OL1.APR–Motorola–SC– Page
Ball grid array package challenges quad flatpack
Since the latter part of the 1980s, the quad flatpack (QFP) has been a
viable means of packaging chips having pin counts in the hundreds. But the
increasingly stringent performance and density requirements of the 1990s
present a stiff challenge to the QFP. Perhaps a package called the ball
grid array will take up where the QFP leaves off. This patent-pending
package, designed by Motorola's Application Specific Integrated Circuits
Div.in Chandler, AZ, is trade-named Ompac. It is designed to accommodate
high-pin-count application-specific ICs and offer density and performance
benefits over the QFP. The package's contacts reside underneath, like a
pin-grid-array (PGA), rather than on the periphery like a quad flatpack.
But unlike the PGA, the ball-grid-array package's contacts are not
gold-plated, through-hole pins. Instead, they are solder bumps attached to
a copper foil pad on an epoxy glass substrate. Silver-filled epoxy, plated
with gold, is used to attach the die. The die attaches to the substrate
via a gold wire bond. Thermal vias run between the solder bumps to
dissipate heat from the package. The diagram shows a cross-section of the
Ompac package. Compared to a quad flatpack, a 169-pin ball grid array
occupies about 51% less space than a 160-pin quad flatpack. A 235-pin
array occupies 26% less than a 208-pin quad flatpack. Moreover, the Ompac
allows all vias to run inside the package's footprint, next to the pads. The
QFP, on the other hand, may require board routing and vias beyond the
package's lead span. Ompac also presents assembly advantages over a QFP.
The package's specially formed solder balls tend to align themselves
squarely on the pc board, unlike a QFP's thin leads which tend to skew and
create coplanarity problems. The solder balls, which are spaced on a 1.
5-mm pitch, enable the package to tolerate misalignment of up to +/-12
mils on the pc board, compared to just +/-3 mils for a QFP. Moreover,
Ompac can be conveniently packaged in tape-and-reel and inserted by
conventional automated pick-and-place equipment. By contrast, a QFP
requires costlier handling trays to be added to the pick-and-place
machine. The Ompac package also offers improved thermal and electrical
performance over QFPs. For instance, inductance on the 169-pin Ompac was
measured to be 6.72 nH on the longest trace, compared to 11 nH on a QFP.
Unlike a QFP where each lead is equidistantly spaced around the package,
the solder bump array on Ompac enables a designer to optimize a chip
layout so critical signal paths are closest to the center, thus minimizing
parasitic electrical effects. From a thermal standpoint, the presence of
thermal vias improves thermal resistance. A 225-pin package with thermal
vias has a thermal resistance of 20 degreesC/W at a 200- ft/min airflow,
compared with 30 degreesC/W at the same airflow for a QFP. Motorola is
now offering 169-pin ASICs in Ompac, and plans to ship 225-pin ASICs in
the package later this year. For more information, contact Motorola's
Application Specific Integrated Circuits Div., Chandler, AZ, at
602-821-4172 or .
–Spencer Chin
CAPTION:
The Ompac ball grid array from Motorola is a pin grid array with solder
balls instead of gold pins attaching to the pc board.
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