JP3056899B2 - Method of forming blind through hole - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a blind through hole (BTH) used for connecting a pattern of an inner layer and a conductor of an outer layer in a multilayer printed wiring board.

[0002]

2. Description of the Related Art In recent years, a substrate having an inner layer pattern formed by, for example, a subtractive method is arranged on an outer layer side,
2. Description of the Related Art A multilayer printed wiring board in which a prepreg as an interlayer insulating material is arranged on the inner layer side and laminated and pressed has been put to practical use. In this type of substrate, the outer conductor layer and the inner layer pattern are electrically connected via BTH or the like.

Conventionally, a prepreg 23 is provided on the inner layer side of a substrate 20 in which an inner layer pattern 21 and a through hole 22 are formed.
There is known a method of forming a BTH 24 in which one side of a through hole 22 is closed by performing a lamination press (see FIG. 5). In the first method, overflow of resin from the BTH 24 is prevented by using a prepreg 23 having a small resin flow, that is, a non-flow type.

[0004] Also, as a second method different from this,
A method is known in which a normal prepreg 33 is laminated and pressed on the base material 30 on which the inner layer pattern 31 is formed, and then the non-through holes 32 serving as the BTH 34 are formed and plated (see FIG. 6). In this case, the non-through hole 32 is formed by, for example, drilling or counterboring.

[0005]

However, the above two methods have the following problems, respectively. For example, the non-flow type prepreg 23 used in the first method has poor flowability of the resin, so that it is difficult for the prepreg 23 to sufficiently follow the inner layer surface having many irregularities. Therefore, FIG.
The void 25 shown in FIG. When such voids 25 are generated, heat resistance, moisture resistance and the like are deteriorated, and a wiring board excellent in reliability cannot be obtained. Further, the non-flow prepreg 23 has disadvantages in that not only storage conditions and manufacturing conditions are severely restricted but also the cost is increased.

On the other hand, when the flow type prepreg 23a is used with priority on improving the reliability, the resin becomes B
Overflowing through the TH 24, the outer conductor portion 26
Is partially covered (see FIG. 7). For this reason, it becomes difficult to form an etching resist only on a necessary portion on the conductor portion 26. Therefore, it is necessary to remove the resin before forming the outer layer pattern.

As a problem of the second method, for example, it is extremely difficult to accurately control the depth when forming the non-through hole 32. If the depth control is inappropriate, inconveniences such as the tip of the processing tool penetrating the prepreg 33 or not reaching the prepreg 33 easily occur. Therefore, a thicker prepreg must be selected in order to prevent the penetration of the processing tool, and as a result, it is difficult to achieve a thinner wiring board.

In the second method, the outer conductor portion 26 is formed.
In order to achieve an electrical connection between the inner layer pattern 21 and the inner layer pattern 21, it is required to perform plating in the non-through hole 32 after the lamination press. However, it is difficult to sufficiently circulate the plating solution even if the plating solution is treated for the non-through hole 32 whose one side is closed. Further, air easily accumulates in the non-through holes 32, and the throwing power of the plating is also deteriorated.

Further, since the step of forming the non-through holes 32 as described above is performed separately from, for example, the step of forming the through-holes and the like, there is a problem that the number of steps inevitably increases.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for forming a BTH capable of reliably preventing resin from overflowing from the BTH and reliably avoiding generation of voids in an inner layer. To provide.

[0011]

In order to solve the above-mentioned problems, according to the present invention, a pattern formed on a side to be an inner layer and a conductor portion on an outer layer side are electrically connected by through holes. After applying a solder resist to at least the vicinity of the through hole on the inner layer side of the material and attaching an adhesive sheet to the solder resist applied surface, a flow type prepreg is laminated and pressed on the adhesive sheet attachment surface side.

[0012]

According to the method of the present invention, the unevenness on the inner layer side of the substrate is eliminated by the solder resist, so that the followability of the adhesive sheet and the prepreg to the substrate is improved. Therefore, generation of voids in the inner layer is reliably avoided. In addition, since the portion that becomes BTH is closed by the adhesive sheet, overflow of the resin from the prepreg side to the BTH side is also prevented.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is embodied in a multilayer printed wiring board will be described in detail with reference to FIGS.

First, a copper-clad laminate in which copper foil was adhered to both sides of an insulating substrate 1 was prepared. Drilling was performed on the copper-clad laminate to form through holes 2 in predetermined portions. Next, the copper-clad laminate was immersed in a plating solution such as electroless copper plating. Copper was deposited in the through-hole 2 by the plating, and the through-hole 2 was used as a through-hole 3 for connecting conductors on both surfaces of the substrate 1. The through hole 3 becomes BTH4 in a later step. Next, the inner layer pattern 5 was formed on a predetermined portion on one side of the substrate 1 by performing etching or the like according to a usual subtractive method.

As shown in FIG. 1, a solder resist 6 is applied to the inner layer side of the base material 1 by screen printing, thereby eliminating irregularities due to the inner layer pattern 5 and forming the inner layer side of the base material 1 in a uniform planar shape. I made it. In this case, the thickness of the solder resist 6 is preferably substantially the same as or slightly larger than the thickness of the inner layer pattern 5. Note that, through this step, the openings 3a of the through holes 3 are formed.
3b, the opening 3a on the inner layer side is somewhat filled with the solder resist 6.

Subsequently, as shown in FIG. 2, an adhesive sheet 4 was adhered to the surface to which the solder resist 6 was applied, thereby completely filling one opening 3a of the through hole 3. That is, through this step, the through hole 3 becomes a non-penetrating BTH 4. Further, in this embodiment, a sheet-like core material 7a having an adhesive layer 7b on both surfaces is used as such an adhesive sheet 7.

Further, as shown in FIG. 3, a flow type prepreg 8 was laminated and pressed on the bonding surface of the adhesive sheet 7. Subsequently, by forming a through hole 9, electroless copper plating, forming an outer layer pattern 10, and applying a solder resist 11, a multilayer printed wiring board 12 as shown in FIG. 4 was produced.

According to the method of this embodiment described above, the unevenness due to the inner layer pattern 5 is eliminated by the application of the solder resist 3, and the inner layer side of the base material 1 has a uniform planar shape. For this reason, the followability of the adhesive sheet 7 and the prepreg 8 to the inner layer side of the base material 1 is improved, and the inner layer pattern 5
The generation of voids in the side walls and the like is reliably prevented. Therefore, heat resistance and moisture resistance are improved, and the reliability of the wiring board is also improved.

According to this method, one opening 3a of the through hole 3 is closed by the solder resist 6 and the adhesive sheet 7, so that the BT
Overflow of the resin to the H4 side is reliably prevented.
Therefore, there is an advantage that a flow-type prepreg 8 which is relatively inexpensive and excellent in handleability can be used.

Further, in this method, since the through holes 3 are formed before the prepreg 8 is laminated, unlike the conventional method, no non-through holes are formed in the laminated state by drilling or the like. Therefore, accurate depth control is not required to prevent penetration of the tip of the drill beforehand, and it is not necessary to thicken the prepreg 8 in advance.

Furthermore, according to this method, through holes 3
The plating solution is supplied to both the openings 3a and 3b. Therefore, the circulation of the plating solution is improved, and the throwing power of the plating is improved.

The present invention is not limited to the above embodiment, but can be modified as follows. For example, (a) the solder resist 6 and the adhesive sheet 7 do not necessarily need to be provided on the entire surface of the substrate 1. Even if these 6 and 7 are provided at least in the peripheral portion of the through hole 3,
The same operation and effect as those of the above-described embodiment can be obtained.

(B) When applying the solder resist 6, means other than screen printing may of course be used. (C) When the diameter of the through-hole 3 is extremely small, it is possible to cope with only the solder resist 6 without attaching the adhesive sheet 7.

[0024]

As described above in detail, according to the method for forming BTH of the present invention, an excellent effect that overflow of resin from BTH can be reliably prevented and generation of voids in the inner layer can be reliably avoided. To play.

[Brief description of the drawings]

FIG. 1 is a partially enlarged front sectional view showing a state where a solder resist is applied to a base material.

FIG. 2 is a partially enlarged front sectional view showing a state where an adhesive sheet is adhered to a solder resist applied surface.

FIG. 3 is a partially enlarged front sectional view showing a state in which a flow type prepreg is laminated and pressed on an adhesive sheet attaching surface.

FIG. 4 is a partially enlarged front sectional view showing a multilayer printed wiring board.

FIG. 5 is a partially enlarged front sectional view showing a conventional BTH forming method (first method).

FIG. 6 is a partially enlarged front sectional view showing a conventional BTH forming method (second method).

FIG. 7 is a partially enlarged front sectional view showing a conventional BTH forming method.

[Explanation of symbols]

1 base material, 3 through holes, 6 solder resist,
7 adhesive sheet, 8 (flow type) prepreg, 4
Blind through hole (BTH).

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-171890 (JP, A) JP-A-3-175696 (JP, A) JP-A-4-206690 (JP, A) JP-A-4- 298098 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H05K 3/46

Claims (1)

(57) [Claims] 1. A solder resist is applied to at least a vicinity of a through hole on an inner layer side of a substrate in which a pattern formed on a side to be an inner layer and a conductor portion on an outer layer side are electrically connected by the through hole; A method of forming a blind through-hole, comprising: adhering an adhesive sheet to the solder resist application surface; and laminating and pressing a flow type prepreg on the adhesive sheet application surface.