Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2001-11-06
2003-09-09
Cuneo, Kamand (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S260000, C174S261000, C174S262000, C174S263000, C361S760000, C361S767000, C361S779000
Reexamination Certificate
active
06617529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit board and electronic equipment using the same and particularly, to a circuit board on which insertion-type electrical parts are mounted by using lead-free solder, and electronic equipment using the same.
2. Description of the Related Art
The construction of a conventional board circuit will be described in detail with reference to
FIGS. 1
to
6
.
FIG. 1
is a perspective view showing a soldered portion of a circuit board on which electrical parts are mounted by using solder,
FIG. 2
is a cross-sectional view taken along a line b-b′ of FIG.
1
and
FIG. 3
is a cross-sectional view that schematically shows an occurrence state of land exfoliation.
FIGS. 4
to
6
are diagrams showing occurrence of land exfoliation confirmed in a verification test, wherein
FIG. 4
is a cross-sectional photograph of A portion (right-and-left inversion) of
FIG. 3
,
FIG. 5
is a cross-sectional photograph of B portion of FIG.
3
and
FIG. 6
is an enlarged photograph of C portion of FIG.
5
.
Conventional circuit board
11
is formed as follows. That is, as shown in
FIGS. 1 and 2
, a paper base, a glass base, a polyester fiber base or the like is impregnated with epoxy resin, phenol resin or the like to form an insulating sheet, and then copper foil is attached onto the insulating sheet by a pressuring and heating treatment to form a copper-coated laminate board. Thereafter, a through hole is formed at a desired place of the board, catalyst is supplied to the side surface of the through hole, and then electroless copper plating is applied to the side surface supplied with the catalyst to carry out rough plating. Thereafter, copper electroplating is applied onto the roughly-plated side surface of the through hole to form a conductor (conductive film). The conductor thus achieved and the copper film on the copper-coated laminate board are connected to each other to form through hole
4
. Thereafter, the conductive film of copper on the copper-coated laminate board is etched to form land
2
.
Finally, solder resist
5
is printed and coated onto the board so that tin-lead solder
12
is not coated onto the portion other than the land
2
on which soldering is conducted, and then exposed to light to form circuit board
11
. At this time, the solder resist
5
takes a role of protecting circuits
7
other than the land
2
on which lead
3
is mounted.
Here, the solder resist
5
on the circuit board
11
is printed so as to have an area larger than that of the land
2
and thus so that no solder resist
5
is covered on the land
2
. This is because formation of fillet
12
a
of tin-lead solder
12
would be disturbed if the solder resist
5
is covered on the land
2
in the soldering process using tin-lead eutectic solder (Sn of 63 weight %, Pb of the remaining weight %) which is most frequently used in a soldering joint process for electronic equipment.
Following the recent high-density mounting design, the land
2
is designed to be as small as possible within the permissible range that the minimum joint strength can be kept. Electronic equipment manufactured by using such a conventional circuit board
11
has no critical problem in reliability because tin-lead eutectic solder takes a role of stress-relaxing thermal expansion mismatch which is caused by joint of foreign matters.
However, environmental pollution of lead has been recently more remarkable because people pay more attention to the environmental pollution, and thus shift to lead-free solder containing no lead has been promoted. The lead-free solder mainly comprises tin, and also contains silver, copper, zinc, bismuth, indium, antimony, nickel, germanium, etc., and it has metal characteristics that are higher in metal tensile strength and creep strength and also less in extension as compared with tin-lead eutectic solder (Sn of 63 weight % and Pb of the remaining weight %, hereinafter referred to as “Pb-63Sn”) which is most frequently used for the solder joint of electronic equipment at present. Therefore, the stress relaxation more hardly occurs at the soldered portion as compared with lead solder. Further, the melting temperature of lead-free solder is equal at 190° C. to 230° C. (190 to 230 degrees Celsius) which is higher than that (183° C.) of tin-lead eutectic solder.
As the lead-free solder are mainly used tin-zinc-based solder (materials that contain a tin-zinc eutectic composition (Sn-9.0 wt % and Znwp) as a main material and are improved in characteristics by varying the amount of zinc or adding other elements are generally referred to as “tin-zinc-based solder”, and Sn-8.0Zn-3.0Bi is known as a representative material), tin-copper-based solder (representatively, Sn-0.7Cu-0.3Ag), tin-silver-based solder (representatively, Sn-3.0Ag-0.5Cu) or the like. The tin-zinc-based solder has a merit that the melting point thereof is equal to a low value (about 190° C.), however, it must be soldered under an inactive atmosphere or under vacuum because it is liable to be oxidized, so that the soldering work efficiency is low.
Further, the tin-copper-based solder and the tin-silver-based solder have little problem in oxidation. However, the tin-copper-based solder has a disadvantage that it has a high melting temperature (about 230° C.) and thus land exfoliation is liable to occur.
The tin-silver-based solder has a merit that the melting point thereof is equal to about 220° C. lower than that of the tin-copper-based solder and it can be reduced to about 205° C. by adding Bi (bismuth). The melting point of the tin-silver-based solder is lowered by increasing the addition amount of bismuth, however, the tin-silver-based solder has a disadvantage that fillet exfoliation occurs if the addition amount of bismuth is increased.
The glass transition temperature of epoxy-based material which is the main material of the circuit board is equal at 125° C. to 140° C. When lead-free solder is used, the difference in solidification and shrinkage temperature between the epoxy-based material and lead-free solder is larger than when tin-lead solder is used, so that the stress applied to the joint portion of the lead-free solder is larger. It is apparent from the metal characteristics of the lead-free solder as described above that if insertion mounting is carried out on conventional circuit board
11
by using the lead-free solder
6
, the land exfoliation which occurs little in the case of the tin-lead solder
12
occurs frequently.
An occurrence example of the land exfoliation will be described in detail with reference to the accompanying drawings.
As shown in
FIG. 3
, if soldering using lead-free solder
6
is carried out by using the conventional circuit board
11
, a large stress is applied to the land
2
due to the difference in solidification and shrinkage temperature between the board material and the lead-free solder
6
, and the land
2
is exfoliated from the circuit board
11
, so that the land
2
is kept to be floated from the circuit board
11
(see the A portion of FIG.
3
and FIG.
4
). At this time, the circuit
7
connected to the land
2
is together lifted up and drawn with the land
2
and thus suffers excessive stress, so that it can be confirmed that the boundary portion between the land end portion
2
a
and circuit
7
is greatly deformed and broken (see the B portion of FIG.
3
and FIGS.
5
and
6
). That is, it is found that occurrence of land exfoliation causes occurrence of wire breaking of the circuit
7
.
The manufacturing of electronic equipment by using the conventional circuit board
11
in which the land exfoliation occurs as described above remarkably lowers reliability to the electronic equipment.
SUMMARY OF THE INVENTION
The present invention has an object to provide a circuit board having high reliability in which neither land exfoliation nor fillet exfoliation occurs even when lead-free solder is used.
Further, the present invention has another object to provide electronic equipment having high reliabilit
Ishizuka Naomi
Kanai Masafumi
Kono Eiichi
Matsumoto Akihito
Matsuoka Hiroshi
NEC Corporation
Patel I. B.
Sughrue & Mion, PLLC
LandOfFree
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