gEDA: Component Packages and PCB Land Pattern - naming Conventions

[Stephen Meier <smeier-at-NOSPAMAlchemyResearch_dot_com>]

Here is my take on the subject.

1. Components --- devices that are often (usually packaged) have 
charateristics that should not be ignored at the schemtic design level.

Issues:

Device susceptibility to soldering temperature.
Is the device Pb free?
Does the device package match the thermal characteristics of the fab 
plastic?

For information about the device package we look to the JEDEC standard JEP95


But JEP95 has package families that are physically similar but made from 
different materials. For example Plastic Leadless Chip Carrier (PLCC) 
(JEP95 MO-047) and a Ceramic Leadless Chip Carrier (LCC) (JEP95 MO-044). 
Both include 0.050" pitch 68 pin leadless devices that would fit the 
same land pattern.

2. Land patterns are also dependent on the assembly solder method (wave 
solder, solder bath, reflow oven). IPC-SM-782-A "Surface Mount Design 
and Land Pattern Standard" includes a note in section 8.1 that states 
"If a more robust pattern is desired for wave soldering devices larger 
than 1608 [603], add 0.2mm to the Y direction and consider reducing the 
X dimension by 30%. Add a "W" sufix to the number e.g. 103W".

The number is the Registered Land Pattern Number of which a 103A pattern 
is the normal pattern for a 3216 [1206] chip resistor.


Proposal:

Symbols for gschem should not include the land pattern they should 
include the package identifier. The package identifier should be from 
the JEP95 or other consortium (European or Japanes) which ever is 
relevent for the device. I would also like to see the gschem symbols 
contain information about Pb content. Gschem should also have a way of 
showing the user what the package is and if it contains Pb in the 
preview mode while selecting devices from the library.

The netlist generator should translate the package identifier to a land 
pattern identifier based upon the assembly soldering method (normally 
reflow for surface mount). The land pattern identifier should be from a 
standard such as IPC-SM-782-A.


Steve Meier