Steve has recommended a DFM checklist by John Maxwell at AVX (see below).
Having read this document thoroughly, I have to caution that it looks to be
dated, and contains inappropriate advice for modern designs and production
methodologies.
For example the recommendation to orient all ICs and chip components in one
orientation only, for ALL REFLOW techniques (which presumably includes
modern forced-convection ovens) not realistic advice, and not necessary.
The component separation guidelines were clearly not created when 0402
chips existed.
The statement that most SMT placement machines use tooling holes for board
registration, rather than fiducial marks and cameras is also wrong and
dated.
I could go on with other concerns, but the examples above should suffice.
Jeff Finger
Manufacturability Specialist
Aimtronics Corporation
>>> Steve R Mikell <[log in to unmask]> 04/01 9:04 AM >>>
[log in to unmask] asked
Is there a standard for the distance between SMD components in the wave
soldering process to prevent shadowing? We are currently basing it on
1.5
times the component height. What is the shape of the "deadspace" behind
these components, rectangular, conical, oval... ?
Any help is greatly appreciated.
::::::::
Dean,
Several people will give you a good "rule of thumb" for your question.
However, I would suggest that you follow good engineering practice and
determine what your customer can tolerate (that's the process boys in the
factory). Many years ago I assisted a couple of engineers from a company
whose design group had created an SMT wave solder design study PWB. The
board
started at one end with typical component layouts that gradually tightened
in
spacing as they moved to the other end. The objective was to try and
determine the smallest spacing that still soldered without defects. The
board
was sent to all of the companies factories to determine what the limits of
the
current factory processes were, with the intent to design only what could
be
built.
Many of us offer comments about design rules, saying how we use them day
to
day, without specifying the particulars of our process. The equipment,
stencils, fluxes, solder pastes, etc... are all important parts of the
process
and directly impact the design rules. Just as important, in the wave
solder
SMT case, will be the shape of the lands used. The use of longer but
narrower
lands and selective use of robber pads will also impact the
manufacturability
of the design.
For some good reading, try http://www.avxcorp.com/tech/smt.htm
Steve Mikell
Lead Ind. Eng. Plt 13
SCI Technology
################################################################
TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################
|
