Hot Air Solder
Leveling (HASL) is, in fact, an art. The ability and knowledge of
the operator is very important to the quality of the finished
product. In addition, a thorough understanding of each process
parameter, and periodic routine maintenance, will also contribute to
a desirable finish.
Todays hot air
leveling systems can handle the entire process automatically. By
accurately controlling the various process parameters, and
mechanical functions, these systems can achieve a high degree of
repeatability. A complete in-line system will automatically pre
clean, flux, solder, cool and post clean the panels. In general,
these systems can achieve production rates of 150 200 panels per
hour. Semi-automatic systems are also available, automatically
performing only the solder leveling process. The pre-clean, fluxing,
and post clean process can be accomplished either manually, or in
separate off-line units. These semi-automatic systems can achieve
production rates of 60 120 panels per hour.
Operator ease and
accessibility are critical with hot air leveling systems. The
operator must spend time cleaning and calibrating the machine in
order to keep it operating efficiently. Solder and flux residues can
accumulate very rapidly and impair the machines performance. Ease
of accessibility aids the operator in maintaining the machine at its
optimum performance level. Accessibility is also important, because
if the system is not readily accessible, an operator can become
discouraged with performing routine maintenance tasks.
Serviceability with respect to repairs and maintenance can be summed
up by saying, The simpler the better.
The most common
method of fixturing a panel in vertical HASL system is through the
use of a pneumatically operated clamp. To prevent solder from
adhering to the clamp, they are usually manufactured out of
stainless steel or titanium. Traditional clamping systems only clamp
the panel at the top, and passive side guides are used to further
assist the fixture of panels. These guides help maintain the
structural integrity of panels of various sizes. Other systems offer
the ability to fixture the panel at a 45º angle. This 45º fixture
supports the top and bottom of panels, greatly enhancing the
structural integrity of the panel. The 45º fixturing method has a
tremendous advantage to that of traditional clamping systems.
Key advantages of the 45º-fixturing method:
Eliminates solder short20 mil quad packs and below
Decreases the variation of solder thickness on Quad
Packscoplanarity is greatly improved.
Eliminates bowing of panel, giving a more uniform
front to back finish, bottom panel support breaks the surface
tension of the solder pot.
The uniformity and wet-ability of copper
surface depends mainly on the following factors:
A. The cleanliness of
B. The viscosity,
surface tension and activity of the flux
C. Pad Orientation
D. Copper Pad Width
E. Air Pressure
F. Clamping, 45º vs.
Pad orientation has a large effect on solder
thickness variation. The greater the variation in pad orientation,
the greater the variation in solder thickness across the panel. In
general, HASL systems have a great leveling effect on vertical pads.
Solder will tend to flow down on vertical pads, creating a puddling
area on vertical pads. Horizontal positioned pads hold more solder
than vertical pads. The solder thickness of horizontal pads is
approximately equal to the solder thickness of puddled areas on
The greater the pad
orientation varies between horizontal and vertical, the greater the
likelihood of variations in solder thickness. One of the most
prominent examples of pad orientation variation is the surface mount
Quad Packs. Thus, some compromises in solder thickness may be
required in order to solder level surface mount panels. In addition
to pad orientation, the individual pad dimensions are also a factor.
The surface mount pad width has a large effect on the solder
thickness. Narrow pads have a greater tendency to retain solder,
than do wide pads. Larger pads with their greater surface area tend
to allow an increased solder flow, leading to a decrease in solder
thickness. See Figure 1
From the above graph,
you can see the clear relationship that exists between narrow pads
(thicker coating) and wider pads (thinner coating). This is a
universal relationship among all liquids. The tighter the area the
liquid is confined to, the more height you will get to the drop, or
bead, of liquid applied. The issue of prime importance here,
however, is not the solder level within a given pad widthbut rather
the delta between the vertical and horizontal pads of similar width.
In the graph above, you will notice that this delta (which we call
variations) starts out low, in the 8 Mil pad widths, and rises as
you approach the 24 Mil pads. This is also normal in solder
leveling, and occurs whether you use horizontal or vertical leveling
systems. Once again, the delta or variation between the
horizontal and vertical pad thickness is the main issue here.
In todays market,
the whole issue of horizontal to vertical variation has been reduced
to the SMT users need for solder coplanarity, on all sides of a
quad pack. Since two of these sides are vertical, and two
horizontal, the need for minimal height differences is important.
Historical data gives
us the ability to see what happens to the solder thickness
characteristics, utilizing different air knife pressure settings.
Using pads in the 16 to 18 Mil width range, we sorted by the amount
of PSI used by the air knife. Keeping all the normal parameters
constant, the results of this study are shown in Figure 2.
As might be expected,
the higher the PSI used in the air knives, the lower the solder
thickness and an increase in thickness variation. While the above is
a representation of nominal values, we caution against using high
PSI to solve thickness problems. There are aberrations that occur
once you go above 100 PSI, and you will find that the thickness
above this range is too erratic for normal processing purposes.
previously, 45º clamping is superior to traditional 90º clamping.
The premise was, that the traditional 90º clamping of panels
influenced the horizontal and vertical pads is such a major
contributor to solder level differential, than the 45º clamping had
to be an improvement.
Tests were performed on a population of panels divided into three groups.
The panels were all identical in terms of circuitry, and the focus
of attention was on the leveling performance of the 20-mil pitch
Quad Packs. One group ran at 75 PSI, the second at 80 PSI, and the
third at 85 PSI. For each of these settings, the nominal solder
thickness values for horizontal and vertical pads are charted in
Figure 3. For the test, normal parameters were kept constant, with
the exception of the PSI settings. In addition, the front to back
PSI differential between air knives was kept to a 2 to 3 PSI
As Figure 3 illustrates, by clamping the panel at
a 45º angle, the Quad Pack horizontal and vertical pad variation was
greatly reduced. The results at 75 PSI were improved over the
traditional clamping method at 75 PSI as shown on the previous chart.
This is worth noting as it correlates with the improvements seen at 80
and 85 PSI. At 80 PSI we are starting to make progress. A 300 micro inch
variation has been reduced to roughly 180 micro inches. At 85 PSI, the
solder thickness of 287 micro inches on the horizontal pads, and 272
micro inches on the vertical pads show a reduction in variation of 15
micro inches on the 20 mil Quad Pack. Below are the results of four
individual tests that were performed, confirming the consistency of the
45º clamping process.