To understand the causes for uneven surfaces from HASL it helps to
understand the process.  First of all their are two types of HASL - vertical
and horizontal.  My comments are directed at the vertical process -

The product that is subjected to HASL is a panel coated with soldermask.
The soldermask exposes bare copper where you want to apply solder. This
panel is coated with flux and optionally preheated.

The panel is place in a clamp and submerged into a vat of molten solder.  It
is held in the solder for a dwell period then removed from the solder.

As the panel is retracted from the solder pot it is blasted with heated air
- this blast of air blows the solder off of the panel. (Thus the name hot
air solder level)

This process is very violent and there are a lot of variables that have to
be identified and an attempt is made to controll the solder removal.

Some of the process variables are easily controlled.  At least they are
easily set.  Air pressure, dwell time in the solder pot, angle of the air
knives,  speed of entry and retraction.  Each on of these items (and there
are many more) will have an effect on the leveling result.

There are a number of things that can't be controlled - the physical
orientation of the pads or size of the pads for example.  For example, if
you have a board with a quad pack  on it you will generally see that the
solder is higher in one direction than the other.  In other words the pads
going up and down will have thicker solder than the pads going left and
right.  (This may be opposite depending on the orientation of the panel when
it was leveled)


This x-y phenomenon is due to the surface tension of the solder and the air
currents in the HASL process, and to a certain point the flux used, the
speed on the in-out and the angle that the panel was placed in the machine.
(To address the X-Y pad phenomenom some levelers are designed to clamp the
panel at a 45  degree angle.

Oh, and gravity will have a lot to do with the thickness - this is
especially evident when you have a single pad that has both thin and thick
solder.

Bottom line is that there are a lot of reasons for thin/thick solder.  The
only way I know to eliminate the problem is to to go to a solder alternative
(ENIG, silver, OSP etc) or specify a horizontal leveler.

Good luck,

Pete







>From: Karen Ebner <[log in to unmask]>
>Reply-To: TechNet E-Mail Forum <[log in to unmask]>,              Karen Ebner
><[log in to unmask]>
>To: [log in to unmask]
>Subject: [TN] HASL Leveling
>Date: Wed, 15 Jun 2005 14:46:06 -0400
>
>What are the most probable causes for an uneven surface of a HASL finish
>pad on PCB??
>
>
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>Karen Walters Ebner

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