Mike,
can't help you with the OSP-HASL dilemma, but as far as the printing is
concerned, I would look at something else that generates the shorts. We are
using on some fine pitch even 100% aperture (0.012" for .020" pitch) and
still have no shorts. The apertures you mention are OK, you shouldn't blame
the shorts on them.
There are a few major things you should consider:
- the printer itself; does it perform the alignment stencil-PCB
automatically, or the alignment is done manually. Manual won't get you rid
of shorts, no matter what you do!
- have the operator wipe the fine pitch area on both bottom and the top of
the stencil every 5th or 6th board
- the paste itself: powder size, how long out of the fridge, etc
- printing speed
- snap-off and squegee pressure
- you mean shorts after print, or after reflow? If you see them after reflow
and not after print, maybe you manipulate the boards too much, or check the
insertion and manual correction.
Hope this helps,
Ioan
> -----Original Message-----
> From: Mike Lang [SMTP:[log in to unmask]]
> Sent: Monday, May 08, 2000 10:26 AM
> To: [log in to unmask]
> Subject: [TN] OSP and Fine Pitch
>
> Hi gang,
>
> A few general questions regarding fine pitch SMT assembly:
>
> 1) My research indicates the "industry starting point" for fine pitch
> (0.020" pitch) solder pad and stencil design is 0.012" wide PCB solder
> pads and 0.010" wide laser cut stencil apertures in 0.005" or 0.006" thick
> stencils. I find that using these parameters I still tend to get some
> solder shorts. Any comments about better pad / aperture designs?
>
> 2) I am using HASL, but recognize the benefits of going to OSP (flatter
> among other things). My concern is the bare copper left on the PCB since
> the paste does not fully wet out the pad. Does anyone have comments, or
> better yet, test data which demonstrates what is acceptable?
>
> 3) Another HASL / OSP concern... PCB is double sided reflow using no
> clean, and then has a small quantity of hand soldered (GASP!) through hole
> components assembled. Will the PCB remain solderable for hand soldering
> after 2 reflow cycles? Again, I expect bare copper in places since the
> solder will not naturally flow out to cover all surfaces of the hand
> soldered pad, top and bottom, especially after two reflow cycles.
>
>
> Thanks!
>
> Lang
>
> ##############################################################
> TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
> ##############################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with
> following text in
> the body:
> To subscribe: SUBSCRIBE TECHNET <your full name>
> To unsubscribe: SIGNOFF TECHNET
> ##############################################################
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> additional
> information.
> If you need assistance - contact Keach Sasamori at [log in to unmask] or
> 847-509-9700 ext.5315
> ##############################################################
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################
|
