The BGA package is composed of three basic parts: the bare chip, a BGA substrate, and an interconnection matrix (see figure). Depending on the package style, the bare chip may be affixed to the BGA substrate either face-up or face-down. The interconnection matrix then connects the bare chip to the BGA substrate using wire-bond, tape-automated-bonding (TAB), or direct attach flip-chip style connections. The BGA substrate, which is really a miniature multi-layer PCB with teeny-weenie traces and microscopic through-hole vias, conveys the signals to the underlying printed circuit board through an array of solder-bump attachment pads on its bottom surface. A metal cover or plastic encapsulation is then used to seal the package.

Here’s the reasons why I love the BGA packaging concept:

1. BGA packages are inherently low-profile. There is nothing to it but the chip, some interconnections, a thin substrate, and a plastic encapsulant. No big pins, and no leadframe. The total installed height above your PCB can be as little as 1.2 mm (0.05").
   
   
2. Their low profile and small size means that the total loop area, from a signal on the chip, through the interconnection matrix onto your PC, and back into the chip through the power/ground pins is very small, as little as 1/2 to 1/3 the size of the same loop on a QFP or SOIC package of equivalent pincount. This smaller loop area means less radiated noise, and less crosstalk between pins.
   
   
3. With all those bumps, you can devise very effective distributed arrays of power and ground bumps. Problems with ground bounce diminish in almost direct proportion to the number of power and ground bumps used.
   
   
4. Most BGA packages have big, fat, easy-to-work-with solder bumps, much bigger than the ones used for flip-chip connections. By way of contrast, flip chip techniques, which use solder-balls placed directly on the face of a silicon die, require solder bumps with much smaller dimensions, which can lead to troublesome and finicky manufacturing problems. Flip-chip has given the solder-ball technique a somewhat dark, mysterious and wholly undeserved reputation, one which I hope will be redeemed by the rise in popularity of the BGA.
   
   
5. BGA packages are sturdy. Compare this with a 20-mil pitch QFP. On the BGA, there are no leads to bend or break. It’s like a little brick.
   
   
6. With a BGA package, I can place a lot of the power and ground bumps in the interior bumps, leaving the I/O traces to the more routable positions around the edges. This is just one of the ways you can use the pre-routing inherent in the BGA substrate to straighten out an otherwise messy I/O routing situation.
   
   
7. Advanced BGA packages can cram all the solder-bumps right under the chip, with very little package overhang. That’s as good as can be done for miniaturization.
   
   
8. The bumps on the bottom look cool, and feel neat.
   
   
9. No fancy PCB technology is involved. It’s not like C4 or direct attach flip-chip technology, where you have to carefully match the thermal coefficient of expansion between the PCB and the chip in order to prevent die cracking. With a BGA package, the interconnection matrix provides sufficient mechanical compliance to relieve thermal stress on the die. No expansion mis-matches, and no hassles.
   
   
10. It is an inherently thin package, with reasonably good cooling properties. With the die mounted face-up, most of the heat flows down and out through the ball-grid array. In packages that mount the die face-down, the back side of the die is in intimate contact with the top of the package, an ideal arrangement for heatsinking.

I'm sure there are plenty of other good reasons to love the BGA. Let me hear yours. In the mean time, if you want more information about BGA packaging, check out the book edited by John Lau, Ball Grid Array Technology, McGraw-Hill, 1995, ISBN 0-07-036608-X. It's a keeper.

All Publications by Dr. Howard Johnson except as noted.
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