Dacia

In my experience and from what I heard so far from the industry members
of LEADFREE mixing is possible.

1) lead free components with tin-lead solder
No problem  except with BGA's. As a matter of fact this combination has
been used already for a long time since several producers of components
introduced lead free finishes years ago without notifying their
customer. The problem with lead free BGA's persists. We did work on
that. However, the publication is in the review process since April this
year. In short we can say that soldering a lead free ball with lead
containing solder gives a metallurgical bond since the ball goes into
solution with the solder without melting. How much of the ball is solved
in the solder depends on the solder parameters. The balls alloy
partially with the solder thus being separated into a zone consisting of
lead free solder and a zone with a mixture of lead free solder and lead
containing solder. In the latter the lead is concentrated along the
grain boundaries. Main problem is that the balls do not collapse and
coplanarity becomes an issue. Right now the specimen are in the slow
thermal cycles and up to 2000 cycles we didn't observe a faster
degradation compared to the other specimen.

2) Lead containing components with lead free solder
Again, alloying works fine also with BGA's. It is true that on some
occasions reduced wetting was observed. However, I doubt that in the
field a controller could identify that consistently. Here we also have
lead contamination along the grain boundaries in BGA solder joints but
through the entire ball. The lead of the components other than BGAs was
very hard to find in the solder joints. Thermal cycling see 1).

So, in short, mixing lead free and lead containing technology works
from the metallurgical point. The one open point is still the
reliability of the solder joints.

Yes,  principally  FR4 can be used for the lead free process. What I
heard from companies working on that is that up to 4 internal layers it
works fine. Thicker boards need higher process temperatures and high TG
materials might be necessary. However, as you said, you need to try
that.

Yes you can run lead free and lead processes in the same equipment if
your equipment is suitable for the lead free process. Logistics might
become a problem especially with MSL sensitive components.

Best regards

Guenter

EMPA
Swiss Federal Laboratories  for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1 823 4054
mail:     [log in to unmask]

-------------------------------------------------------------------------------Leadfee Mail List provided as a service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Leadfree
To temporarily stop/(start) delivery of Leadree for vacation breaks send: SET Leadfree NOMAIL/(MAIL)
Search previous postings at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-------------------------------------------------------------------------------