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X. Saint-Martin: Bull, Les Clayes Sous Bois, France,
Y. Stricot: ,
M. Auray**: **Bull Electronics Europe, Angers, France and
C. Floury**: **Bull Electronics Europe, Angers, France
ACKNOWLEDGMENT: *This paper was presented at the 10th European MicroelectronicsConference, held in Copenhagen, Denmark, in May 1995.
INTRODUCTION
During 1994, Bull has carried out industrial assembly of ball grid arrays (BGAs) on electronic boards devoted to UNIX servers. The first BGA used was a 256 I/O ceramic one (CBGA) housing the 601 or 604 Power PC. BGA use was then extended to plastic BGAs (PBGAs) and tape BGAs (TBGAs), along with a continuous pin count increase up to 600.
BGAs are declared to be 'surface mount compatible'. However, although state-of-the-art surface mount procedures and equipment make it possible to deal with these new packages in a better way than with very fine pitch peripheral packages, a considerable effort has to be made in order to achieve a successful industrial implementation of BGA assembly on boards.
The present paper describes the methodology used in the authors' company to implement BGA assembly on the manufacturing line: package architecture analysis, engineering studies, development of equipment and procedures for board assembly, technology and manufacturing qualification including test boards. The paper mainly addresses the assembly of high pin count BGAs (256 to 625 I/Os).
WHY BGAS?
The advantages of BGAs over more conventional packages such as peripheral or PGA are now well known. The first advantage is space saving: size gain is becoming more and more significant as pin count increases, as shown in Figure 1, which takes the board real estate into account. This advantage can be easily upgraded through the use of 1 mm pitch BGAs, which is already achievable with current BGA technologies. In the case of high power chip dissipation, however, thermal management must be carried out to maintain this advantage.
The second main advantage of BGAs is ease of board routability: the 1.27 mm pitch, along with 0.35 mm diameter vias, permits dense routing inside the printed circuit board compared with PGAs or peripheral packages.
The third advantage is the cost/performance ratio: BGAs (mainly PBGA and TBGA) can enable cost-reductions up to 30% compared with PGAs, while providing a shorter electrical path between devices. Very high assembly yield provides...