Connection structure and electronic circuit board

Details Connection structure and electronic circuit board Connection structure and electronic circuit board

2002-02-04

2003-12-02

Cuneo, Kamand (Department: 2827)

Electricity: conductors and insulators

Conduits, cables or conductors

Preformed panel circuit arrangement

C174S263000, C174S265000, C174S266000, C361S772000, C361S773000

Reexamination Certificate

active

06657135

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a connection structure in which an electronic component is connected to a board by flow soldering, and an electronic circuit board that includes the connection structure.
BACKGROUND ART
As a conventional process for manufacturing an electronic circuit board by connecting an electronic component(s) onto a board (or a substrate) such as a printed circuit board, there is a process that employs the so-called flow soldering.
FIG. 13
shows a schematic partial sectional view of an electronic circuit board manufactured by the conventional flow soldering process. FIG.
14
(
b
) shows an enlarged view of a part shown in
FIG. 13
, and FIG.
14
(
a
) shows a schematic plan view from top of the part shown in FIG.
14
(
b
) while excluding a fillet.
According to the conventional flow soldering process in a general way, as shown in FIG.
13
and
FIG. 14
, first a lead
65
(for example, an electrode) extending from an electronic component
67
is inserted to a through hole
62
formed in a board
61
to penetrate therethrough from the front surface (top in the drawing) to the back surface (bottom in the drawing) of the board
61
. The board
61
has a land
63
(
FIG. 13
) made of, for example, a copper foil. The land
63
is formed along a wall surface of the through hole
62
and on front and back surfaces of the board
61
around the through hole
62
, and portions of the land
63
on these three surfaces are called a wall surface land portion
63
c
, a front surface land portion
63
a
and a back surface land portion
63
b
, respectively (FIG.
14
(
b
)). The land
63
is connected to a wiring pattern (not shown) formed on the front surface or the back surface of the board
61
. The board
61
is covered by a solder resist
64
(indicated with hatching in
FIG. 13
) on the front surface and back surface thereof except for the land
63
.
Then a solder material that has been molten by heating is applied in the form of a wave(s) to the board
61
from the back surface of the board
61
. The molten solder material rises up an annular space between the through hole
62
and the lead
65
inserted to the through hole
62
(see, FIG.
14
(
a
)), and spreads over the front surface land portion
63
a
and the back surface land portion
63
b
while wetting the surface thereof. The solder material solidifies as its temperature lowers, thereby forming a connection portion
66
as shown in FIG.
13
and FIG.
14
(
b
) (not shown in FIG.
14
(
a
)). The solder material does not deposit on the surface region that is covered by the solder resist
64
.
The connection portion
66
made of the solder material, the so-called fillet, is formed as described above (hereinafter, it is simply referred to as the fillet).
Thus the lead
65
of the electronic component
67
and the land
63
formed on the board
61
are electrically and mechanically connected thereby making an electronic circuit board
70
.
DISCLOSURE OF INVENTION
In the past, the electronic circuit board manufactured as described above generally employs an Sn—Pb based solder material that contains Sn and Pb as main components, particularly an Sn—Pb eutectic solder material. However, lead included in the Sn—Pb based solder material has a possibility of causing environmental pollution if it is wasted improperly. Accordingly, as an alternative of such lead-containing solder material, a solder material containing no lead, the so-called lead-free solder material has begun to be used on an industrial scale. However, use of the lead-free solder material in the flow soldering process for manufacturing the electronic circuit board leads to such a problem that an upper fillet portion
66
a
and/or a lower fillet portion
66
b
peels off (or is lifted off) the front surface land portion
63
a
and/or the back surface land portion
63
b
, respectively, as shown in FIG.
14
(
b
), resulting in insecure connection between the land and the fillet. This phenomenon, which is generally called “lift-off”, is undesirable for an electronic circuit board that is required to have high bonding strength between the lead of the electronic component and the land in order to provide high reliability of the electronic circuit board. The lift-off hardly occurs in a case of using the Sn—Pb eutectic solder material and there has been no concern over the problem, but recent finding shows that the lift-off frequently occurs in a case of using the lead-free solder material thus raising a concern about it. As the transition from the conventional Sn—Pb based solder material to the lead-free solder material is promoted, it is very important in the manufacture of electronic circuit boards to prevent the lift-off that is a characteristic problem accompanying the use of the lead-free solder material.
The lift-off that occurs when using the lead-free solder material is generally supposedly to be arisen by following two types of cause.
One type of cause relates to the composition of the lead-free soldering material itself used in flow soldering. When using an Sn—Ag—Bi based alloy, for example, as the solder material, an Sn—Bi eutectic solder material among all of metal elements (Sn, Ag, Bi) that constitute the Sn—Ag—Bi based alloy and all of alloys (an Sn—Ag based alloy, an Sn—Bi based alloy and an Ag—Bi based alloy) that can be formed from any combination of these metal elements has a melting point (about 138° C.) that is lower than a melting point (about 200° C.) of the original Sn—Ag—Bi based alloy. A metal and/or an alloy that has a melting point lower than the melting point of the original alloy will be hereafter referred to simply as a low-melting point metal or a low-melting point alloy. In the case of the Sn—Ag—Bi based alloy, the Sn—Bi eutectic alloy is the low-melting point metal.
In this case, as the Sn—Ag—Bi based alloy supplied in a molten state to the board gradually solidifies, the Sn—Bi eutectic alloy that has a melting point lower than that of the original Sn—Ag—Bi based alloy migrates to a region not yet solidified due to the temperature gradient in the fillet, and is concentrated at the region. As a result, the Sn—Bi eutectic alloy is segregated by concentration toward the hottest region of the fillet, namely a portion that solidifies last. The hottest region in an upper portion of the fillet (i.e. an upper fillet portion) is a region near an interface between the land made of copper foil that is a good heat conductor and the fillet (hereafter also referred to simply as a land/fillet interface), and the low-melting point alloy
75
such as the Sn—Bi eutectic alloy is concentrated in the vicinity of the land/fillet interface as shown in FIG.
14
(
b
) to segregate. In the process of solidification of the fillet, in case a tension (indicated schematically by arrow of dashed line for only the upper fillet portion
66
a
in FIG.
14
(
b
)) due to shrinkage by solidification is exerted in the low-melting point alloy near the land/fillet interface on the board surface where the solder material is still molten and has an insufficient strength, cracks are generated at a periphery of the land/fillet interface, and proceed toward an inside (or a central portion) of the fillet
66
a
and
66
b
as the solidification proceeds from the periphery to the inside. Since the cracks are generated in the fillet as described above, a peripheral edge
71
of the fillet is supposed to peel off a peripheral edge of the land
72
, as shown in FIG.
14
(
b
), thus causing the lift-off.
Another type of cause relates to combination of a material of the lead
65
and the solder material of the fillet
66
. In case the molten solder material that makes contact with the lead
65
is made of an Sn-0.7Cu eutectic alloy (an alloy formed from 0.7% by weight of Cu and the balance of Sn), for example, there is no low-melting point metal or low-melting point alloy having a melting point that is lower than a melting point (about 227° C.) of the original Sn—Cu eutectic alloy among all of the metal elements constituting the solder material and all of the all

Affiliated with

Also associated with

Rating

Connection structure and electronic circuit board does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Connection structure and electronic circuit board, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Connection structure and electronic circuit board will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3122628