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Subject: Re: solder mask

Date: 07/02/03 at 5:24 AM
Posted by: KARTHIK PVS
E-mail: karthiktilak@yahoo.com
Message Posted:

In Reply to: solder mask posted by Aleksander on 07/01/03 at 12:22 PM:

1) The gangway soldermask is the best for fine pitch components i.e for 0.8mm and less. In case of QFPs, PLCCs which have a fine pitch gangway soldermask is the best way of going ahead. Anyway, we would not be routing inbetween the pads, so the gangway soldermask is the best.

As per IPC-SM-840 which deals with the soldermask issues, gangway is the best for fine pitch components. One more thing is that soldermask application also depends on the capability of the manufacturer. There are manufacturers who can create soldermask openings which is a 1:1 replica of all the pads which need to be opened up on the board. In appliance domain they prefer to have soldermask 10 mil greater than the pad size i.e 5 mil clearance from each side of the pad. In ost of the high speed digital boards its 1:1 replica of the pads. So o question of clearance.

We will not be able to maintain the soldermask dam requirements in case of very fine pitch components. So its better to apply soldermask all over the pads(gangway type)

2) We should never try to reduce the pad size ( keeping annular ring requirements and the solderability in mind) as it will decrease the joint strength and hence decrease the reliability of that component.

3) Usually for the SMD footprints the finger shaped pads are used for denoting the pin number 1. It is just an issue of convention and convinience.

Sometimes in some CAM tools ( when gerbers are being viewed)the arcs( which is a collection of points) of the finger pads may not get represented properly which needs an editing of the D-codes and gerber files corresonding to those finger pads.

This will be a kind of re-work.

Finally it is the call to be taken by the designer as to which type of pads he should use for his footprints and try to maintain them uniformly over the board.

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