Reworking of a ball grid array module

Details Reworking of a ball grid array module Reworking of a ball grid array module

1999-03-22

2001-05-08

Ryan, Patrick (Department: 1725)

Metal fusion bonding

Process

Repairing, restoring, or renewing product for reuse

C228S264000, C228S022000

Reexamination Certificate

active

06227434

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the removal of the solder connection balls from a substrate during the rework process of electronic packages.
BACKGROUND OF THE INVENTION
A recent development introduced the use of Ball Grid Array (BGA) technique in the manufacturing of electronic modules which can be of the Single Chip Module (SCM) type or Multi Chip Module (MCM) type. These modules are provided with a plurality of conductive pads for electrical connection with electronic circuits (such as mother boards, back planes, application boards). The electrical connection is achieved by little spherical portions of solder alloy which give the name of Ball Grid Array (BGA) to this kind of electronic module.
FIG. 1
is an example of a section of a BGA module of the SCM type. On the lower face of a substrate
101
there is a plurality of conductive pads
103
each pad provided with a solder ball
105
which will be put in contact with an electronic circuit and reflowed, thereby realizing the electrical connection. These solder balls are usually and conveniently made of an eutectic alloy containing Sn (63% by weight) and Pb (37% by weight). This alloy offers the advantage of having a relatively low melting point (183 C), known as eutectic point. On the upper face of the module there is the active element
107
. The active element usually needs to be protected against atmospherical agents and against the heat of the reflow cycles. As represented in
FIG. 1
the active element can be covered by a molded cap
109
. Other methods of protection can be used instead, e.g. an epoxy resin underfiller.
The BGA technology has a number of advantages over the traditional technologies such as Pin Grid Arrays in terms, for example, of reliability, robustness and cost of manufacturing.
The substrates of the BGA modules can be ceramic or, more recently, organic Printed Circuit Board laminates. These last modules are usually called Plastic Ball Grid Arrays. The definition “Plastic” indicates the flexible nature of the PCB as opposed to a ceramic substrate.
During the manufacturing process it may happen that one or more of the connection balls are damaged or lost. Furthermore, once the module has been mounted on the electronic board (second level packaging), the solder balls could be proved not to provide an adequate connection between the module and the circuit board, on which the BGA module has been mounted. In such instance the BGA module must be removed from the board and it is often lost. Considering the increasing costs of the semiconductor devices, it would be obviously desirable to rework and reuse as many modules as possible once they have been found to be damaged.
The modules could be checked and fixed manually, but this activity would be far too laborious and expensive.
In order to re-introduce the reworked module in the manufacturing process, all the solder balls must be completely removed and no traces of solder material should be left on the module surface. It is known to rework ceramic packages by using hot water vapour in order to reflow the solder balls causing detachment from the substrate. However in the case of BGA package with an organic laminate the temperatures cannot be too high, because this would damage the substrate. Another problem is that the solderable surface on the module substrate could be jeopardized by the oxidation caused by the exposure to air. Furthermore the reflowing of the solder balls does not guarantee the detachment of all the solder material.
It is an object of the present invention to provide a technique which alleviates the above drawbacks of the prior art.
SUMMARY OF THE INVENTION
According to the present invention we provide a method for completely removing the solder balls from the substrate of a Ball Grid Array electronic module for further reworking, including the steps of: heating the solder balls until they reach their melting point by immersing the module in a heated water soluble oil bath; while the module is immersed in the water soluble oil bath, engaging the module substrate with at least one rotating brush.
Furthermore, according to the present invention we provide a system for completely removing the solder balls from the substrate of a Ball Grid Array electronic module for further reworking, including: a water soluble oil bath having heating means adapted to heat the oil to a temperature higher than the melting point of the solder balls; support means for immersing the module in the oil bath; at least one rotating brush engaging the module substrate.


REFERENCES:
patent: 3359132 (1967-12-01), Wittmann
patent: 3923002 (1975-12-01), Vanyi
patent: 4531986 (1985-07-01), Barajas
patent: 4635584 (1987-01-01), Obermann
patent: 5298082 (1994-03-01), Weitz
patent: 5458281 (1995-10-01), Downing et al.
patent: 5766674 (1998-06-01), Hirosawa
Lawrence Van Vlack. Elements of Materials Science and Engineering: Sixth Edition. 1990, p. 578.

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