint. The test appeared to be a more 25 accurate indicator of good and bad joints than the preparation method was at preparing samples. The fact remains that the test easily identified severe defects and apparently identified marginally defective joints with good accuracy.
Figure 3 shows the spectral response of a joint containing minor inclusions. The broadening of the 200 and 270 peaks as well as the much greater effect on the amplitude of higher fre~uency peaks is evident.
~L9 5~ G
Figure 4 also shows the same effects due to a crack at the heel of the lead in the solder joint. The stiffness has been severely reduced causing a large increase in amplitude of the higher 5 frequency peaks.
The samples tested were fairly light leads and appeared to be most responsive to frequencies in the neighborhood of 500-600 khz. O~her lead sizes and solder joints might require lower or higher 10 ranges to be responsive.
The spectral responses of solder joints are truly characteristic of the conditions therein but the difference between the responses of a good joint and a bad joint can be subtle for some joints 15 which are close in their degree o defect.
Classifying the joint condition is therefore largely a matter of learning and experience on the part of the operator of the testing equipment. The experienced operator can use this test to identify 20 poor solder joints with a high degree of accuracy.
The principle on whlch the invention relies is that each solder joint presents the inspection apparatus with electrical properties (particularly impedance) characterizing the physical condit~on of the joint. Observation and comparison of the 10 impedance at various frequencies with that of known solder joints enables the characterization of the sample solder joint consistent with previously known similar joints.
For exampl~, one mechanism of response of 15 the lead wire and solder joint (as shown in Figure 1) to the vibrations may be related to the behavior of an elastic rod constrained along its length. If the rod is long and slender and constrained to eliminate flexural modes, it will act like a mechanical filter 20 to pass only those signals at frequencies at or near its longitudinal resonances. Hence, as the frequency of the applied wave sweeps through at least one resonant frequency of the joint, a peak will occur in the spectral response (impedance 25 versus frequency) curve. The shapes and heights of the resonance peaks will be a function of the conditions of the lead and the joint. Losses at grain boundaries and interfaces between solder and inclusions or voids, will affect 30 the shape. Defects which affect the stiffness of the joint will also affect amplitude o the spectral curve. Generally the broadened peaks indicate unsatisfactory conditions in the solder joint. Depressed peaks at the lower 35 frequencies and magnified peaks at the higher .
., : .
~ , ' frequencies also seem to indicate unsatisfactory conditions in the joint.
Figure 2 shows a typical spectral response of the output impedance (as measured by the voltage 5 drop) of the receiver in the inspection of so~ne good solder joints as the acoustic transducer applies vibrations to the solder joint in a sweep over the range of 150 khz-650 khz. The two peaks on the left are at about 200 khz and 270 khz. Several 10 small peaks appear in the 520 khz area.
The spectral responses of some defective solder joints are shown in Figures 3 and 4. The curves selected represent rather extreme cases.
Other marginally defective joints may produce 15 spectral responses that are not always significantly different from the good responses ma]cing it appear that the test is very subjective. However, we believe that this is occasioned only somewhat by the apparatus limitations and the test itself but 20 probably more by the limitations in manually preparing the "good" and "bad" samples. In other words, some of the attempts at making good and bad samples actually did not produce the desired good or bad joint. The test appeared to be a more 25 accurate indicator of good and bad joints than the preparation method was at preparing samples. The fact remains that the test easily identified severe defects and apparently identified marginally defective joints with good accuracy.
Figure 3 shows the spectral response of a joint containing minor inclusions. The broadening of the 200 and 270 peaks as well as the much greater effect on the amplitude of higher fre~uency peaks is evident.
~L9 5~ G
Figure 4 also shows the same effects due to a crack at the heel of the lead in the solder joint. The stiffness has been severely reduced causing a large increase in amplitude of the higher 5 frequency peaks.
The samples tested were fairly light leads and appeared to be most responsive to frequencies in the neighborhood of 500-600 khz. O~her lead sizes and solder joints might require lower or higher 10 ranges to be responsive.
The spectral responses of solder joints are truly characteristic of the conditions therein but the difference between the responses of a good joint and a bad joint can be subtle for some joints 15 which are close in their degree o defect.
Classifying the joint condition is therefore largely a matter of learning and experience on the part of the operator of the testing equipment. The experienced operator can use this test to identify 20 poor solder joints with a high degree of accuracy.
Claims (7)
1. A method for qualitatively inspecting the integrity of a solder joint joining first and second conductors comprising (a) applying acoustic vibrations to the solder joint over a range of frequencies which includes at least one natural resonant frequency of the solder joint, (b) receiving the acoustic vibrations, as modulated thereby, from the solder joint with a receiving transducer converting acoustic vibrations into electrical signals, (c) observing the spectral response of the receiving transducer, and (d) comparing the spectral response of the receiving transducer with previously determined spectral responses characteristic of either unacceptable or acceptable solder bonds whereby to determine the acceptability of the solder bond.
2. The method of Claim 1 wherein the range of frequencies falls between about 20 hz and 1 Mhz.
3. The method of Claim 2 wherein the acoustic vibrations are swept through a frequency range of about 150 khz-650 khz.
4. The method of Claim 1 wherein the range of frequencies includes at least two natural resonant frequencies of the solder joint.
5. The method of Claim 1 wherein the acoustic vibrations are applied to the first conductor or the solder joint and are received from the second conductor.
6. The method of Claim 1 wherein the acoustic vibrations are applied to the first conductor or the solder joint and are received from the first conductor or the solder joint.
7. The method of Claim 6 wherein the acoustic vibrations are both applied and received by the receiving transducer.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000376912A CA1159546A (en) | 1981-05-05 | 1981-05-05 | Inspection of solder joints by acoustic impedance |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000376912A CA1159546A (en) | 1981-05-05 | 1981-05-05 | Inspection of solder joints by acoustic impedance |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1159546A true CA1159546A (en) | 1983-12-27 |
Family
ID=4119894
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000376912A Expired CA1159546A (en) | 1981-05-05 | 1981-05-05 | Inspection of solder joints by acoustic impedance |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1159546A (en) |
-
1981
- 1981-05-05 CA CA000376912A patent/CA1159546A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |