Re: Designing for and soldering a tiny BGA

rickman wrote:
On Apr 11, 8:18 pm, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
rickman wrote:
On Apr 11, 4:33 pm, linnix <m...@xxxxxxxxxxxxxxxxxx> wrote:
On Apr 11, 1:17 pm, zwsdot...@xxxxxxxxx wrote:
On Apr 11, 3:20 pm, linnix <m...@xxxxxxxxxxxxxxxxxx> wrote:
difficult and IMHO very ill-advised to attempt fanout of a dense-pitch
MBGA without at least four layers. I know it's possible to do it in 2,
but it's not ideal.
If you need most of the pads, 2 is impossible.
Not quite impossible, just a very bad idea. I have seen commercial
products where they put a via off-center inside the pad and use mask-
defined pads to keep the paste JUST off the via's hole, then using
very fine space/trace rules you can get two signals out between every
pair of pads. Very nasty.
Perhaps for larger pitch. But for 0.5mm pitch, you need 0.1mm to
0.2mm lines. I would rather wire-bond (0.1mm wires) than using such
nasty PCBs. One possiblity is to wire-bond a jumper block opposite
the BGA, to flip the pads inside out. By the way, I just got a quote
for bonding wires at $0.001 each. So, for higher volume, wire bonding
is the way to go.
I think this debate shows the difficulty of designing a PWB for fine
pitch BGAs. It should only be attempted after a great deal of
research in how not to make coasters.
As to the comments by zwsdot about Eagle vs. FreePCB, if I am not
mistaken, his comments show that he has not used FreePCB. If you take
a look at the package, I think you will find very few shortcomings vs.
Eagle or other available packages. My main point was actually about
Eagle. It has some very significant disadvantages as zwsdot mentioned
in addition to the point I was making about the licensing. Although
at one time Eagle was the package of choice for hobbyists, I think
that time has passed and there are any number of PWB layout packages
that will do a very good job without all the drama of Eagle.
AFAIK FreePCB does not contain a schematic editor. It is very nice to
have that and the layout integrated. Personally I am using Eagle and
found it rather nice. But Eagle has one serious shortcoming: No
hierarchy. That's why I am looking for something better but haven't
found anything better yet. Tried gschem, KiCad and some others but they
all have an issue or more.

If that is important to you, then Eagle it is. I have not found a
strong need for that. I would like to have simultaneous integrated
schematic for back annotation, to be able to do it on the fly rather
than batch mode. I brought this up once and did not find anyone who
thought this was a good idea. They seem to feel it is better to have
the two separate so that you can use whatever combination of tools
that you prefer.


That's what Eagle does, nicely integrated schematic and layout handling. But no hierarchy and once you are past a dozen pages with no top layer *** the circuit might still be readable to you but not much to others. And you even might have trouble yourself when you revisit it 5-10 years down the road.


There are also other tools that support combined schematic and
layout. I have not tried them because they are primarily Unix/Linux
based and support for Windows is minimal. One fine day I will get a
Linux machine up and running and give some of this a try. But in the
mean time, FreePCB is an excellent tool and I much prefer it to Eagle
with its bizarre interface and licensing issues.


I've tried gschem. You can run it nicely in a virtual engine. I use VirtualBox from Sun plus Ubuntu. However, gschem has an IMHO serious problem with reference designators when you need a multi-part chip such as a quad opamp with the supplies piped out and not inherently connected to VDD/VEE.

KiCad came close to ideal but for some reason that completely eludes me it has an ugly schematic frame hardcoded it. I've also still got DOS-OrCad and was thinking about a Windows version but they crashed a bit much on me at clients, plus out of principle I do not like the forec-feeding of a service contract where I never needed one.


Ian: If you are on a tight budget also for production try to avoid BGA
like the plague. Only sophisticated and professional (as in expensive)
assembly houses can handle those. This is not the stuff to try in a
modified toaster oven.

If, by expensive, you mean that they charge for their time vs being
able to "do it yourself", then yes, assembly houses are not cheap.
But one that can do BGAs is no more expensive than others. In fact,
my last board was designed with all leaded devices for ease of
debugging and assembly. I picked the low cost assembly house and it
turns out they also do BGAs. So my next job may well use non-leaded
parts, either QFN or BGA.


Let's hope they last. I've seen cases where BGAs worked initially but then some failed in the field.

--
Regards, Joerg

http://www.analogconsultants.com/

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