Matthew,
Are these capacitors being wave or reflow soldered? Assume wave soldered
if assembly dept is asking for reduction in pad width. Some have found
that reduced pad widths are better for their process. Check out
http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfilesname/F3102Fce.PDF/$file/F3102Fce.PDF
. (I hope the link works.) Here a mfgr has a pad width for wave
soldering at about 70% of the width for reflow soldering.
Also, if wave soldering, is the dwell time too long in the wave? Preheat
cycle too short? Parts along board edges or near connectors perpendicular
to board surface? Rough handling? Same type of part failing each time?
Here's a question for everybody: IPC-SM-782A has reduced width pad
geometries for resistors, but not capacitors. Back in unused portion of
brain a memory keeps saying that the no-rev version of SM-782 had reduced
width for res-s and caps. Anybody care to comment? Any reason for the
change (if it really was so)?
Roger M. Stoops, C.I.D., PCB Designer
Trimble
Engineering and Construction Division
5475 Kellenburger Rd.
Dayton, OH 45424-1099 USA
Ph: +01 937.233.8921 or +01 937.233.4574 ext 288
Fax: +01 937.233.7511
Matthew Lamkin
<matthew.lamkin@PROTEC-FIRE-DETECTI To: [log in to unmask]
ON.PLC.UK> cc:
Sent by: DesignerCouncil Subject: [DC] Capacitor pad reduction ?
<[log in to unmask]>
08/27/01 10:09 AM
Please respond to "(Designers
Council Forum)"; Please respond to
Matthew Lamkin
Hello everyone,
I wonder if I can tap into your experience of cracking of ceramic
capacitors.
I have a situation where the shop floor are having problems with cracked
capacitors, due to
several possible causes.
However, the blame for the problem is being thrown at design with an
instruction to reduce
the width of the pads (1206/0805/0603 caps only) to be less than the width
of the component.
Now there are many documents out there advocating using the IPC SM-782-A
pads, and many that
advocate reducing the pads.
However, I have the IPC specs which give me actual dimensions to follow
whereas the documents
that advocate reduced pad widths do not.
Can anyone give me some advice on what size these pads should be when
reduced to prevent MLCC cracking?
Can anyone supply a URL of a document that gives any good information on
this?
I have many documents from AVX/SYFER etc on the subject but they do not
quote any figures on reducing the width.
Thankyou -
Matthew Lamkin
Protec Fire Detection PLC.
England, U.K., Planet Earth.
P.S. Come on Steve, I'm timing you now, to see how long it takes until you
quote a website....he, he....
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