Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2002-01-14
2003-07-29
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S248100
Reexamination Certificate
active
06598779
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electronic component mounting methods, and more specifically relates to an electronic component mounting method used for manufacturing a surface-mounted unit in which a surface-mount component and an IC are disposed on a single wiring substrate.
BACKGROUND ART
A first example of the background art of the present invention will be described below. In a manufacturing process of a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate, the surface-mount component and the IC (also referred to simply as electronic components) are mounted on the wiring substrate by, for example, the following method. An IC provided with bumps formed of a Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315° C.) is prepared, and is connected and fixed to a predetermined electrode formed on a wiring substrate. In addition, a surface-mount component is connected and fixed to a predetermined electrode formed on the wiring substrate using a eutectic solder (for example, Sn63/Pb37, melting point: 183° C.).
According to this method, the IC and the surface-mount component are mounted on the wiring substrate by the following processes.
(1) Flux is first applied to a substrate forming the IC (IC substrate). Then, the IC provided with bumps is disposed on the wiring substrate and a reflow process (peak temperature: 355° C.) is performed.
(2) The wiring substrate is cleaned, and a eutectic solder paste for mounting the surface-mount component is supplied to an electrode for receiving the surface-mount component using a dispenser, etc. Then, the surface-mount component is disposed on the wiring substrate and a reflow process (peak temperature: 220° C.) is performed.
(3) The wiring substrate is cleaned again, and an underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate and is cured.
Thus, a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate is completed.
Next, a second example of a known method for mounting a surface-mount component and an IC on a wiring substrate will be described below. In this method, the surface-mount component and the IC are both connected and fixed to electrodes formed on the wiring substrate using a eutectic solder (for example, Sn63/Pb37, melting point: 183° C.).
According to this method, the IC and the surface-mount component are mounted on the wiring substrate by the following processes.
(1) A eutectic solder paste for mounting the surface-mount component is supplied to an electrode for receiving the surface-mount component by, for example, a printing process using a metal mask.
(2) The surface-mount component is disposed on the electrode formed on the wiring substrate.
(3) Flux is applied to the IC provided with bumps of eutectic solder.
(4) The IC provided with bumps of eutectic solder is disposed on an electrode for receiving the IC.
(5) The wiring substrate on which the IC and the surface-mount component are disposed is subjected to a reflow process (peak temperature: 220° C.).
(6) The wiring substrate is cleaned, and an underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate and is cured.
Thus, a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate is completed.
However, the above-described first example has the following problems.
(a) Since two reflow processes and two cleaning processes are performed, a long processing time is required. In addition, since two kinds of solders having different melting points are used, two reflow furnaces are required. Thus, production efficiency is low and high equipment costs are incurred.
(b) When the solder bumps on the IC are formed of a Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315° C.), the IC must first be mounted and fixed from the viewpoint of the heat resistance of the surface-mount component. Thus, a typical printing process cannot be used for supplying the solder paste for mounting the surface-mount component, and the efficiency is thereby reduced.
(c) The Pb-based, high-temperature solder has a low solder wettability, and self-alignment cannot be ensured unless the reflow process is performed in a reducing atmosphere or flux having a high degree of activity is used.
(d) When the surface-mounted unit (sub-module unit) including the IC and the surface-mount component is mounted on a motherboard, there is a risk that the eutectic solder connecting the surface-mount component will re-melt and the surface-mount component will be displaced or become separated. Thus, reliability cannot be ensured.
(e) With reference to
FIG. 15
, for example, an electrode
52
is formed on a wiring substrate
51
, and a solder resist
56
is applied to the electrode
52
at a region around a solder bump
54
which is formed on an IC
53
and which is connected and fixed to the electrode
52
. In the case in which a reflow process is performed after the solder resist
56
is applied, a resist based on resin such as epoxy resin, epoxy acrylate resin, etc., is normally used as the solder resist
56
. However, a resin-based solder resist has a low heat resistance, and the adherence to the electrode
52
is reduced because of the heat applied in the reflow process. Thus, solder
54
a
, which is the solder bump
54
in a molten state, flows under the resist
56
, and the height of the solder bump
54
becomes insufficient (the solder bump
54
becomes shorter). As a result, sufficient cleaning cannot be performed in the subsequent process and the underfill resin (not shown) cannot be easily injected into the space between the bottom surface of the IC
53
and the wiring substrate
51
. Accordingly, there is a problem in that the connection reliability is reduced.
(f) There is a risk in that solder balls generated in the second reflow process cannot be removed completely in the cleaning process. In such a case, the solder balls may travel under the bottom surface of the IC and obstruct the process of applying (injecting) the underfill resin.
(g) When the Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315° C.) is used, the amount of Pb, which causes pollution, is increased. Thus, preferably, the Pb-based, high-temperature solder is not used from the viewpoint of environment protection.
In addition, with regard to the above-described second example, there is a risk in that, when the surface-mounted unit (sub-module unit) including the IC and the surface-mount component is mounted on a motherboard, the eutectic solder connecting the IC and the surface-mount component will re-melt and the surface-mount component will be displaced or become separated. Thus, there is a problem in that reliability cannot be ensured. In the above-described case, the IC will not be displaced or become separated since it is retained by the underfill resin.
In order to solve the above-described problems, an object of the present invention is to provide an electronic component mounting method by which a surface-mount component and an IC can be efficiently and reliably mounted on a single substrate.
DISCLOSURE OF INVENTION
In order to attain the above-described object, according to the present invention, an electronic component mounting method for manufacturing a surface-mounted unit in which a surface-mount component and an IC are disposed on a single wiring substrate, comprises the steps of:
(a) supplying a high-temperature solder paste on an electrode for receiving the surface-mount component, the electrode being formed on the wiring substrate;
(b) provisionally fixing the surface-mount component on the electrode with the high-temperature solder paste;
(c) applying flux to an electrode for receiving the IC, the electrode being formed on the wiring substrate and the IC being provided with eutectic solder bumps, or applying the flux to the eutectic sol
Hirota Jitsuho
Morimoto Ryoichi
Elve M. Alexandra
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Stoner Kiley
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