Metal fusion bonding – Process – Applying or distributing fused filler
Reexamination Certificate
2001-08-09
2003-11-18
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Applying or distributing fused filler
C228S200000
Reexamination Certificate
active
06648216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and an apparatus for “flow soldering” (which is also referred to as “wave soldering”) wherein electronic components are mounted onto (or bonded to) a board (or a substrate) by means of a lead-free solder material.
2. Description of Related Art
In recent years, it has been strongly desired to increase the reliability of an electronic circuit board which is contained in a downsized electronic device while still maintaining and a high performance of the electronic device. Therefore, there is an increasing demand to increase a reliability property such as the thermal shock resistance and the mechanical strength of a connecting portion which is formed by soldering an electronic component onto a board in the field of mounting the electronic components.
Moreover, while concern about the protection of global environment is increased in a worldwide scale, a regulation or legal system to control industrial waste treatments is being arranged. Although an Sn-Pb based solder material, which contains Sn and Pb as its main components (e.g. a so-called “63 Sn-37 Pb” eutectic solder material), is generally used in an electronic circuit board which is incorporated in an electronic device, lead contained in such solder material may cause environment pollution if it is subjected to an inadequate waste treatment. As a result, research and developments are carried out as to a solder material which does not contain lead (i.e. a so-called lead-free solder material) as an alternative to a solder material which does contain lead.
A conventional flow soldering process for producing an electronic circuit board by connecting a electronic components to a board such as a printed board as well as an apparatus for such process will be described with reference to drawings below.
FIG. 3
shows a schematic view of the conventional flow soldering apparatus.
At first, a board is prepared prior to soldering, wherein through holes are formed through the board, and an electronic component is located thereon by inserting a lead (e.g. an electrode) of the component into the through hole from an upper surface of the board. In such a board, a land which is made of copper or the like is formed on a region (A+B+C, see
FIG. 4
) consisting of a surface (A) which defines the through holes as well as an upper surface portion (B) and a lower surface portion (C) of the board, where portions (B) and (C) surround the through hole, and such a land is connected to a circuit pattern on the upper surface of the board. On the other hand, regions of the upper surface and the lower surface of the board, except for the lands, are covered by a solder resist.
Next, the board is subjected to a pre-treatment in which the lower surface of the board on which no electronic component is located thereon is applied with flux by means of a spray fluxer (not shown). The pre-treatment is conducted in order to improve wetting and spreading of the solder material on a surface of the land by removing an oxide film (such as a film formed by natural oxidation) which is inevitably formed on the land.
Then, referring to
FIG. 3
, a thus prepared board (not shown) is put into the flow soldering apparatus
60
while the upper surface on which the electronic component is located is kept upward (with regard to the drawing), and the board is mechanically transferred in a direction of the arrow
61
inside the flow soldering apparatus
60
with a substantially constant velocity by means of a conveyer. In the flow soldering apparatus
60
, the board is first heated in a preheating zone by means of a preheating unit (or preheater)
62
in order to make the flux applied to the board, according to the pre-treatment, effectively display its activity ability.
Thereafter, when the board is conveyed into a solder material supplying zone located above solder wave nozzles
64
and
65
, the solder material (not shown), which is in a molten state by heating beforehand in a solder material supplying unit
63
, is supplied to the board from its lower side through the primary wave nozzle
64
and the secondary wave nozzle
65
in the form of a primary wave and a secondary wave respectively. The solder material thus supplied goes up from the lower surface of the board by means of the capillary action through an annular space between the surface of the through hole (i.e. the land) and the lead which is inserted through the through hole from the upper surface of the board. Thereafter, the solder material naturally cools by releasing its heat to surrounding areas of the board with its natural cooling rate, so that the solder material thereby solidifies to form a connecting portion of the solder material (or a so-called “fillet”). In this step of supplying the solder material (or the step of flow soldering), the primary wave functions so as to sufficiently wet the surfaces of the lead and the land with the solder material, and the secondary wave functions so as to remove the solder material on regions covered with a solder resist. As a result, the solder material does not form a bridge by remaining on the board and thereby solidifying between the lands (the bridge is not desirable because it causes a short circuit), and the solder material does not form a cornute projection, thereby controlling (or conditioning) the form of the fillet.
As described above, the fillet (or the connecting portion) made of the solder material is formed to electrically and physically (or mechanically) connect the lead of the electronic component and the land formed in the board. p The fillet made of the solder material as described above is required to have a sufficiently large connecting strength between the lead of the electronic component and the land of the board in order to provide a high reliability of the electronic circuit board. However, referring to
FIG. 4
, if the electronic circuit board
70
is produced by using the lead-free solder material according to the conventional flow soldering process as described above, the solder material having been wetted and spread on the surface of the land
73
(which is located to cover an inside surface which defines the through hole
72
perforated through the board
71
as well as regions which surround the through hole
72
on the upper side and the lower side of the board
71
) partially peels off at an interface between the solder material and the land as indicated by the arrow
80
upon the solidification of the solder material. There thus arises a problem in that the connection between the land
73
and the fillet
74
made of the solder material becomes insufficient and thereby a high connecting strength between the lead
75
and the land
73
cannot be obtained.
Such a phenomenon of the peeling-off of the fillet
74
from the land
73
is generally referred to as a “lift-off” phenomenon, which frequently occurs when the lead-free solder material is used, although it scarcely occurs when the Sn-Pb based solder material is used. The lift-off phenomenon notably occurs especially in the cases where the lead-free solder material which contains Sn and Bi (such as an Sn-Ag-Bi based material) is used and in the cases where the lead-free solder material for connecting a lead which is plated with an Sn-Pb based material is used.
As a reason for the occurrence of the lift-off, it could be generally considered that the solder material used for the flow soldering and/or a metal material which can elute into the solder material upon soldering (e.g. a plating metal for the lead) forms a weak alloy having a lower melting point than that of the initial solder material and a composition which is different from that of the initial solder material (hereinafter, such an alloy is merely referred to as a “low-m.p. alloy”) upon the solidification of the solder material from its molten state.
When the molten solder material is at a high temperature it loses its heat mainly via the lead
75
which comes from the electronic component (not shown). In the solder materi
Hirano Masato
Sakai Yoshinori
Yamaguchi Atsushi
Matsushita Electric - Industrial Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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