Re: [SI-LIST] : BGA vs Leaded - summary

Rich.EVANS@st.com
Tue, 9 Mar 1999 09:03:25 -0600

Here is a summary from a slightly different view:

BGAs have a lot of advantages, and accompanying disadvantages ---

a BGA gives considerable flexibility in design and pinout, compared
to a leaded packge.

a BGA generally gives smaller overall size for the same number of
connections -- if your PWB technology (=$$) can support it

a BGA cannot have bent leads, so assembly yields usually go up

Electrical stuff:

Most low-cost BGAs have bondwires. Bondwires are not necessarily
bad. If you put the return paths adjacent to the signals, it
can be well controlled. If you are clever, bondwires can also
be quite short, like 1 mm.

Most low-cost BGAs do not have internal planes. Be sure that you
need them... if the trace is only 1 or 2 mm long, is the plane really
doing anything for its characteristics? You might want the planes
to distribute power, though.

Most low-cost BGAs have electro-plated gold. This means that they
have plating stubs on every signal. Inner-row signals have longer
stubs than outer-row signals. This is important to include in
any electrical analysis.

Flip-chip is good, but not low-cost. Remember also that flip-chip
replaces the bondwires with trace, so the total path length is about
the same. Generally, anything that you can do to reduce the path
length reduces your SI concerns.

A disadvantage of the flexibility of BGAs is that it is difficult
to generalize about their electical performance. Performance is
really determined by the specific design.

We do a lot of BGAs at ST, you may notice that our focus is
on low cost!

Maybe the moral of the story is that you should talk to your package
supplier/designer a lot, and figure out how to use BGA's
flexibility to your advantage. Low cost, high performance, and
small size could all happen at the same time, but only if you
work at it!

Rich

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