Phil,

We have built a number of PCAs with lead-free and leaded balls with mixed
success..

The first thing to consider is the type of metal in the solder paste...This
is what forms the joint...You must first reflow the solder paste...

Next, what thermal profile will the PCB take... This determines the amount
of stress the joints will see after the solder solidifies and before the PCB
material reaches its final rest state....At this point you may also want to
look at the type of PCB surface finish...HASL seems more forgiving than
ENIG..

Lastly the solder paste thickness and metals content...These give you joint
volume and flux amount which reduces ball surface tension...This plays a
very big role in ball collapse...

Assuming you are using convection driven reflow ovens..

and we have a HASL PCBs with standard PCB material, running 90%-91% 63-37
metal water-soluble solder paste, with 6+mil thick stencils, we are
successful at 10C over our current peak with standard profiles.


Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48

-----Original Message-----
From: DUTTON Phil [mailto:[log in to unmask]]
Sent: Thursday, February 10, 2005 8:18 PM
To: [log in to unmask]
Subject: [TN] Mixing Lead free BGAs with normal BGAs

Hello,

We've run into a problem where a PB Assembly will have a mixture of
BGAs, some with regular tin/lead solder balls, some with lead free
solder balls.
This raises the question as to what temperature profile to run. Do we
need to ask our loader to do a Pb free profile first, followed by a Pb
profile? Or will it be ok to solder both together using the regular,
lower temperature Pb profile?
Personally I'd like to avoid the extra run through reflow.

thanks,

Phil Dutton C.I.D.

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