Hi Murlidhara,



HASL operation itself being very old in the industry, also has its
advantages & disadvantages. The spec. for the HASL thickness within a pad &
across various other pads varies from customer to customer, depending on
their process, application etc., & that is the reason the HASL thickness
spec. in general has a wide variation from 50 or 100 micro inches minimum to
1000 or 1500 micro inches maximum, that is the nature of the beast.

In general, since the HASL operation is dependent on the pad size - smaller
fine pitch devices tend to get thicker solder & as the pad size increases
the solder distribution across the pad would tend to get more even & also
you would get the thinnest solder deposit at the centre of a large pad. So
writing a spec. for HASL has to be very general. Also if one uses vertical
HASL compared to horizontal then you would different results.

The other thing is where on the pad you measure the thickness - normally the
edges, especially the two corners of a pad would tend to be thicker compared
to the centre of the pad. The east & west pads of a QFP device would be
thicker than the north & south pads depending on the direction of the board
through the HASL machine.

Hence most of the customers would spec. in such a way that you measure the
thickness across various pad sizes starting from a fine pitch device to a
large pad on the board & the thickness would have to be measured across the
geometric centre of the pad or taking the average of the four corners &
centre of the pad. Some of the critical customers would also ask for
measuring the highest & lowest point within a pad & the variation should not
be greater than 250 micro inches, or a different number depending on the
device, application & process used.



Hope this helps.



Thanks,

Krishnan

SANMINA-SCI



-----Original Message-----
From: Murulidhara [mailto:[log in to unmask]]
Sent: Saturday, February 05, 2005 4:11 PM
To: [log in to unmask]
Subject: [TN] Hot air leveing for SMD - specification



Dear Technetters,





I want to know wheteher there is any specification for hot air leveling
operation for PCBs with SMD?





What is the level difference allowed from pad to pad and what is the level
difference allowed within a pad?





Thanks in advance.





Regards,





Murulidhara.S





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