JP2019021768A - Electronic device - Google Patents

Abstract

To provide an electronic device in which deterioration of solder life due to stress can be restrained, even when a filler added to resin is settled upon resin curing.SOLUTION: A print circuit board 3 is formed by laminating a core layer 3a on an upper surface and a surface layer 3b on a lower surface. Electronic components 4 are mounted on both surfaces of the print circuit board 3 by a solder 8. The print circuit board 3 is housed in an enclosure, and is filled with liquid sealant added with a filler so as to form resin layers 5, 6. On the lower surface of the print circuit board 3, a low distribution layer 6a is formed in contact with the electronic component 4 by settlement of the filler. The low distribution layer 6a has a large linear coefficient of expansion compared with the resin layer 6 containing the filler, and the stress of thermal strain due to temperature change is applied to the solder 8. Since the degree of elasticity (Young's modulus) is set low, the surface layer 3b can absorb stress, and thereby stress to the solder 8 is reduced and deterioration of solder life can be suppressed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electronic device.

  As an electronic device, there is a configuration in which an electronic component is mounted on a printed board or the like and resin-sealed in a state where the electronic component is disposed in a case. The reason for resin sealing is to improve waterproofness, dustproofing, and vibration resistance by sealing a printed circuit board on which electronic components are mounted with resin. In this case, generally, in the sealing resin, an inorganic filler is added to the resin in order to reduce distortion due to thermal expansion with the printed circuit board, solder, and case to be adhered.

  However, when a liquid sealing material is used as the resin layer, the filler settles during the period from the injection of the resin into the case to the curing, so that a layer that does not contain the filler may be formed on the upper surface of the resin. . When an electronic device in which electronic components are arranged on a printed board by double-side mounting is resin-sealed, a layer that does not include filler is formed at the interface between the resin and the printed board on the back side, that is, the lower side of the printed board. For this reason, the distortion of the thermal expansion of the resin layer which does not contain a filler is large, and there is a problem that the life of the solder provided on the portion where the electronic component is mounted is reduced.

  On the other hand, there is a technique in which the sedimentation of filler in the resin can be suppressed by changing the material composition. However, in the actual process, since there is a process of curing the resin at 100 ° C., it is practically impossible to suppress the filler sedimentation so as to leave the filler up to the height of the soldering portion. In addition, the filler sedimentation can be suppressed to some extent by increasing the viscosity in the process, but if the resin viscosity increases, a problem that the castability deteriorates occurs.

Japanese Patent Laid-Open No. 2008-181989 JP 2009-203431 A

  The present invention has been made in consideration of the above circumstances, and the purpose of the present invention is to make it possible to suppress a decrease in solder life due to the influence of stress even when the filler added to the resin settles during resin curing. To provide an apparatus.

  The electronic device according to claim 1 is filled with a printed circuit board on which electronic components are mounted using solder, a housing in which the printed circuit board is stored, and a cover that covers both surfaces of the printed circuit board. And the printed circuit board is configured by laminating two or more base materials having different elastic moduli in the thickness direction, and the base material corresponds to the density distribution of the filler of the resin layer. Be placed.

  By adopting the above configuration, the printed circuit board stored in a state where the casing is filled with the resin layer is configured by laminating two or more base materials having different elastic moduli in the thickness direction, and the resin layer Since the base material is disposed corresponding to the density distribution of the filler, it is possible to suppress the life of the solder in the portion where the electronic component is mounted from being affected by the stress due to thermal strain.

  This is because the filler-containing resin layer may have a filler density distribution in the curing process after addition, and the linear expansion coefficient varies depending on the filler density distribution state. This is because the stress exerted on the solder of the mounting part of the electronic component can be dispersed by adopting a configuration in which two or more base materials having different rates are laminated.

1 is a longitudinal side view of a printed circuit board showing a first embodiment. Overall profile side view Action explanatory drawing explaining the stress relation of this embodiment (the 1) Action explanatory drawing explaining the stress relation of this embodiment (the 2) Action explanatory drawing explaining the stress relation of a comparative example (the 1) Action explanatory drawing explaining the stress relation of a comparative example (part 2) Action explanatory diagram showing comparison results Correlation diagram of solder crack rate between this embodiment and comparative example Vertical side view of a printed circuit board showing a second embodiment

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.

  In FIG. 2 showing the overall configuration, a housing 1 is made of a resin having a rectangular container shape with an open upper surface, and is internally formed with recesses 1a and 1b in two steps, and a recess 1a at the bottom of the recess 1a. The recess 1b is provided in a narrower range. The housing 1 is formed with a lead portion 1c that protrudes to the outside, and a metal lead 2 is embedded in the lead portion 1c. One end of the lead 2 is led out to the outside, and the other end bent upward and directed upward projects upward from the bottom surface of the recess 1a.

  The printed circuit board 3 has a plurality of electronic components 4 mounted on both sides. The electronic component 4 is various electronic components such as a transistor, a resistor, a coil, and a capacitor. Here, the electronic component 4 is surface-mounted by solder as described later. The printed circuit board 3 is stored in the recess 1a of the housing 1, and the lower surface faces the recess 1b.

  A resin layer 5 is filled in the recess 1a of the housing 1, and a resin layer 6 is filled in the recess 1b. Fillers are added to the resin layers 5 and 6, and the filler settles during the resin curing process, so that the filler can be distributed in a dense and dense manner. In this case, low distribution layers 5a and 6a having a coarse filler and a low distribution density are formed on the upper layers of the resin layers 5 and 6, respectively.

  FIG. 1 shows a detailed cross-sectional configuration of the printed circuit board 3. In FIG. 1, the printed circuit board 3 has a configuration in which a core layer 3a and a surface layer 3b are stacked as a plurality of base materials. The core layer 3a is a material for forming a general printed circuit board, and the Young's modulus, which is an elastic modulus, is, for example, 27 to 30 GPa. Further, the surface layer 3b is provided on the lower surface side in FIG. The surface layer 3b is set to have a lower elastic modulus than a material forming a general printed circuit board, and has a Young's modulus of, for example, 5 to 10 GPa.

  Further, the linear expansion coefficient α in the XY direction, that is, the in-plane direction, of the core layer 3a and the surface layer 3b is, for example, 13. The core layer 3a constitutes a main part of the printed circuit board 3, and the surface layer 3b is formed thinner than the core layer 3a.

  The electronic component 4 mounted on the printed circuit board 3 has, for example, a rectangular parallelepiped shape, and electrodes 4a are formed on the connection terminal portions at both ends. The two electrodes 4a of the electronic component 4 are mounted in a state where they are electrically connected by lands 7 and solder 8 patterned copper foil provided on the surfaces of the core layer 3a and the surface layer 3b of the printed circuit board 3. . The solder 8 is provided so as to be in contact between the surface of the land 7 and the electrode 4 a of the electronic component 4. For example, a lead-free solder for vehicle use is used as the solder 8, and the linear expansion coefficient α is, for example, 21.

  For the resin layers 5 and 6, for example, a liquid sealing material in which a filler such as silica is added to an epoxy resin is used. When the resin layers 5 and 6 are formed, the electronic components 4 are stored and mounted in the recesses 1 a of the housing 1 on both sides, and are held in an electrically connected state with the leads 2. In this state, a resin liquid sealing material is poured into the recesses 1a and 1b of the housing 1 and filled. This is a so-called potting method for forming a resin layer. The poured liquid sealing material is cured with the passage of time and formed as resin layers 5 and 6.

  At this time, since the resin layers 5 and 6 are divided by the printed circuit board 3, the filler added to the inside of the liquid sealing material settles in the recess 1a and the recess 1b, respectively, and becomes an upper layer portion. Low distribution layers 5a and 6a having a low filler density are generated. A portion of the resin layer 5 in contact with the electronic component 4 mounted on the core layer 3a on the upper surface of the printed circuit board 3 is formed as a resin in which filler is sufficiently added, and the low distribution layer 5a is formed of the electronic component 4. It is formed in the surface layer part of the upper surface away from. On the other hand, a portion of the resin layer 6 in contact with the electronic component 4 mounted on the surface layer 3b on the lower surface of the printed circuit board 3 is formed such that the low distribution layer 6a having a low filler distribution covers a part of the electronic component 4. ing.

  The resin layers 5 and 6 have a linear expansion coefficient α of, for example, 10 to 12 in a state where the filler is sufficiently added inside. However, in the low distribution layers 5a and 6a where the filler distribution is coarse, the linear expansion coefficient α increases to a range of 40 to 50, for example.

  In this case, since the linear expansion coefficient α of the core layer 3a and the surface layer 3b is approximately the same as that of the resin layers 5 and 6, the printed circuit board 3 is displaced with respect to the displacement of the resin layers 5 and 6 due to temperature change. The stress caused by thermal strain on the solder 8 of the electronic component 4 is small.

On the other hand, since the linear expansion coefficient α of the low distribution layers 5a and 6a is larger than the linear expansion coefficient α of the core layer 3a and the surface layer 3b of the printed board 3, the solder of the electronic component 4 mounted on the surface layer 3b side. 8 is in a state of being greatly subjected to stress due to thermal strain. Therefore, since the Young's modulus, which is the expansion coefficient of the surface layer 3b, is set low, the displacement due to temperature is small, but stress can be absorbed by deformation.
In general, the value of the thermal strain ε has a relationship of the following equation (1) with respect to the linear expansion coefficient α and the temperature change amount ΔT.
ε [ppm] = α [ppm / ° C.] × ΔT [° C.] (1)

Using this relationship, for example, assuming that the temperature change in the environment of use such as a vehicle is severe, calculating the temperature fluctuation range with a temperature change amount ΔT from 25 ° C. to 125 ° C., the thermal strain ε is expressed by the following equation ( It can be calculated as in 2).
ε = ((40-50) -21) × 100 = 1900-2900 ppm (2)

  On the other hand, as shown in FIG. 3, the Young's modulus of the surface layer 3b of the printed circuit board 3 on which the electronic component 4 is mounted is set low as described above. As a result, as shown by the white arrow in FIG. 4, the surface layer 3b is greatly deformed as shown by the black arrow in FIG. 4 in response to the stress of the thermal strain ε from the low distribution layer 6a toward the solder 8. The stress component is dispersed in the surface layer 3b. As a result, the stress applied to the solder 8 is reduced. In FIG. 4, the display of the resin layer 6 is omitted.

  As a comparative example corresponding to the prior art, for example, as shown in FIG. 5, it is assumed that the surface layer 3b is not provided and the printed board 3 remains the core layer 3a. In this case, since the Young's modulus of the core layer 3a is high, the core layer 3a corresponds to the stress of the thermal strain ε directed from the low distribution layer 6a to the solder 8 indicated by the white arrow in FIG. Since the amount of deformation is small as shown by the arrow, the stress component of thermal strain is not dispersed so much. As a result, as shown in FIG. 6, the life of the solder 8 is reduced because the stress received by thermal strain increases. In FIG. 6, the display of the resin layer 6 is omitted.

  As described above, the Young's modulus set for each part in the case of the configuration of the present embodiment in which the surface layer 3b shown in FIG. 3 is provided and the case of the configuration of the comparative example in which the surface layer 3b shown in FIG. FIG. 7 collectively shows the linear expansion coefficient, stress, and the like.

  As a result, even when both the core layer 3a and the surface layer 3b of the printed circuit board 3 have a small linear expansion coefficient α, a layer with a low Young's modulus is provided as in the surface layer 3b, so that solder due to thermal strain is provided. The stress to 8 can be dispersed and reduced on the substrate side.

  FIG. 8 shows whether or not the crack rate of the solder 8 of the surface-mounted electronic component 4 can be reduced when the elastic modulus, that is, the Young's modulus is changed for the surface layer 3b of the printed circuit board 3 as described above. The measurement result by an inventor is shown. Here, the solder crack rate indicates that the larger the value (%), the greater the degree of cracking and the shorter the solder life. The solder crack rate of 100% indicates that the solder portion is disconnected (open) due to the crack.

  As can be seen from this result, in the conventional comparative example, the Young's modulus corresponding to the core layer 3a of the printed circuit board 3 is 27 to 30 GPa, and the solder crack rate cannot be reduced even at 20 GPa or 25 GPa below that, That level is 100%. And like this embodiment, when the Young's modulus is set to about 10 GPa or 15 GPa as the surface layer 3b, it can be greatly reduced to 30% or 40%.

  Therefore, when the Young's modulus of the surface layer 3b is preferably set in the range of 10 to 15 GPa, the solder crack rate can be reliably reduced, and considering the level between the comparative example and the upper limit, the Young's modulus is about 17 GPa. It can be seen that this effect can be expected. That is, the above-described effects can be obtained by setting the Young's modulus of the surface layer 3b in the range of 10 to 17 GPa.

  According to this embodiment, even when the low distribution layers 5a, 6a are generated in the resin layers 5, 6 due to sedimentation of the filler, the Young's modulus of the surface layer 3b of the printed circuit board 3 in contact with the low distribution layer 6a is determined as the core. By setting it lower than the layer 3a, the stress due to thermal strain can be dispersed. As a result, a decrease in the solder life of the solder 8 of the electronic component 4 mounted on the surface layer 3b side can be suppressed.

  Further, when the electronic component is sealed with the resin layer 6 without cleaning the flux residue after the soldering of the electronic component 4, the flux residue remaining between the electronic component 4 and the surface layer 3b of the printed circuit board 3 can be used. Adverse effects can be mitigated.

  That is, in general, the flux residue pushes up the electronic component 4 due to thermal expansion and reduces the solder life. However, by setting the elastic modulus (Young's modulus) of the surface layer 3b to be low, this can be achieved by deforming the surface layer 3b. Therefore, there is an effect of suppressing the solder life reduction of the solder 8. In other words, there is an effect that the restriction that the flux residue when the electronic component 4 is soldered must be removed can be relaxed.

(Second Embodiment)
FIG. 9 shows the second embodiment. Hereinafter, parts different from the first embodiment will be described. In this embodiment, a printed circuit board 10 is provided instead of the printed circuit board 3. The printed circuit board 10 is provided with a surface layer 10b on both sides with a core layer 10a as a center. The core layer 10a is equivalent to the core layer 3a, and the surface layer 10b is equivalent to the surface layer 3b.

  Even when such a configuration is adopted, the surface layer 10b on the side where the printed circuit board 10 is in contact with the resin layer 5 has a linear expansion coefficient α equivalent to that of the resin layer 5, and therefore the solder of the electronic component 4 mounted on this surface 8 is not greatly affected by the thermal strain stress.

  Therefore, the same operational effects as those of the first embodiment can be obtained also by the second embodiment. In addition, since the printed circuit board 10 employed in the present embodiment has the surface layer 10b on both surfaces, there is an advantage in the manufacturing process that mounting work and potting work can be performed without distinction between the upper and lower surfaces.

(Other embodiments)
In addition, this invention is not limited only to embodiment mentioned above, In the range which does not deviate from the summary, it is applicable to various embodiment, For example, it can deform | transform or expand as follows.

In the embodiment, the case where the electronic component 4 is mounted on both surfaces of the printed circuit board 3 has been described. be able to.
In the embodiment, the case where the solder 8 is a lead-free solder for a vehicle is shown, but the present invention is not limited to this, and other solders can also be used.

  In the embodiment, the example of the two-layer printed circuit board 3 and the three-layer printed circuit board 10 has been described. However, a configuration in which the surface layer 3b or 10b having a low Young's modulus is provided in a portion where the low distribution layer 6a contacts is adopted. If so, a printed circuit board in which four or more base layers are laminated can also be adopted.

  Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments and structures. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure.

  In the drawings, 1 is a casing, 3, 10 is a printed circuit board, 3a and 10a are core layers (base materials), 3b and 10b are surface layers (base materials having a low elastic modulus), 4 is an electronic component, and 5 and 6 are resins. Layers 5a and 6a are low distribution layers, and 8 is solder.

Claims (7)

Printed circuit boards (3, 10) on which electronic components (4) are mounted using solder (8);
A housing (1) in which the printed circuit board is stored;
A resin layer (5, 6) filled in the casing so as to cover both sides of the printed circuit board and including a filler;
The printed circuit board is configured by laminating two or more base materials (3a, 3b, 10a, 10b) having different elastic moduli in the thickness direction, and the base material corresponds to the density distribution of the filler of the resin layer. Electronic device to be placed.   In the printed circuit board (3), a base material (3b) having a low elastic modulus among the base materials is disposed on a surface of the resin layer on a side where a low distribution layer (6a) having a rough filler is in contact. Item 2. The electronic device according to Item 1.   The electronic device according to claim 1, wherein the resin layer is obtained by curing a liquid sealing material to which the filler is added.   The electronic device according to any one of claims 1 to 3, wherein the electronic component is mounted on both sides of the printed circuit board.   5. The electronic device according to claim 1, wherein a base material (10 b) having a low elastic modulus among the base materials is disposed on both sides of the printed circuit board (10).   The electronic device according to any one of claims 1 to 5, wherein an elastic modulus of a base material (3b, 10b) having a low elastic modulus among base materials of the printed circuit board is in a range of 5 to 17 GPa.   The electron according to any one of claims 1 to 6, wherein the resin layer is hardened in a state where the filler is uniformly mixed, and has a linear expansion coefficient set to be substantially the same as that of the base material of the printed circuit board. apparatus.

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