Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2000-06-19
2001-07-24
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S246000, C228S180220, C438S613000, C438S614000
Reexamination Certificate
active
06264097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for forming or creating a solder ball on a substrate or circuit board (work), e.g., BGA (Ball Grid Array), etc.
2. Description of the related Art
FIG. 12
illustrates a perspective view of a BGA
51
.
A plurality of solder balls
45
(bumps) is arranged on the BGA
51
. The solder balls
45
(bumps) are arranged with an equal interval, i.e., a determined pitch P. As a method for forming the solder balls
45
(bumps), a known method of screen printing solder paste on an electrode of a work uses a printing mask and forms the solder balls (bumps) on the electrode by re-flow processing of the solder paste. Due to recent increases in the density of electronic parts and decreased pitch P of the solder balls (bumps), there are now more requests for reducing a size of an opening of the printing mask.
When the BGA
51
is attached to another substrate or circuit board by soldering, a height H of the solder balls (bumps) should be higher. If the height H of the solder balls (bumps) is higher, a difference in a height of the substrate or circuit board, to which the BGA
51
is attached by soldering, due to cambering and curbing of the substrate or circuit board can be better absorbed. A difference in a height on a surface of the substrate or circuit board due to cambering and curbing of the substrate or circuit board of the BGA
51
can also be better absorbed. Further, a difference in thermal expansion can be absorbed. Hence, all the solder balls
45
(bumps) can be electrically connected without failure. Therefore, an amount of the solder paste coated on the electrode of the BGA
51
must be increased as much as possible, and the height H of the solder balls (bumps) must be made higher.
FIG. 13
illustrates a procedure of forming the solder balls
45
(bumps) by screen printing according to the related art. An electrode
33
is provided on a work
31
,and a resist
34
is provided around the electrode
33
. An opening
23
of a printing mask
21
is provided to correspond to the electrode
33
. When the printing mask
21
is placed on the work
31
and a squeegee
25
is moved in a direction of the arrow, solder paste
24
is printed on a surface of the work
31
.
In
FIG. 13
, (a) shows a printing state, and (b) shows a state after printing when the printing mask
21
is detached from the work
31
. In
FIG. 13
, (c) shows a state in which the solder ball
45
(bump) is formed from the solder paste
24
by re-flow processing. For increasing a height H of the solder ball (bump) in (c), a thickness W of the printing mask in (a) must be increased. However, when the thickness W of the printing mask is increased, side walls of the printing mask and the solder paste
24
stick to each other more firmly. Therefore, when the printing mask
21
is detached from the work
31
, the solder paste
24
is also detached from the work
31
. As a result, the solder paste
24
does not remain on the work
31
, and a printing condition becomes insufficient. When the thickness W of the printing mask is increased, it is possible to lower a viscosity of the solder paste
24
to solve this problem. However, when the viscosity of the solder paste
24
is lowered, the solder paste
24
spreads around the electrode
33
after screen printing and contacts with adjacent solder paste. This causes problems, e.g., a failure in an electric connection, generation of a solder bridge, etc.
SUMMARY OF THE INVENTION
According to a preferred embodiment of this invention, when a solder ball (bump) is formed on a work by screen printing, an amount of the solder paste coated on an electrode of the work is increased as much as possible for forming a higher solder ball (bump).
Further, according to a method for forming a solder ball (bump) in this invention, even if the amount of the solder paste is increased as much as possible, the solder bridge does not tend to be generated.
According to one aspect of this invention, a method is provided for forming a solder ball, in which the solder ball is formed on an electrode of a work by using a solder paste. The method includes forming a first solder layer by placing a first solder paste on the electrode of the work, forming a second solder layer by placing a second solder paste on the first solder layer formed on the electrode of the work after forming the first solder layer, and forming the solder ball by heating, melting and solidifying the first solder layer and the second solder layer in re-flow processing after forming the second solder layer.
Further features and applications of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
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Cooke Colleen P.
Dunn Tom
Micro-Tec Company Ltd.
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