AN1231 Motorola / Freescale Semiconductor, AN1231 Datasheet - Page 5

AN1231

Manufacturer Part Number

AN1231

Description

Plastic Ball Grid Array (PBGA)

Manufacturer

Motorola / Freescale Semiconductor

Datasheet

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that it can typically be placed onto printed circuit boards and

assembled using existing surface mount equipment. This is

not true for many other new and high pin count packaging

technologies such as TAB, DCA, fine pitch QFPs, PGA, etc.

Most require new or upgraded process equipment and in

some cases new processes or manual assembly. It has the

added advantage of being compatible with existing handling

systems. Some pick and place equipment may require a vi-

sion system updgrade to recognize BGA balls. Open tooled

handling media, namely trays and tape and reel (heat seal or

C-pak) are available for many of the JEDEC PBGA body

sizes. Handling damage is significantly reduced by package

robustness due to the absence of fragile leads.

FLUXING/SCREEN PRINTING

spraying, dispensing, or foaming) must be applied to the PC

board solder pads prior to assembly. This is necessary to not

only reduce oxides formed on the solder pad, but also on the

solder ball. Slight solder ball oxidation may occur during ex-

posure to burn-in, storage, and dry baking in non-inert atmo-

spheres. Typically, the method chosen to apply flux is done

MOTOROLA FAST SRAM

One of the greatest advantages of the PBGA package is

Either solder paste (cream), paste flux or liquid flux (i.e.,

Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì Ì

SURFACE MOUNT ASSEMBLY

b) conventional non-soldermask defined pad. Note the greater volume of solder and greater

effective joint diameter for the non-soldermask defined pad to achieve the same stand-off.

Note: When encountered, fractions of traces were rounded down. Line spacing assumed to be equal to trace width.

Board Technology

Figure 3. Comparison of PBGA Solder Joints with: a) soldermask-defined pad and

Î Î Î Î Î Î Î Î Î

Widths in Mils)

(Line/Space

Î Î Î Î Î Î

Table 3. Number of Escape Traces That Can Be Routed Between PBGA

Pads Given Device Pitch, Pad Diameter, and Board Line/Space Widths

3/3

4/4

5/5

6/6

8/8

1.5/23

Number of Escape Traces Routed Between Pads Given Ball Pitch/Pad

Diameter (mms/mils). Shaded Columns Represent NSMD Pads.

1.5/25

1.5/31

Î Î Î Î Î Î Î Î Î Î

to maintain compatibility with current processes. Due to the

fact that the solder ball is comprised of eutectic or near-eu-

tectic solder and its entire volume is molten during reflow, it is

not necessary to add solder volume to the joint with solder

paste. The 30 mil diameter ball provides enough volume to

give a 16 to 24 mil average stand-off across the device de-

pending on package and device/board solder pad configura-

tion. This is enough collapse to ensure that no opens will

occur due to device or board warpage at elevated tempera-

tures (more discussion on device warpage in the Coplanarity

section). Applying an amount of solder paste equal to 14% of

the ball volume (i.e., eight mil thick stencil, 25 mil diameter

apertures, final solder volume = 1/2 solder paste volume) will

generally increase the stand-off by one to two mils. In some

cases additional solder volume may be advantageous to

increase stand-off and subsequent device solder joint reli-

ability. However, applying larger amounts of solder paste with

the use of thicker stencils and/or larger apertures has the po-

tential to result in joint voiding, especially when combined

with fast oven ramp rates and volatile fluxes. Voids are

formed when flux volatilizes and is entrapped within the joint.

Voids form at the bottom (motherboard interface) of the ball,

but end up at the joint/package interface due to buoyancy

effects. These voids have been shown not to be a reliability

risk. Figure 4 shows a pad from an PBGA test board with

1.5/33

1.27/23

1.5/25

1.27/31

1.27/33

AN1231

AN1231 Motorola / Freescale Semiconductor, AN1231 Datasheet - Page 5

AN1231

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