Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-08-17
2004-05-18
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S832000, C029S843000, C029S852000, C174S262000, C174S260000, C361S774000, C361S768000, C228S180210, C228S180220
Reexamination Certificate
active
06735857
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims benefit of priority to Japanese Application No. JP 2000-248624 filed Aug. 18, 2000, the entire content of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of mounting whereby a BGA (ball grid array) such as an FBGA (fine ball grid array) is soldered onto a printed circuit board.
2. Description of the Related Art
A prior art example of a method of mounting etc. whereby a BGA is soldered onto a printed circuit board is described using
FIG. 1
to FIG.
5
.
First of all, the method of mounting the BGA is described in FIG.
1
and FIG.
2
.
FIG. 1
is a view showing the condition prior to mounting the BGA on the printed circuit board. The general structure of a BGA is that BGA-side pads
5
constituting electrodes are arranged in grid fashion on the underside of a BGA body
4
and solder balls
6
are connected to the surfaces of the BGA-side pads
5
(these electrodes are referred to as bump electrodes).
In order to mount a BGA on a printed circuit board, solder paste
3
is applied beforehand onto printed circuit board-side pads
2
constituting electrodes arranged in the same arrangement as the BGA-side pads
5
on the printed circuit board
7
, and mounting is effected such that the solder balls
6
are superimposed on the printed circuit board-side pads
2
. In
FIG. 1
, numeral
1
are through-hole upper lands, numeral
8
are through-hole internal portions, and numeral
9
are through-hole bottom lands. These
1
,
8
and
9
are connected to the printed circuit board-sides pads
2
, and then the printed circuit board-sides pads
2
are respectively connected to every circuit (not shown) via through-holes.
After mounting, the BGA and printed circuit board are heated in a reflow device or the like, not shown, so that soldering is effected by melting the solder
6
and solder paste
3
.
FIG. 2
is a view showing the condition after the BGA has been mounted on the printed circuit board.
The molten solder
6
and solder paste
3
fuse to a single mass which wets and spreads out over printed circuit board-side pads
2
, thereby forming a solder fillet (condition in which the BGA-side pads
5
and printed circuit board-side pads
2
are connected through bump electrodes) between the BGA-side pads
5
and printed circuit board-side pads
2
, which are thus soldered together.
In this process, if the amount of solder paste
3
is too large, the solder spreads out, effecting joining between the printed circuit board-side pads
2
i.e. solder bridges
12
are produced. Also, if the amount of solder paste
3
supplied is too little or BGA solder balls are missing, soldering is not effected between the BGA-side pads
5
and the printed circuit board-side pads
2
, giving rise to solder gaps
11
(condition in which the amount of solder is too little).
In the case of the solder joints of ordinary mounted components, not shown, the presence of solder gaps or solder bridges can be ascertained by visual observation of the solder fillets formed between the leads or electrodes and the pads of the printed circuit board of the mounted components, since the mounted components are on the outside. Also, the condition of the solder fillets can be automatically inspected using a soldering appearance inspection device which makes use of a camera and/or laser to make this visible to the eye.
FIG. 3
is a view of a transmission image obtained using X-rays of solder joints of a BGA and printed circuit board pads. The lead contained in solder
6
does not easily transmit X-rays and so shows up as dark patches in the transmission image. The transmission image of normal solder joints appears circular, but in the condition where there is a solder bridge
12
, an oval-shaped transmission image is produced between adjacent pads.
Also, in the case of a solder gap
11
, since the amount of solder is small, a transmission image of a smaller circular shape than the transmission image of a normal soldered joint is produced. Thus, since the BGA solder fillets are sandwiched between the printed circuit board
7
and BGA body
4
, they are not visible to the naked eye and it is necessary to observe their transmission images using X-rays etc. Thus the inspection step of a printed circuit board on which a BGA was mounted was performed divided into separate steps for the BGA section and the other mounted components.
FIG. 4
is a view showing a condition in which the BGA is removed using a repair device.
In the ordinary method of repairing soldering defects of mounted components, not shown, in the case of solder bridges, the location thereof is heated using a soldering iron and the solder is removed by melting. Also in the case of solder gaps, the location of the gap is heated with a soldering iron and repair is effected by supplying solder thereto.
However, in order to repair a BGA, an expensive special-purpose repair device is required. When solder bridges
12
of adjacent printed circuit board-side pads
2
occur or solder gaps
11
of BGA-side pads
5
and printed circuit board-side pads
2
occur, even though these occur at only a single location of a BGA having some hundreds of soldering joint locations, the entire BGA must be removed and a new BGA re-mounted.
As shown in
FIG. 4
, regarding the method of removing the BGA, removal from the printed circuit board is effected by heating printed circuit board
7
from below using a heater
16
and raising the BGA body
4
by suction using a suction nozzle
15
, after melting all of the solder of the BGA by using a heated air current nozzle
14
to heat the BGA by blowing onto it, from above the BGA, a current of hot air from a heated air current generator, not shown. Since the solder of the BGA-side pads
5
of this BGA that has been removed has become nonuniform, it cannot be reused. Solder remaining behind on the printed circuit board-side pads
2
is flattened using a solder suctional removal device, not shown.
Next, the method of remounting the BGA is that a new BGA is prepared, and positional location is performed by suctional attachment of the BGA using suction nozzle
15
, after which, in the same way as in the case of removal from the printed circuit board, heating is effected from below printed circuit board
7
using a heater
16
and heating of the upper side of the BGA is effected by blowing onto the BGA by a heated air current nozzle
14
a current of hot air from a heated air current generator, not shown, thereby melting all of the solder
6
of the BGA to effect soldering.
FIG. 5
is a view showing the condition in which heat radiating fins
17
of the BGA are mounted.
BGA packages are often employed in computer CPUs or MPUs and the amount of heat generated on passage of current is increasing as the speeds of the CPUs or MPUs are increased.
However, with the conventional method of BGA mounting, a soldering portion gap is present between the BGA body
4
and printed circuit board
7
, so heat removal by transfer of heat to the printed circuit board was not possible. Heat radiating fins
17
or the like were therefore necessary to allow the heat to escape. Since the heat radiating fins
17
were mounted on the top of the BGA body
4
, they presented an obstacle to reducing the thickness of the product.
As described above, in inspecting solder joints of a BGA using the conventional BGA mounting method, the inspection had to be divided into two inspection processes, namely, inspection of the soldered joints of the ordinary mounted components and inspection of the BGA soldered joints. For inspection of the BGA solder joints, an expensive X-ray apparatus was necessary. This therefore increased the inspection costs due to increase in the number of inspection steps and tended to prolong the lead time for manufacturing printed circuit boards. In addition, there was a problem that an expensive repair device was necessary for repairing defects in BGA soldered joints. Also, since heat is generated on passage of cur
Fukuzawa Kozo
Saito Yuji
Arbes Carl J.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Trinh Minh
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