Hi Ioan,
What we have found is the FR-4 stiffener is a) often not flat enough so when
you reflow the BGA you can have issues b) being so thin it does not provide
enough support in assembly and/or end use of the product. Often we wind up
using stainless steel stiffeners and we pick a stainless steel type which
considers all the CTE's in the build plus the BGA. Usually the BGA rules
because flex being so thin the thermal mass is not the overriding factor. We
often have the stainless steel stiffeners built by wire EDM to hold a very
tight flatness which is obviously not inexpensive. You also may have to
consider the CTE's of the adhesive used to bond the stiffener to the flex-
sometimes we use a polyimide, sometimes an acrylic sometimes even a lo-flow
pre-preg(polyimide or FR-4). Finally the new IPC-6013 specification
tightened up the stiffener requirements a lot. Before a stiffener was a
mechanical support only- now there are all kinds of requirements for voids
etc. How one determines this for a stiffener you can't see through I have no
idea so for my company we wrote most of our critical customers and took
exception to 3.3.2.13 or we sample - but doing a x-section in one area of a
stiffener quite frankly tells me zilch and how we apply this spec to
stiffeners put on with pressure sensitive adhesives is beyond me ( forgive
the rant on the last part). 
So in summary if we use FR-4 the BGA is usually small but we will probably
wind up doing some kind of under fill or corner fill unless in the final
package everything is well supported,  and for high end applications we use
metal.
Good luck.
Regards Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: February-04-10 9:21 AM
To: [log in to unmask]
Subject: Re: [TN] Laser soldering of BGA

Technos,

There are 2 main ideas I can gather from your answers:

1. Why do I bother with laser when I could reflow? Well, as I said, reflow
seems to warp either the component or the PCB and having no idea about laser
soldering myself, I thought it would be a good idea to inquire. Since it
seems to have no benefit, I believe I better get back to reflow. But not
before asking another one: the BGA is soldered onto flex that is backed by a
12 mil rigid something; are there lasers that could solder from the bottom,
through the substrate, rather than through the part, thus minimizing the
heat seen by the assembly?

2. Steve K, could you please elaborate regarding the metal stiffener vs.
FR-4. To my surprise, it seems that brass stiffeners yield better results
than FR-4 indeed; shouldn't they give a bigger CTE mismatch?

Thanks,

Ioan Tempea, ing.
Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
T | 450.967.7100 ext.244
E | [log in to unmask] 
W | www.digico.cc

N'imprimer que si nécessaire - Print only if you must

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