JP4852310B2 - Optical transmission module - Google Patents

Description

The present invention relates to an optical transmission module and a semiconductor device that convert an optical signal into an electrical signal or convert an electrical signal into an optical signal.

  Optical transmission modules are highly demanded to increase the optical transmission speed as the amount of communication increases. The reason for this is that by using the existing optical fiber as it is and increasing the amount of transmission, it is possible to suppress the expensive investment of laying a new optical fiber. At present, the mainstream optical transmission speed has shifted from 2.5 Gb / s to 10 Gb / s. The need for 40 Gb / s transmission is increasing as the next generation.

With reference to FIG. 1, the configuration of an optical transmission module having a transmission rate of 10 Gbit / s will be described. Here, FIG. 1 is a block diagram of an optical transmission module having a transmission rate of 10 Gbit / s. In FIG. 1, on the light receiving side, the light transmitted from the optical fiber 14 is switched to an electrical signal by a photoelectric conversion module (photodiode module: PDM) 19 , and 16 demultiplexers (DMUX) ICs 22 It is replaced with a parallel signal and processed as an electrical signal.

On the other hand, on the light transmission side, 16 parallel electrical signals are aggregated into one signal by a multiplexer (MUX) IC 20 and transmitted to the optical fiber 15 as an optical signal by an electro-optic conversion module (laser diode module: LDM) 21. Is done.
In the 10 Gb / s optical transmission module, as indicated by the 300-pin MSA, the electrical signal portion is composed of 16 622 MHz × 16 lines. The electrical signal of the optical transmission module 50 is transmitted and received using the connector 23. The PDM 19, DMUX IC 22, LDM 21, MUX 20, and connector 23 are each mounted on a printed circuit board 27 to constitute an optical transmission module 50.

  In transmission / reception of an optical transmission module, high-precision design is performed to maintain a transmission waveform of light for highly reliable transmission. Specifically, on the receiving side through which a 10 Gb / s signal passes, between the PDM and DMUX on the transmitting side, between MUX and LDM on the transmitting side, crosstalk noise from the outside, an increase in loss in signal propagation, Ingenuity to suppress the occurrence of reflection as much as possible has been made in the wiring design on the printed circuit board.

  Patent Document 1 describes a semiconductor device in which a resin having a small elastic coefficient and a thermal expansion coefficient substantially equal to that of solder is filled between a flip chip mounted chip and a substrate.

JP 62-316447 A

However, it has been found that it is difficult to transmit a 40 Gb / s signal while maintaining a predetermined signal crystal quality by the design device on the printed circuit board that can be realized at 10 Gb / s.
Therefore, a promising technique is to connect a 40 Gb / s electrical signal directly between PDM and DMUX and between LDM and MUX using a coaxial cable with low attenuation and high noise resistance. As a result, it has become possible to transmit the signal without significantly reducing the signal waveform quality of 40 Gb / s.

  By the way, in order to install the coaxial cable in the MUXIC or DMUXIC, it is usually a ceramic PKG (package), and the PKG size (substrate plane and height) is increased. Conventionally, PGA (bin grid array) has been mainly used for mounting a ceramic PKG having a coaxial cable on a substrate. This is because priority is given to maintaining the authenticity of the connection. However, in order to transmit a 622 MHz electrical signal as an electrical signal with high quality, the connection between ICPKG and the printed circuit board must be a BGA (ball grid array) that can shorten the connection distance compared to PGA.

  On the other hand, ceramic PKGBGA always has a problem of connection reliability. This is because the thermal expansion coefficient (about 7ppm / ° C) of the alumina ceramic that is usually used is very different from the thermal expansion coefficient (14-16ppm / ° C) of the printed circuit board. This is to cause fatigue and shorten the connection life.

  Communication equipment is required to have a reliability of more than 10 years, and large ceramic BGAPEG requires underfill as a means to satisfy this requirement. However, no reliable mounting means has been disclosed so far for ceramic BGAPKG equipped with a coaxial cable.

  The present invention solves a highly reliable mounting method for a ceramic BGA package solder joint. According to the present invention, an optical transmission module and a semiconductor device having a highly reliable ceramic BGA package IC can be obtained.

  The above-described problem is implemented by mounting a multiplexer IC that multiplexes a plurality of electrical signals to generate a serial electrical signal, and an electro-optical conversion module that converts the serial electrical signal into an optical signal and sends the optical signal to an optical fiber. The multiplexer IC and the electro-optic conversion module are connected by a coaxial cable to transmit serial electrical signals. The multiplexer IC is connected to the printed circuit board by a plurality of solder balls, and the plurality of solder balls are surrounded by an underfill flow prevention frame. The periphery of the plurality of solder balls is achieved by the optical transmission module filled with the underfill so as to have a generally catenary profile between the side surface of the multiplexer IC and the inner surface of the underfill flow prevention frame. it can.

In addition, a photoelectric conversion module that converts an optical signal received from an optical fiber into an electrical signal and a demultiplexer IC that separates the electrical signal and generates a plurality of parallel electrical signals are mounted on a printed circuit board, and the photoelectric conversion is performed. module and the demultiplexer IC are connected by a coaxial cable to transmit electrical signals, the demultiplexer IC are connected with a printed circuit board and a plurality of solder balls, the plurality of solder balls surrounded by underfill flow prevention frame, around the plurality of solder balls by an optical transmission module that is filled by the underfill to have a generally catenary-shaped profile between the side surface and the underfill inner surface of the movement preventing frame demultiplexer IC, can be achieved .

  Furthermore, a semiconductor is mounted on a ceramic carrier, a plurality of solder balls are formed on the bottom surface of the ceramic carrier and connected to the semiconductor, and a C chamfer of 0.5 mm or more or an R chamfer of 0.2 mm or more is provided on the entire circumference of the bottom surface. This can be achieved by a semiconductor device.

  According to the present invention, an optical transmission module having a highly reliable ceramic BGA package IC and a ceramic BGA package IC can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings using examples. Note that the same reference numerals are assigned to the same parts, and description thereof is not repeated.
An embodiment will be described with reference to FIGS. Here, FIG. 2 is a block diagram of the optical transmission module. FIG. 3 is a partial cross-sectional view of a ceramic BGA portion with a coaxial cable of an optical transmission module. FIG. 4 is a plan view of a ceramic BGA portion with a coaxial cable of the optical transmission module.

  In FIG. 2, input light having a transmission rate of 40 Gbit / s from the optical fiber 14 is received by a photoelectric conversion module (PDM) 9. The PDM 9 converts an optical signal into an electric signal, and transmits a 40 Gbit / s normal phase signal and a reverse phase signal to the demultiplexer IC (DMUX) 12 via the coaxial cable 8. The DMUX 12 converts 40 Gbit / s electric signals into 622 MHz × 64 signals and outputs the converted signals from the 300-pin connector 13.

  On the other hand, 622 MHz × 64 electrical signals input from the outside to the 300-pin connector 13 are received by the multiplexer IC (MUX) 10. The MUX 10 converts 622 MHz × 64 electrical signals into a 40 Gbit / s normal phase signal and a reverse phase signal, and transmits them to an electro-optical conversion module (LDM: Laser Diode Module) 11 via two coaxial cables 8. The LDM 11 converts a 40 Gbit / s electrical signal into a 40 Gbit / s optical signal and outputs it from the optical fiber 15. Input / output electrical signals of the optical transmission module 100 are input / output using the connector 13. The PDM 9, DMUX IC 12, connector 13, LDM 11, and MUXIC 10 are mounted on the printed circuit board 7 to constitute the optical transmission module 100.

  The optical transmission module 100 shown in FIG. 2 includes both the PDM 9 and the LDM 11. However, in this specification, the optical transmission module includes an optical transmission module including only one of PDM and LDM. An optical transmission module including only the PDM is an optical transmission module, and an optical transmission module including only the LDM, which is an optical reception module.

Figure 3 is a diagram showing a mounting state of MUX10, DMUX12 of the optical transmission module described with reference to FIG. The MUX 10 and the DMUX 12 are composed of the ceramic substrate portion 1 and the metal portion 2 shown in FIG. 3, the coaxial connector portion 3 is formed on a part of the metal portion, and the coaxial cable 8 is attached to the coaxial connector portion 3. It is the structure which was made. Further, solder balls 4 are installed under the ceramic substrates 1 of the MUX 10 and DMUX 12 to make electrical connection with the printed circuit board 7. An underfill flow prevention frame 5 provided in advance is installed on a printed circuit board around the ceramic substrate, and an underfill material is filled and solidified between the ceramic substrate 1 and the underfill prevention frame. Here, the underfill 6 is formed so that the solder ball 4 has a substantially catenary profile between the side surface of the ceramic substrate 1 and the inner surface of the underfill flow prevention frame 5. Here, the suspended line shape can be expressed as a downwardly convex quadratic curve shape.

  Hereinafter, description will be made using specific numerical values. In FIG. 3, the transmission speed of the signal transmitted through the coaxial cable is 40 Gbit / s, the size of the ceramic substrate is 20 mm × 20 mm × 2.5 mm, and the size of the metal part is 22 mm wide × 25 mm long × 3.0 mm thick. A total of 361 solder balls are arranged in a 19 × 19 area array at a pitch of 1 mm. The underfill flow prevention frame has an outer shape of 25 × 25 × 2 mm (the height of the lower part of the coaxial connector is 1 mm), the thickness of the frame is 1 mm, and the material is the same as that of the printed board. In FIG. 3, the distance between the ceramic substrate 1 and the underfill flow prevention frame 5 is 2.5 mm. Here, as for the underfill flow prevention frame, the flow prevention frame was installed immediately below the PKG structure for the installation portion of the coaxial cable on the PKG, and the gap was provided between the PKG and the underfill flow prevention frame for the other portions.

In determining the distance between the underfill flow prevention frame 5 and the ceramic substrate 1 and the height of the underfill flow prevention frame 5, the following examination was performed.
First, from the viewpoint of high-frequency transmission, the wiring distance from the MUXIC 10 and DMUXIC 12 to the connector needs to be as short as possible, and the principle of wiring is to eliminate dead space such as an underfill flow prevention frame. However, passive components such as capacitors are installed around the MUX 10 and DMUX 12, but when these components are partially covered by the underfill, the underfill cure shrinkage stress is applied to damage the solder joints. .

  On the other hand, if the underfill does not form a smooth fillet from the side surface of the ceramic substrate as will be described later, the stress at the interface between the ceramic and the underfill increases and the possibility of peeling is high. Further, if the underfill is not applied to the side surface of the ceramic, a stress concentration portion is generated at a connection point between the underfill and the ceramic substrate, and in this case, there is a high possibility that peeling occurs. Accordingly, it is essential to form the underfill by forming a smooth slope from the side surface of the ceramic substrate in order to ensure the temperature cycle reliability of the solder balls. In that case, if an underfill start point is provided in the side surface portion 1 mm from the bottom surface of the ceramic substrate in the case of 1 mm pitch BGA, the bottom of the underfill spreads to about 10 mm from the end portion of the ceramic. In this case, an area where parts cannot be placed on the periphery of the ceramic substrate size of 20 × 20 mm (40 × 40 mm) is required (a floor area that is four times the floor area of the ceramic substrate is required). Will be forced. In FIG. 3, the undermount flow prevention frame can greatly reduce the component mounting prohibited area to 25 × 25 mm (1.6 times the floor area of the ceramic substrate).

  In addition, when the underfill is applied using a shrunk, if the underfill filling amount is too small as shown in FIG. 5, it is difficult to wet the side surface of the ceramic substrate of the underfill. As a result, cracks enter from the corners of the ceramic during the temperature cycle test and break the underfill. Furthermore, as a result, the underfill at the interface between the underfill and the ceramic substrate starts to peel off, causing a problem that the temperature cycle life of the BGA solder is shortened. In this case, the peeled surface reversely causes stress concentration in the BGA solder, and the temperature cycle life may be shortened compared with the case where there is no underfill.

  On the contrary, when the underfill filling amount is excessive, the fillet shape disappears as shown in FIG. 6, and there is a problem that the underfill is formed in a state close to a right angle from the ceramic substrate. In the case as shown in FIG. 6, the heat shrinkage stress generated between the underfill and the ceramic substrate during the temperature cycle test is almost perpendicular to the connection interface. As a result, the stress that peels the underfill from the ceramic is maximized. As a result, the underfill begins to peel from the side surface of the ceramic substrate during the temperature cycle, and the peeling progresses toward the bottom surface side of the ceramic substrate. Eventually, stress concentration occurs in the BGA solder, resulting in a mode similar to that shown in FIG. 5, and the temperature cycle life may be shorter than when there is no underfill.

As described above, it has been found that the fillet shape of the underfill has a great influence on the actual operation reliability of the ceramic BGA connection portion. On the other hand, as shown in Patent Document 1, it is known that the underfill material (Young's modulus (longitudinal elastic modulus), thermal expansion coefficient) has a great influence on the actual operation reliability of the ceramic BGA connection. However, information indicating specific numerical values for the underfill fillet shape and the underfill material is not disclosed, and there is no actual implementation of highly reliable ceramic BGA. In the configuration of FIG. 3, a numerical simulation was performed under the conditions shown in Table 1, and the relationship between the underfill material, the generated stress, and the generated strain was grasped. Table 1 shows that underfill has two levels (large and medium), the Young's modulus of the underfill material is 7.9 GPa and 4.0 GPa, and the thermal expansion coefficients are 30 ppm / ° C and 60 ppm / ° C. The relative comparison of stress and strain is shown.

Table 1
Under coefficient of thermal expansion Young's modulus A point stress B point stress C point strain Fill α E △ σA △ σB | △ εC |
Level (ppm) (GPa) (MPa) (MPa) (ppm)
-------------------------------------------------- ----------------
Large 60 7.9 175 849 1100
Large 30 7.9 75 469 675
Large 60 4.0 86 459 433
Large 30 4.0 44 257 64
Medium 60 7.9 169 349 1159
Medium 30 7.9 77 163 649
Medium 60 4.0 83 177 410
Medium 30 4.0 46 82 90

The representative values of the generated stress and generated strain in Table 1 are the stress at the corner A of the ceramic that is the base point of the underfill peeling that occurs during the temperature cycle in FIG. 5, and the stress at the B point that is the start point of the underfill fillet. It was set as the strain of C point of BGA solder. Since the stress at points A and B is subject to underfill peeling, the underfill stress is the representative value, and the stress at point C is the solder fatigue life, so the solder strain is the representative value. did.

  From Table 1, it was found that the stress at the point B (underfill start point) is lowered and hardly peeled even if the underfill fillet is inside even in the same material structure. There is no difference between the stress at the corner A of the ceramic and the strain at the BGA solder C point. From this, it has been found that the formation of the underfill fillet is important by the coating with the underfill coating amount being medium. Next, focusing on the stress at the corner A of the ceramic, it has been found that the stress becomes smaller as the Young's modulus of the underfill is smaller than in Table 1 or the thermal expansion coefficient is smaller. It was also found that when the Young's modulus and the thermal expansion coefficient change at the same rate, the change in Young's modulus is more effective for reducing the stress.

  Furthermore, when attention is paid to the strain at the BGA solder C point, when both the Young's modulus and the thermal expansion coefficient are reduced, the solder strain is reduced. The degree of thermal expansion coefficient was found to be more effective. In order to more clearly determine the material selection, FIG. 7, FIG. 8, and FIG. 9 were created. Here, FIG. 7 is a diagram for explaining the stress at the point A when the underfill is large. FIG. 8 is a diagram for explaining the stress at point B when the underfill is large. FIG. 9 is a diagram for explaining the distortion at the point C when the underfill is large.

  7 to 9, the thermal expansion coefficient and Young's modulus of the underfill, which are the material selection parameters, are shown as x-axis and y-axis, respectively, and the stress values at points A and B and the strain at point C are shown as contour lines. In each figure, the target allowable value is indicated by a bold line from the reliability data of the underfill coated part of the small ceramic BGA so far. This target value is 380 MPa or less for point A stress, 175 MPa or less for point B stress, and 0.4 ppm or less for point C strain. The reliability criterion is a condition that passes 500 cycles in the temperature cycle test of −40 to 85 ° C. of the telcordia GR-468-CORE standard that the optical transmission module should satisfy.

  From these results, the inventors determined that the maximum Young's modulus and thermal expansion coefficient of the underfill material are determined by the strain at the point C, and that the Young's modulus is 5 GPa and the thermal expansion coefficient is about 35 ppm / ° C. On the other hand, the minimum values of Young's modulus and thermal expansion coefficient of the underfill material are Young's modulus of 2 GPa and thermal expansion coefficient of about 20 ppm / ° C. based on the knowledge of the inventors. Therefore, the Young's modulus of the underfill material is 2 to 5 GPa and the thermal expansion coefficient is 20 to 35 ppm / ° C. The maximum value of the Young's modulus and the thermal expansion coefficient of the underfill material may be a Young's modulus of 6 GPa and a thermal expansion coefficient of about 40 ppm / ° C.

  Further, in FIG. 3, the corner chamfering is as small as 0.05 or less in normal ceramic cutting. In this case, stress concentration occurs in the underfill at point A, which is several times larger than the stress value in numerical calculation. For this reason, there was a problem that the temperature cycle life was short during the temperature cycle test because cracks were likely to occur at the underfill A point.

  On the other hand, stress concentration of the underfill generated at point A can be avoided by chamfering R0.2 or more at the corner of the ceramic. Further, as chamfering, C chamfering of C0.5 or higher has the same effect as R chamfering R0.2.

  Here, C chamfering is a process of cutting intersecting surface portions at 45 °. For example, it is C1 in the case of 45 ° cut at a location of 1 mm each. R chamfering is a process of rounding intersecting surface portions. If the radius of the round shape is 1 mm, it is R1.

It is a block diagram of an optical transmission module with a transmission rate of 10 Gbit / s. It is a block diagram of an optical transmission module. It is a fragmentary sectional view of the ceramic BGA part with a coaxial cable of an optical transmission module. It is a top view of the ceramic BGA part with a coaxial cable of an optical transmission module. It is a fragmentary sectional view of the ceramic BGA part with a coaxial cable of an optical transmission module (underfill filling amount is too small). It is a fragmentary sectional view of the ceramic BGA part with a coaxial cable of an optical transmission module (underfill filling amount is excessive). It is a figure explaining the stress of A point in case of underfill size. It is a diagram for explaining a stress point B in the case of A Ndafiru large. It is a figure explaining distortion of C point in case of underfill size.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ceramic substrate, 2 ... PEG frame, 3 ... Coaxial connector part, 4 ... BGA ball, 5 ... Underfill flow prevention frame, 6 ... Underfill, 7 ... Printed circuit board, 8 ... Coaxial cable, 9 ... Photoelectric conversion module (PDM), 10 ... Multiplexer IC (MUX), 11 ... Electro-optical conversion module (LDM), 12 ... Demultiplexer IC (DMUX), 13 ... Connector, 14 ... Input optical fiber, 15 ... Output optical fiber , 19: photoelectric conversion module (PDM), 20: multiplexer IC (MUX), 21: electro optical conversion module (LDM), 22: demultiplexer IC (DMUX), 23: connector.

Claims (3)

An optical transmission module in which a multiplexer IC that multiplexes a plurality of electrical signals to generate a serial electrical signal and an electro-optical conversion module that converts the serial electrical signal into an optical signal and sends it to an optical fiber are mounted on a printed circuit board. In
The multiplexer IC and the electro-optic conversion module are connected by a coaxial cable to transmit the serial electric signal,
The multiplexer IC is composed of a ceramic substrate part and a metal part, and has a coaxial connector part to which the coaxial cable is attached on a side surface of the metal part,
The multiplexer IC, the printed circuit board and are connected by a plurality of solder balls disposed in the lower portion of the ceramic substrate portion,
Wherein the plurality of solder balls, surrounded by underfill flow prevention frame,
The size of the ceramic substrate is 20 mm × 20 mm × 2.5 mm thick,
The size of the underfill flow prevention frame is 25 mm × 25 mm × 2 mm height, and the lower part of the coaxial connector part is 1 mm high,
The periphery of the plurality of solder balls is filled with underfill so as to have a generally catenary profile between the side surface of the multiplexer IC and the inner side surface of the underfill flow prevention frame ,
The underfill has a Young's modulus of 2 to 5 GPa and a thermal expansion coefficient of 20 to 35 ppm / ° C. An optical transmission module in which a photoelectric conversion module that converts an optical signal received from an optical fiber into an electrical signal and a demultiplexer IC that generates a plurality of parallel electrical signals by separating the electrical signal are mounted on a printed circuit board ,
The photoelectric conversion module and the demultiplexer IC are connected by a coaxial cable to transmit the electrical signal,
The demultiplexer IC is composed of a ceramic substrate part and a metal part, and has a coaxial connector part to which the coaxial cable is attached on a side surface of the metal part,
The demultiplexer IC, the printed circuit board and are connected by a plurality of solder balls disposed in the lower portion of the ceramic substrate portion,
Wherein the plurality of solder balls, surrounded by underfill flow prevention frame,
The size of the ceramic substrate is 20 mm × 20 mm × 2.5 mm thick,
The size of the underfill flow prevention frame is 25 mm × 25 mm × 2 mm height, and the lower part of the coaxial connector part is 1 mm high,
Around the plurality of solder balls is filled substantially by underfill to have a catenary profile between the inner surface of the the side of the demultiplexer IC underfill flow prevention frame,
The underfill has a Young's modulus of 2 to 5 GPa and a thermal expansion coefficient of 20 to 35 ppm / ° C. The optical transmission module according to claim 1 or 2, wherein
An optical transmission module, wherein a chamfer of 0.5 mm or more or an R chamfer of 0.2 mm or more is provided on the entire circumference of the connection surface on which the plurality of solder balls are formed.

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