JPH04315909A - Layer thickness measuring method - Google Patents

Abstract

PURPOSE:To measure each thickness of a copper wiring layer and a solder plating layer laminated on an insulating layer in order, by using ultrasonic waves. CONSTITUTION:Supersonic waves are injected to a printed wiring plate 2 from a transmitter side piezoelectric element 12 at the incident angle theta1 set in the scope 55 deg.<=theta1<90 deg., and the reflected wave to the incident waves is received by a receiver side piezoelectric element 14. A reflected wave spectrum is form from the reflected wave in an FFT analyzer 24. In this case, when the incident angle theta1 is set within the above scope, two dip frequencises appeared on the reflected wave spectrum have the characteristics relating to the thicknesses of a copper wiring layer 6 and a solder plating layer 8 of a printed wiring plate 2 respectively. As a result, by finding this relation experimentally or theoretically beforehand, the layer thicknesses of the copper wiring layer 6 and the soler plating layer 8 can be found respectively by detecting the two dip frequencies when the layer thickness is measured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a layer thickness measuring method for measuring the thickness of a solder plating layer and a copper wiring layer in a printed wiring board, for example.

0002

[Prior Art] In the manufacturing process of printed wiring boards,
First, a copper wiring layer is formed on the surface of a substrate whose top layer is an insulating layer such as a polymer, and then solder plating is applied to the surface of this copper wiring layer to form a solder plating layer. Before solder plating is applied here, the thickness of the copper wiring layer (
As a conventional method for non-destructively measuring layer thickness, there is known a measuring method using ultrasonic waves proposed by the inventors of the present application in Japanese Patent Laid-Open No. 63-285406. below,
The principle of this method will be briefly explained.

[0003] When broadband impulse ultrasound is applied to the surface of an object having a copper wiring layer formed on the surface of an insulating layer from above the object through an ultrasonic propagation medium such as water at an incident angle θ, the copper wiring layer is formed on the surface of an insulating layer. A surface wave closely resembling a Lamb wave (hereinafter referred to as a pseudo-Lamb wave) is excited along the surface of the object at a specific frequency component that is inversely proportional to the layer thickness of the layer. Then, in the reflected wave spectrum reflected from the surface of the object, only the intensity of the frequency component that excited the pseudo Lamb wave decreases, so that a minimum intensity portion appears in the reflected wave spectrum. The frequency of this minimum intensity part (dip frequency) depends on the layer thickness, so by determining the relationship between the dip frequency and layer thickness experimentally or theoretically in advance, the The layer thickness of the copper wiring layer can be determined from the dip frequency on the reflected wave spectrum.

0004

Since the above method is carried out before the solder plating layer is formed, it is necessary to measure the thickness of the solder plating layer separately from the thickness of each copper wiring layer. However, from the viewpoint of quality control of printed wiring boards and rationalization of measurement work, it is desirable to simultaneously measure the solder plating layer thickness and the copper wiring layer thickness by a rapid non-destructive method after the solder plating is formed. Therefore, if an attempt is made to simultaneously measure the solder plating layer thickness and the copper wiring layer thickness using the above-mentioned ultrasonic measuring method, the following problems will occur.

When a solder plating layer is formed on the surface of the copper wiring layer, the layer to be measured on the insulating layer of the board has a multilayer structure, so the surface waves excited on the surface of the solder plating layer are This is significantly different from the pseudo-Lamb waves excited on the surface of the wiring layer. Further, the dip frequency appearing in the reflected wave spectrum is not simply inversely proportional to the thickness of the solder plating layer or the copper wiring layer, but is generally a complex function. Therefore, it is impossible to uniquely measure the solder plating layer thickness and the copper wiring layer thickness from the reflected wave spectrum.

The present invention has been made in view of the above problems, and its purpose is to provide a layer thickness measuring method that can simultaneously measure the solder plating layer thickness and the copper wiring layer thickness using ultrasonic waves. It is in.

[0007]

[Means for Solving the Problems] In order to achieve the above object, in the layer thickness measurement method of the present invention, when the ultrasonic wave is incident on the specimen and the reflected wave is received, the transmission means transmits a signal to the upper surface of the specimen. A stroke in which at least one of the incident angle and the receiving angle from the subject by the receiving means is set to an angle of 55 degrees or more and less than 90 degrees, and the intensity of the received reflected wave is determined as a function of the frequency of the ultrasonic wave. a step of forming a reflected wave spectrum shown as , and a step of detecting frequencies f1 and f2, which are the frequencies of two different minimum intensity parts on the reflected wave spectrum and whose magnitudes are f1 < f2, Based on the relationship between the copper wiring layer thickness given in advance and the strength minimum part frequency, the copper wiring layer thickness of the test object is determined from the detected frequency f1, and the thickness of the copper wiring layer given in advance is determined from the solder plating layer thickness and the strength minimum part frequency. The present invention is characterized by comprising the step of determining the solder plating layer thickness of the object from the detected frequency f2 based on the relationship with the frequency.

In this case, the ultrasonic waves transmitted from the transmitting means may be broadband impulse waves, burst waves transmitted while sweeping the frequency, or plane waves or focused waves.

[0009]

[Operation] In the process of making the present invention, the inventors of the present application have proposed that ultrasonic waves be incident on the surface of a test object, on which a copper wiring layer and a solder plating layer are sequentially formed on an insulating layer, through an ultrasonic propagation medium. The frequency f of a surface wave excited by an ultrasonic wave incident at an angle θ was theoretically determined by numerical calculation using the method shown in the following document (1). In this case, many different modes of surface waves are excited. Furthermore, similar calculations were performed while successively changing the layer thicknesses of the copper wiring layer and solder plating layer of the test object, and the results of the calculations were examined, and the following new findings were obtained. That is, it has been found that surface waves excited at an incident angle in the range of 55 degrees to 90 degrees are classified into two completely different modes. More specifically, in one mode, the frequency strongly depends only on the layer thickness of the solder plating layer,
It is hardly affected by the layer thickness of the copper wiring layer. On the other hand, in the other mode, the frequency strongly depends only on the layer thickness of the copper wiring layer, and almost does not depend on the layer thickness of the solder plating layer.

Therefore, when an ultrasonic wave is incident on the surface of an object through an ultrasound propagation medium at an incident angle in the range of 55 degrees to 90 degrees, the spectrum of the reflected wave from the object with respect to the incident wave is as follows. Among the layer thicknesses of each layer of the test object, the frequency of the minimum intensity part (dip frequency) that depends only on the copper wiring layer thickness
f1 and a dip frequency f2 that depends only on the thickness of the solder plating layer appear independently.

[0011] Therefore, these two dip frequencies f1
, f2 (f1 < f2 ),
Based on the relationship between the copper wiring layer thickness and the dip frequency and the relationship between the solder plating layer thickness and the dip frequency given in advance, the solder plating layer thickness and the copper wiring layer thickness of the test object can be determined.

[0012] Although it is possible in calculation to excite a surface wave with an incident wave having an incident angle of 90 degrees, that is, horizontal to the surface of the object to be examined, it is impossible in reality. Therefore, the physically possible range of the above incident angle is 55 degrees or more and less than 90 degrees.

Furthermore, two types of modes similar to those described above can be obtained by setting the receiving angle (detection angle) for receiving the reflected wave from the object within the above range instead of the incident angle.

Literature (1): L. M. Brekhovsk
ikh, ”Waves in Layered Me
dia” pp41-61, Academic Pres.
s, New York, 1980.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a schematic configuration of a layer thickness measuring apparatus for carrying out the layer thickness measuring method of the present invention.

In the figure, a printed wiring board 2 as an object to be tested has a copper wiring layer 6 and a solder plating layer on the upper surface of an insulating substrate (substrate) 4 having a single layer structure made of an insulating material such as a polymer. 8 are sequentially stacked.

The ultrasonic sensor 10 placed above the subject 2 is a transmitting/receiving independent sensor comprising a transmitting piezoelectric element (transmitting means) 12 and a receiving piezoelectric element (receiving means) 14. This ultrasonic sensor 10
It is supported by an attitude control device 16 so that the inclination relative to the main body can be adjusted. The ultrasonic wave transmitting surface 12a of the piezoelectric element 12 and the ultrasonic wave receiving surface 14a of the piezoelectric element 14 are both flat. Water 18 as an ultrasound propagation medium is interposed between the transmitting surface 12a, the receiving surface 14a, and the subject 2. The transmitting side piezoelectric element 12 has a transmitting surface (plane) 12a.
Incident angle θ as the angle between the normal line of the object 2 and the normal line of the object 2
1 is set within a range of 55 degrees or more and less than 90 degrees. Further, the receiving side piezoelectric element 14 has a receiving surface (plane) 14a.
The reception angle (
The detection angle θ2 is set to be an acute angle. These incident angle θ1 and detection angle θ2 can be set to arbitrary angles by the attitude control device 16.

For example, an impulse signal source (not shown) is connected to the transmitting side piezoelectric element 12 as its drive signal source, and a pulsed broadband impulse signal P is applied from this signal source. As a result, high-frequency ultrasound as a broadband impulse wave is transmitted from the transmission surface 12a, and is incident on the surface of the subject 2 through the water 18 at an incident angle θ (55°≦θ<90°). A broadband reflected wave from the subject 2 in response to the incident wave is received by the receiving surface 14a of the receiving side piezoelectric element 14 via the water 18, and is converted into an electrical signal.

Note that a burst signal source may be used instead of the impulse signal source as the drive signal source to cause the transmitting piezoelectric element 12 to emit a burst wave of a specific frequency. In this case, by sweeping the frequency, the receiving piezoelectric element 14 can receive reflected waves in various bands.

[0020] The receiving side piezoelectric element 14 includes an amplifier 20, A
/D converter 22 and FFT analyzer 24 are connected in this order. The electrical signal converted from the reflected wave by the receiving piezoelectric element 14 is amplified by an amplifier 20, and then converted from an analog signal to a digital signal by an A/D converter 22. This digital signal is given to an FFT analyzer 24. The FFT analyzer 24 performs fast Fourier transform on the given digital signal.
A spectrum of the reflected wave is formed by performing frequency analysis using (transform, FFT). Further, the FFT analyzer 24 searches the formed reflected wave spectrum, detects the frequency of the minimum intensity portion (dip frequency), and outputs the detected value to an appropriate display device, recording device, or the like.

Next, the layer thickness measuring method of the present invention will be explained using the above-mentioned measuring device.

First, prior to measuring the subject 2, two or more reference subjects (not shown) are prepared. It is assumed that these reference objects have the same structure as the object 2 and that the thicknesses of the copper wiring layer and the solder plating layer are known. With respect to this reference object, the measuring device of FIG. 1 measures 55°≦θ
From the reflected wave spectrum when the ultrasonic wave is incident at an incident angle θ1 set in the range of <90°, two different dip frequencies f1' and f2'(f1'<f2
′) is detected. Among these dip frequencies, frequency f1' depends on the layer thickness of the copper wiring layer, and frequency f2' depends on the layer thickness of the solder plating layer. As a result, the relationship between the copper wiring layer thickness and the dip frequency and the relationship between the solder plating layer thickness and the dip frequency can be experimentally determined.

Next, measurements are performed on the subject 2. The measuring device of FIG. 1 is applied to the surface of the object 2 through the water 18 at an incident angle θ1 set to be the same as that of the reference object.
Dip frequencies f1 and f2 (f1 < f2) are detected from the reflected wave spectrum when an ultrasonic wave is incident at. Among these dip frequencies f1 and f2, for the dip frequency f1, the layer thickness d1 of the copper wiring layer 6 of the test object 2 is determined according to the relationship between the dip frequency f1' and the copper wiring layer thickness determined experimentally above. You can ask for it. Similarly, regarding the dip frequency f2, the layer thickness d2 of the solder plating layer 8 of the subject 2 can be determined according to the relationship between the dip frequency f2' and the solder plating layer thickness.

By the way, the above relationship is not determined by experiment, but by theoretically calculating the reflected wave spectrum when an ultrasonic wave is incident on the object 2 at an incident angle θ1, for example, using the method of the above-mentioned document (1). It may also be determined by

For example, in FIG. 2A, the layer thickness d1 of the copper wiring layer 6 of the test object 2 is 50 μm (constant), and the layer thickness d2 of the solder plating layer 8 is 10 μm (solid line), 20 μm (broken line), The frequency of the surface wave, calculated as 30 μm (dot-dash line), is shown as a function of the angle of incidence. In FIG. 2A, it can be seen that mode A has hardly changed, but higher-order modes B and B' have changed significantly. Table 1 shows the relationship between the thickness of the solder plating layer and the frequency of mode B for the case of an incident angle of 65 degrees.

Table 1 Layer thickness of solder plating layer (μm) 10
20
Dip frequency (Mhz)
44.7 24.2
From these Figure 2(A) and Table 1, mode B
It can be seen that the frequency decreases as the thickness of the solder plating layer 8 increases.

FIG. 2(B) shows the solder plating layer 8 of the test object.
The layer thickness of the copper wiring layer 6 is set to 40 μm, 5
The surface wave frequencies calculated as 0 μm and 60 μm are shown as a function of the angle of incidence. This figure 2 (B
), mode B remains almost unchanged, but mode A
It can be seen that there has been a big change. Here, Table 2 shows the relationship between the layer thickness of the copper wiring layer 6 and the frequency of mode A in the case of an incident angle of 65 degrees.

[0028]
Table 2 Copper wiring layer thickness (μm) 40
50 60
Dip frequency (MHz)
10 7.5
6 From these Figure 2(B) and Table 2,
It can be seen that the frequency of mode A decreases as the layer thickness of the copper wiring layer 6 becomes thicker. Therefore, two modes A
, B are obviously related to the layer thickness of the solder plating layer and the layer thickness of the copper wiring layer, respectively.

Next, two types of printed wiring boards 2 (hereinafter referred to as test objects 2a and 2b) were measured by the layer thickness measuring method described above.
, the dip frequency f1 of the two modes A and B
, f2, the layer thickness d1 of the solder plating layer 8 and the layer thickness d2 of the copper wiring layer 6 are measured. Here, the incident angle θ1 is 65°, the frequency band of the piezoelectric elements 12 and 14 is 5 MHz to 40 MHz, and the broadband impulse signal applied to the piezoelectric element 12 has a voltage of 125 V and a pulse width of 15 ns. Furthermore, as shown in FIG.
~53 μm copper wiring layer 6, layer thickness 5 μm or 10-20 μm
A solder plating layer 8 in the form of a tin-lead alloy having a thickness of μm was sequentially laminated. In this case, the copper wiring layer 6 has a layer thickness of 18 μm.
m (for 1/2 ounce) copper foil 6a, layer thickness 10-15
A first copper plating 6b having a thickness of 15 to 20 μm and a second copper plating 6c having a layer thickness of 15 to 20 μm are sequentially laminated. In addition, the subject 2a
, 2b and the sound speeds of longitudinal waves and transverse waves in the ultrasonic propagation medium (water) are shown in Table 3.

Specific gravity ρ Sound speed of longitudinal wave Cl (m/s) Sound speed of transverse wave Ct
(m/s) Water 1
1492
- solder
9.35 2745
1185
copper 9.11
4616
2307 Glasse 2
3000
1500 Poxy The results of this measurement example are shown in Table 4.

Table 4 Object Layer thickness d1 Layer thickness d2 Dip frequency f1 Dip frequency f2

Mode A (MHZ) Mode B
(MHZ) 2a 18
43 9
26
2b 15 43
9 30
As is clear from Table 4,
The dip frequency f1 of mode A does not change if the layer thickness d1 of the copper wiring layer 6 is the same, and the dip frequency f2 of mode B becomes lower as the layer thickness d2 of the solder plating layer 8 increases. Therefore, according to the layer thickness measurement method of the present invention,
The layer thickness d1 of the copper wiring layer 6 and the layer thickness d2 of the solder plating layer 8
can be measured independently. The fact that the layer thickness d1 of the copper wiring layer 6 and the layer thickness d2 of the solder plating layer 8 in Table 1 are accurate values can be confirmed by fracturing the specimens 2a and 2b after the measurement and observing the fractured surface with an optical microscope. Confirmed by observation.

[0032] In the above embodiment, the minimum intensity on the reflected wave spectrum is at an incident angle θ of 55°≦θ1 <90°.
A similar intensity minimum occurs when the reception angle θ2 is 55°≦θ2<90°.
You can also get it as Therefore, instead of setting the incident angle θ1, the reception angle θ2 may be set within the above range, and the same effects as in the above embodiment can be obtained.

Furthermore, except for the setting of the incident angle θ1 or the reception angle θ2, the layer thickness measuring method of the present invention does not depend on the details of the method of transmitting and receiving ultrasonic waves or the method of determining the reflected wave spectrum. That is, the measuring device for implementing the present invention is not limited to that used in the above embodiments, and various modifications are possible.

For example, in the above embodiment, the ultrasonic sensor 1
0, both the transmitting side and the receiving side are piezoelectric elements 1 for plane waves.
2 and 14, but as in the sensors disclosed in JP-A-2-251751 and JP-A-2-251752, one of them is used as a focused wave piezoelectric element and the other as a plane wave piezoelectric element. You may also use the Or,
The ultrasonic wave transmitted from the transmitting piezoelectric element 12 may be a broadband impulse wave or a burst wave. Further, the means for forming the reflected wave spectrum is not limited to the FFT analyzer 24, and various electronic devices commonly used for frequency analysis can be used.

Furthermore, the object 2 to be tested is not limited to a printed wiring board, and the present invention is applicable to any object in which a copper wiring layer and a solder plating layer are sequentially laminated on an insulating layer. . Furthermore, the structure of the substrate (substrate) 4 of the test object 2 is not limited to a single layer structure consisting of an insulating layer, but may be a multilayer structure in which an insulating layer and a wiring layer are laminated such that the uppermost layer is an insulating layer. The same effects as in the above embodiment can also be achieved.

[0036]

As explained above, according to the layer thickness measuring method of the present invention, even though the layer to be measured has a multilayer structure of copper wiring layers and solder plating layers, these copper wiring layers The layer thicknesses of the solder plated layer and the solder plated layer can be measured simultaneously using ultrasonic waves without affecting each other.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram schematically showing an apparatus for implementing the layer thickness measuring method of the present invention.

[Figure 2] Includes (A) and (B), where (A) shows the calculated value of the surface wave frequency as a function of the incident angle for changes in the solder plating layer thickness when the copper wiring layer thickness of the test object is constant. (B) is a diagram showing the calculated value of the frequency of the surface wave as a function of the incident angle with respect to the change in the thickness of the copper wiring layer when the thickness of the solder plating layer of the test object is constant.

FIG. 3 is a cross-sectional view showing the layer structure of a printed wiring board as an example of a test object.

[Explanation of symbols]

10... Ultrasonic sensor, 12... Sending side piezoelectric element, 14...
Receiving side piezoelectric element, 24...FFT analyzer.

Claims (5)

[Claims] Claim 1: A copper wiring layer and a solder plating layer are provided on the upper surface of a base having a single layer structure consisting of an insulating layer or a multilayer structure in which an insulating layer and a wiring layer are laminated such that the uppermost layer is an insulating layer. This is a method for measuring the layer thickness of the solder plating layer and the layer thickness of the copper wiring layer on a test object consisting of sequentially laminated layers. The incident wave is made incident on the upper surface of the object through a propagation medium, and the reflected wave from the object in response to the incident wave is received by the receiving means through the medium. and the reception angle from the subject by the receiving means are set to an angle of 55 degrees or more and less than 90 degrees, and the intensity of the received reflected wave is shown as a function of the frequency of the ultrasonic wave. a step of forming a reflected wave spectrum, a step of detecting frequencies f1 and f2, which are the frequencies of two different intensity minimum parts on the reflected wave spectrum and whose magnitudes are f1 < f2, and Based on the relationship between the copper wiring layer thickness and the intensity minimum frequency, the thickness of the copper wiring layer of the test object is determined from the detected frequency f1, and the thickness of the copper wiring layer and the intensity minimum frequency given in advance are determined. A layer thickness measuring method comprising the step of determining the solder plating layer thickness of the object from the detected frequency f2 based on the relationship. 2. Claim 1, wherein the ultrasonic waves transmitted from the transmitting means are broadband impulse waves.
Layer thickness measurement method described in . 3. The layer thickness measuring method according to claim 1, wherein the ultrasonic waves transmitted from the transmitting means are burst waves transmitted while sweeping the frequency. 4. The layer thickness measuring method according to claim 1, wherein the ultrasonic waves transmitted from the transmitting means are plane waves. 5. The layer thickness measuring method according to claim 1, wherein the ultrasonic waves transmitted from the transmitting means are focused waves.