JP2004228261A - Printed circuit board - Google Patents

Abstract

An object of the present invention is to provide a printed circuit board in which, in a flow method using lead-free solder, the solder sufficiently spreads on the upper surface side land electrodes and does not cause lift-off and land peeling.
Abstract: Three or more mini-vias are formed on an outer peripheral portion of an upper surface side land electrode. Due to the heat transfer effect of the mini-via 14, heating and cooling of the outer peripheral portion of the upper surface side land electrode 12a are promoted, so that solder wettability is improved and occurrence of lift-off is prevented. In addition, the reinforcement effect of the mini-via 14 prevents occurrence of land peeling. The mini via may be arranged outside the land electrode 12a, and the mini via and the land electrode may be connected by a metal layer.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a structure of a printed circuit board, and is intended to cope with a lead-free solder.
[0002]
[Prior art]
The methods commonly used to mount electronic components on printed circuit boards include the reflow method applied to surface-mounted electronic components and insertion-mounted electronic components (hereinafter referred to as "insert-mounted components"). There is a flow method applied to In the reflow method, solder paste etc. is supplied to the soldering parts such as electrodes in advance by screen printing etc., and with the surface mounted electronic components mounted thereon, the solder is heated and melted with infrared or hot air, and printed. This is a method of soldering surface mount electronic components to a circuit board. In the flow method, a lead of an insertion mounting component is inserted into a through hole of a printed circuit board, and the printed circuit board is passed through while being in contact with a jet portion of a molten solder bath. This is a method of soldering insertion mounting parts. In the soldering with the tin-lead eutectic solder, which has been most commonly used, both methods are widely used.
[0003]
4A and 4B show a structure near a through hole 11 of a conventional printed circuit board 1 used in a flow method, wherein FIG. 4A is a perspective view showing an appearance thereof, and FIG.
A metallized layer 111 is formed on the inner surface of the through hole 11 into which a lead wire (hereinafter simply referred to as a “lead wire”) of an insertion mounting component is inserted, and upper and lower surfaces around the metallized layer 111 are connected to the metallized layer 111. A side land electrode 12 and a lower surface side land electrode 13 are formed.
In the following description, when the upper surface side land electrode 12 and the lower surface side land electrode 13 are collectively referred to, they are simply referred to as “land electrodes”.
[0004]
In the flow method, the printed circuit board 1 is passed over the jet portion of the solder bath while the printed circuit board 1 is in contact with the jet portion, with the lead of the insertion mounting component inserted into the through hole 11. FIG. 3B shows an initial state of this step. Of the lower surface of the printed circuit board 1 in contact with the jet solder 4, the lower surface side land electrodes 13 and the lower projecting portions of the leads 2 are wetted with the jet solder 4. From this state, a part of the jet solder 4 rises in the through-hole 11 while being wet with the metallized layer 111 of the through-hole 11 and the lead 2 and spreads on the upper surface side land electrode 12.
This process will be described below in relation to the flow of heat.
[0005]
The base material of the printed circuit board 1 is a resin substrate having low thermal conductivity (the thermal conductivity of the resin substrate is about three orders of magnitude lower than that of copper). The heat supplied from the jet solder 4 to the lower surface of the printed circuit board 1 is transmitted to the upper surface side land electrode 12 mainly through the metallized layer 111 of the through hole 11 and the lead 2. For this reason, the temperature starts to rise from the lower part of the through hole 11 and is gradually heated to the upper part. Then, the upper surface side land electrode 12 is heated outward from a part close to the through hole 11. Therefore, the temperature of the outer peripheral portion of the upper surface side land electrode 12 rises most slowly. However, when the solder used is a tin-lead eutectic solder, there is a sufficient temperature difference between the melting point (183 ° C.) and the temperature of the molten solder (eg, 250 ° C.), and the solder wettability. Therefore, the solder can sufficiently spread on the upper surface side land electrode 12, and highly reliable soldering can be performed.
[0006]
At the time of cooling, the main body of the inserted mounting component becomes a heat sink, and the printed circuit board 1 is cooled from below by a cooling means such as a fan. The distribution is reversed from that at the time of heating, and becomes lower near the through hole 11 and becomes highest at the outer peripheral portion.
Lead-containing solder, such as tin-lead eutectic solder, which is used as a solder material for such established mounting technology, has an issue of environmental pollution due to the effect of lead as a component on the human body and elution. For this reason, conversion from lead-containing solder to lead-free solder (lead-free solder) is being promoted worldwide. In the case where the tin-lead eutectic solder is replaced with lead-free solder, the same reflow method or flow method as described above is applied as a component mounting method.
[0007]
However, the lead contained in the solder has the function of lowering the melting point, increasing the fluidity during melting, and lowering the surface tension. This lead-free solder that does not contain lead is a tin-lead eutectic solder , The melting point is higher at 210-230 ° C., the fluidity at the time of melting is lower, and the surface tension is higher. For this reason, in the flow method using lead-free solder, the wetting of the solder in the through-hole 11 becomes slow, and the wetting and spreading of the solder to the upper surface side land electrode 12 become insufficient, and the reliability decreases. In addition, there is a problem that a lift-off phenomenon in which the solder and the land electrode separate from each other and a land peeling in which the land electrode peels from the printed circuit board occur. FIG. 5 is a diagram schematically showing the lift-off phenomenon and the land peeling, and shows a state where the lift-off phenomenon occurs on the upper surface side land electrode 12 and the land peeling occurs on the lower surface side land electrode 13.
[0008]
The former problem is related to the characteristics of lead-free solder that does not contain lead as a component that has an effective function as described above, but the maximum allowable temperature of the soldering process depends on the heat-resistant temperature of materials and components. Is also a problem with being restricted by That is, since the maximum allowable temperature is limited by the heat-resistant temperature of the material or component, the maximum allowable temperature of the soldering process cannot be somewhat higher than 250 ° C. in the case of the tin-lead eutectic solder. The temperature difference between the temperature and the melting point of the solder is あ る い は or less as compared with the case of the tin-lead eutectic solder. For this reason, the temperature of the outer peripheral portion of the upper surface side land electrode 12, which is the slowest in temperature rise during heating, may not be able to rise to the temperature required for soldering (the temperature at which the solder and the land electrode wet). In other words, there is no room for the temperature difference.
[0009]
In order to cope with this problem, Japanese Patent Application Laid-Open Publication No. HEI 11-122556 discloses a technique in which upper and lower openings of a through hole are tapered to improve solder wetting.
The latter problem is that, at the time of cooling, the temperature of the outer peripheral portion of the upper surface side land electrode 12 decreases with the longest delay, and the liquidus temperature and solidus temperature due to the fact that lead-free solder is not eutectic solder. Derived from the temperature difference (10-20 ° C), the solidification time of the solder becomes longer, the components of the solder segregate, and the lead-free solder is harder and less ductile than the tin-lead eutectic solder And the melting point of lead-free solder is 210-230 ° C., which is higher than that of tin-lead eutectic solder. All of the above are related to the lift-off phenomenon, and Are mainly related.
[0010]
In order to cope with this problem, Japanese Patent Application Laid-Open Publication No. H10-163,087 discloses a land electrode in which at least the outer peripheral portion of the land electrode is covered with a solder mask.
[0011]
[Patent Document 1]
JP 2002-76615 A [Patent Document 2]
JP 2001-332851 A
[Problems to be solved by the invention]
All of the above problems are related to the reliability of the printed circuit board on which the insertion mounting component is mounted, and must be solved by all means.
An object of the present invention is to solve these problems and to provide a printed circuit board in which a solder spreads sufficiently on a top-side land electrode and does not cause a lift-off phenomenon and land peeling in a flow method using lead-free solder. It is to be.
[0013]
[Means for Solving the Problems]
The invention according to claim 1 is a printed circuit board on which upper and lower land electrodes are provided at both upper and lower ends of a through-hole, and an insertion mounting component is mounted by a flow method, wherein an outer peripheral portion of the upper surface land electrode is provided. In the vicinity, there is provided a heat transfer / reinforcement means for conducting heat from the lower surface side to the outer peripheral portion of the upper land electrode and supplementing the adhesion strength between the land electrodes.
As described in the section of "Prior Art", in the conventional upper surface side land electrode, the temperature of the portion near the through-hole rises faster, but the temperature rise is delayed toward the outside. For this reason, in the case of a lead-free solder having a high melting point and poor fluidity, promoting the rise of the temperature of the upper surface side land electrode, particularly the temperature of the outer peripheral portion, is to improve the spread and spread of the solder. This is an important point. Therefore, when the heat transfer portion is provided on the outer peripheral portion of the land electrode, the temperature rise of the outer peripheral portion of the land electrode is promoted, and the spread and spread of the solder is promoted. In addition, this portion acts as a heat dissipation path during cooling, hastening the cooling of the outer peripheral portion of the upper land electrode, reducing the temperature difference in the upper land electrode, and preventing the occurrence of a lift-off phenomenon. I will.
[0014]
Furthermore, if the outer peripheral portion of the land electrode is provided with reinforcing means for supplementing the adhesion strength of the land electrode, the tip of the land electrode will be firmly held on the substrate material of the printed circuit board, thereby preventing the occurrence of land peeling. can do.
According to a second aspect of the present invention, in the first aspect, three or more mini-vias are provided as the heat transfer / reinforcement means.
Since the mini-via has a metallized layer with good heat conduction on its inner surface, it has a large effect as a heat transfer means even if its occupied area is small, and the metallized layer is integrated with the outer peripheral portions of the land electrodes on both upper and lower surfaces. Therefore, the effect as a reinforcing means is sufficiently large. However, when the number of mini-vias is two or less, three or more mini-vias are necessary because there is a portion where the effect as the reinforcing means does not reach at all.
[0015]
According to a third aspect of the present invention, in the second aspect of the present invention, the mini-vias are arranged at equal angular intervals.
Arranging the mini-vias at equal angular intervals on the outer periphery of the land electrode will distribute the heat transfer effect and the reinforcing effect most evenly, and therefore the number of required mini-vias is minimized.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of a printed circuit board according to the present invention will be described using an example. The parts having the same functions as those of the prior art are denoted by the same reference numerals.
[First embodiment]
1A and 1B show the structure of a first embodiment of a printed circuit board according to the present invention, wherein FIG. 1A is a perspective view showing the appearance, and FIG. 1B is a sectional view.
The printed circuit board 1a of this embodiment is provided with four mini-vias 14 on the outer periphery of an upper surface land electrode 12a provided on the upper and lower ends of a through hole 11 into which a lead wire of an insertion mounting component is inserted and a lower surface land electrode 13a. I have. On the inner surface of the mini-via 14, similarly to the metallized layer 111 on the inner surface of the through hole 11, a metallized layer 141 composed of an electroless plated layer serving as a base and a copper electrolytic plated layer formed thereon is formed. The upper and lower ends of 141 are integrated with the upper surface side land electrode 12a and the lower surface side land electrode 13a, respectively.
[0017]
The four mini-vias 14 are arranged at 90 ° intervals.
FIG. 3A is a cross-sectional view showing an initial state in which the printed circuit board 1 a of this embodiment is brought into contact with the jet solder 4. The jet solder 4 wets the entire lower surface side land electrode 13a including the lower end of the mini via 14 of the printed circuit board 1a.
Although the metallized layer 141 of such a mini-via 14 occupies a small area, its material is mainly copper, which has a very good thermal conductivity, so that the heat of the jet solder contacting the lower surface of the printed circuit board 1a is reduced. It efficiently transmits to the upper surface of the printed circuit board 1a, raises the temperature of the upper surface side land electrode 12a near the mini-via 14, and promotes the spread of the lead-free solder on the upper surface side land electrode 12a. The heat from the mini-via 14 raises the temperature of the outer peripheral portion of the upper surface side land electrode 12a where the temperature rise is the slowest in the conventional printed circuit board, so that the effect of promoting the wetting and spreading of the lead-free solder is large. When the printed circuit board 1a is cooled, the mini-via 14 transfers the heat of the upper land electrode 12a to the lower surface side to dissipate the heat, thereby increasing the cooling rate of the outer peripheral portion of the upper land electrode 12a. Due to this cooling effect, almost no lift-off occurred in the upper surface side land electrode 12a. Further, since the metallized layer 141 of the mini-via 14 is integrated with the upper surface side land electrode 12a and the lower surface side land electrode 13a, the metallized layer 141 has a function of holding the upper surface side land electrode 12a and the lower surface side land electrode 13a. No longer occurs.
[0018]
As described above, the four mini-vias 14 formed on the outer peripheral portion of the upper surface land electrode 12a as the heat transfer / reinforcement means can improve the spread of the lead-free solder and prevent the lift-off phenomenon and the land peeling.
The size of the mini-via 14 used in this example was Φ0.2 mm, and the solder did not rise on the inner surface.
For reference, the thickness of the copper plating of the metallized layer 141 is about 15 μm, and the thermal conductivity of copper is about three orders of magnitude higher than the thermal conductivity of the resin substrate that is the base material of the printed circuit board 1a. Therefore, the amount of heat transmitted through the metalized layer 141 having a thickness of about 15 μm is much larger than the amount of heat transmitted through the resin substrate of the printed circuit board 1a.
[0019]
When the manufacturing process of the printed circuit board 1a is compared with the manufacturing process of the conventional printed circuit board, a drilling process for the mini-via 14 is added, but the metallized layer 141 can be formed simultaneously with the metallized layer 111 of the through hole 11. The step of forming the metallized layer 141 is not added.
In this embodiment, four mini-vias are arranged at equal angular intervals of 90 °, but the number and arrangement of the mini-vias are related to the pattern arrangement of the printed circuit board, and the number is reduced to four. It is not limited, nor is it limited to equiangular intervals. However, in consideration of the effect, there is a region where the reinforcing effect does not reach two, so that it is most effective to arrange three or more at substantially equal angular intervals.
[0020]
[Second embodiment]
FIG. 2 is a perspective view showing the structure of the second embodiment.
In this embodiment, the arrangement position of the mini-via 14a is located outside the upper land electrode 12b, and a part of the upper metal layer 142 of the mini via 14a is connected to the outer peripheral portion of the upper land electrode 12b. Although the function of the mini-via 14a is the same as that of the first embodiment, the effectiveness is reduced as long as the mini-via 14a has a certain distance from the upper surface side land electrode 12b. In some cases, such an arrangement must be adopted, and in some cases, the effect can be enhanced as a whole by increasing the number of mini-vias 14a.
[0021]
Also in the case of this embodiment, the only additional step for forming the mini-via 14a is the drilling step as in the first embodiment. The patterning of the metal layer on the surface of the printed circuit board can be performed in exactly the same process, regardless of whether the land electrode is used alone or the land electrode and the mini-via coexist.
[0022]
【The invention's effect】
According to the first aspect of the present invention, the outer periphery of the land electrode on which the component is mounted is provided with a heat transfer / reinforcement means for assisting heat conduction from the lower surface to the upper surface and for supplementing the adhesion strength of the land electrode. Occasionally, the temperature rise at the outer peripheral portion of the land electrode is promoted, so that the wet spread of the solder is promoted. During cooling, the outer peripheral portion of the upper land electrode is cooled, and the temperature difference within the upper land electrode is reduced. Therefore, the lift-off phenomenon is prevented, and the outer peripheral portion of the land electrode is firmly held by the substrate material of the printed circuit board, thereby preventing the occurrence of land peeling.
[0023]
Therefore, according to the present invention, in a flow method using lead-free solder, it is possible to provide a printed circuit board in which the solder sufficiently wets and spreads on the upper surface side land electrode and does not cause a lift-off phenomenon and land peeling.
In the invention of claim 2, three or more mini-vias are provided as the heat transfer / reinforcement means. The mini-via has a metallized layer with good thermal conductivity on its inner surface, so it has a large effect as a heat transfer means even if its occupied area is small, and the metallized layer is integrated with the outer peripheral portions of the land electrodes on both upper and lower surfaces. Therefore, the effect as a reinforcing means is sufficiently large. However, when the number of mini-vias is two or less, three or more mini-vias are necessary because there is a portion where the effect as the reinforcing means does not reach at all.
[0024]
According to the third aspect of the present invention, the mini-vias are arranged at equal angular intervals. Arranging the mini-vias at equal angular intervals on the outer periphery of the land electrode distributes the heat transfer effect and the reinforcing effect most evenly, so that the number of necessary mini-vias is minimized, and the cost of providing the mini-vias is reduced. Minimize the rise.
[Brief description of the drawings]
FIGS. 1A and 1B show the structure of a first embodiment of a printed circuit board according to the present invention, wherein FIG. 1A is a perspective view showing the appearance of the printed circuit board, and FIG. FIG. 3 shows a state of contact between a jet solder and a printed circuit board in order to explain the effect of the present invention. FIG. 3A is a sectional view of the present invention, and FIG. FIG. 4 shows a structure in the vicinity of a through hole in a conventional example, (a) is a perspective view showing its appearance, and (b) is a cross-sectional view. FIG. 5 is soldering for explaining a problem of the conventional technique. Sectional view showing the state around the through hole after [Description of reference numerals]
1, 1a, 1b Printed circuit board 11 Through hole 111 Metallized layers 12, 12a, 12b Upper land electrodes 13, 13a, 13b Lower land electrodes 14, 14a Mini vial 141 Metallized layer 142 Upper metal layer 2 Lead 3 Nozzle 4 Jet Solder 5 Solder

Claims (3)

A printed circuit board on which upper and lower land electrodes are provided at both upper and lower ends of a through hole, and electronic components for insertion mounting are mounted by a flow method.
In the vicinity of the outer peripheral portion of the upper land electrode, a heat transfer / reinforcement means for conducting heat from the lower surface to the outer peripheral portion of the upper land electrode and supplementing the adhesion strength between the two land electrodes is provided.
A printed circuit board, characterized in that: Comprising three or more mini-vias as the heat transfer and reinforcement means,
The printed circuit board according to claim 1, wherein: The mini-vias are arranged at equal angular intervals,
The printed circuit board according to claim 2, wherein:

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