Application Brief
SSYA009A - February 2000 – Revised May 2000
MicroStar Junior  Made Easy
Mixed Signal Products
ABSTRACT
This application brief provides a high level overview of the MicroStar Junior package and its
installation and use for thermal dissipation.
What is the MicroStar Junior Package?
The MicroStar Junior package is a polyimide
film-based ball grid array (BGA) package. Like
other BGA packages, the MicroStar Junior
package uses solder alloy balls as the
interconnect between the package substrate and
the board on which the package is soldered.
Figure 1 shows the structure of TI’s MicroStar
Junior package.
Encapsulant
Chip
Wire bond
Die
paste
How does the MicroStar Junior Package
Dissipate Heat?
The MicroStar Junior Package dissipates heat
by using the shaded balls in the middle of the
package, indicated in Figures 2(a) and 2(b), for
thermal dissipation through vias to the ground
plane. Only the outside row(s) of balls is used for
electrical connections in this package. This
eases routing requirements as compared with
other larger BGA packages. For simplicity, the
remainder of this document will concentrate
solely on a single outer-row signal connection
package. Although, the same basic rules apply
for the dual outer-row signal connection
package.
GQE PACKAGE
(TOP VIEW)
Cu pattern
Index
Index
Single Outer-Row
(a)
Dual Outer-Row
(b)
Via
Solder Alloy ball
Flex substrate
Figure 1. Structure of TI’s MicroStar
Junior Package
Figure 2. Top View of MicroStar
Junior Package – Outer Row(s)
routed for signal
MicroStar Junior is a trademark of Texas Instruments.
1
SSYA009A
MicroStar Junior Assembly Process
1. For single outer-row signal routing, prepare
the PCB with a top side land pattern as
shown in Figure 3. Each circular land,
represented by shaded circles, corresponds
to a ball on the outside edge of the package
as shown in Figure 4. The diameter of the
lands is shown in Table 1.
0.5 mm
Ball Pitch
Copper Land
0.28 mm
(non solder
mask defined)
2. Place vias between the positions where the
balls will contact the PCB, as shown in
Figures 6 and 7. Do not place the vias
directly under the ball locations. These vias
should be 0.30-mm (max) in diameter and
extend from the top mounting surface to the
internal ground plane(s) as shown in
Figure 5.
Cross-sectional view
Internal
Copper
Plane
Via (0.30 mm)
Layer 1
Layer 2
Figure 5. Via Structure
Via
0.30 mm (max)
Internal
Copper
Plane
Solid
Copper Fill
Via
0.30 mm (max)
Figure 3. PCB Layer 1 (top view)
MicroStar Junior package
Package
Ball Via
Copper Land
Top Side Thermal Pad
A
B
PCB
Figure 6. PCB Ground Plane Layers
(top view)
(Not to scale)
A = Via diameter on package
B = Land diameter on PCB
Ratio A/B should equal 1.0 for optimum reliability.
Figure 4. Package and PCB Land Pattern
(side view)
Via
0.30 mm (max)
Table 1. Ball Pitch vs Diameter of Lands
2
BALL PITCH
LAND DIAMETER
0.5 mm
0.28 mm
0.65 mm
0.33 mm
MicroStar Junior  Made Easy
Figure 7. Drill Pattern
SSYA009A
3. When connecting the vias to the ground
plane, do not use the typical web or spoke
via connection methodology. Web
connections have a high thermal resistance
that is useful for slowing heat transfer during
soldering operations. This makes the
soldering of vias that have web connections
easier. However, in this application, low
thermal resistance is desired for the most
efficient heat transfer. Therefore, the vias
should make their connection to the internal
ground plane with a complete connection
around the entire circumference of the plated
hole as shown in Figure 8.
The 0.30-mm vias should be plated with at
least 1 ounce of copper so the holes will be
completely filled. For thinner copper, smaller
vias should be used. Be aware that the
viscosity of the plating solution may prevent
adequate flow through small vias. Thereby
leaving the center of the via unplated or
open. If the copper does not completely fill
the via, epoxy or solder mask should be used
to fill the remaining holes to prevent wicking.
Exposed
Copper
Land
Solder
Mask
Via
Covered With
Solder Mask
Figure 9. Solder Mask (top view)
5. Apply solder paste to the exposed thermal
lands shown in Figure 10. Proper placement
of the solder paste is important as this will
properly center the IC during the solder
reflow operation.
Solder
Paste
Solder
Mask
Solid Via
RECOMMENDED
Web or Spoke Via
NOT RECOMMENDED
Via Connection
Figure 8. Via Connection
4. As shown in Figure 9 and Figure 11, the top
side solder mask should leave the lands
exposed. The exposed copper lands should
be aligned with the balls and sized according
to the land diameters sizes listed in Table 1.
The 0.3-mm vias between the lands should
be covered with solder mask.
Figure 10. Solder Paste (top view)
6. When these preparatory steps are in place,
the MicroStar Junior IC is simply placed in
position and run through the solder reflow
operation as any standard surface-mount
component. The result is a part that is
properly installed.
NOTE: This is a thermally enhanced PCB
construction and care must be taken to ensure
solder reflow temperatures are achieved at the
component location. It is best to verify that the
PCB vendor has the process capabilities to
achieve a proper thermally conductive solution.
MicroStar Junior  Made Easy
3
SSYA009A
Signal Pads
Top Side Thermal Pad
Solder Mask
Signal Pads
Solder Mask
Vias
PCB
GND Plane
PCB
Power Plane
Power Plane
Bottom Side Thermal Pad
PCB
Solder Mask
Figure 11. PCB Side View
For detailed information on MicroStar BGA
packages, see SSYZ015A MicroStar BGA
Packaging Reference Guide.
4
MicroStar Junior  Made Easy
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