Hot air solder levelling—is it a HASL?
between circuitry and the resist aper- achieve its true capability with tin/lead and The coverage in the plated through hole
tures. This was due to only limited temper- lead-free alloys. should be with a minimum of 3 µm on
ature being applied to the laminate prior Figures 5-7 show examples of what some the knee of the plated through hole. The
to resist application. With the old tin/lead may call poor levelling; however it’s not solder levelling process should not affect
reflow processes the laminate is exposed to levelling or an alloy issue. The first case the minimum hole size requirements.
soldering temperatures which exceed the it’s due to solder mask residues; in the The thickness of the coating should be
laminate’s glass transition temperature. second and third examples, poor bond- tested using XRF measurement for thick-
This causes expansion and contraction due ing, contamination on the surfaces of the ness and coating composition. Regular
to the stress in the laminate which is no copper prior to bonding or resin issues in solder samples should be taken to control
longer held by the copper foil. the laminate. the build up of copper in the solder bath
The levelling of PCB panels consists of The quality and consistency of the which will affect the solder reflow and
a pre-clean, fluxing, hot air levelling and solder levelling process could be far better levelling temperature and flow rates. Excess
a post-clean. Pre-cleaning is usually done than some of the examples often circu- copper will impact the balance of other
with a micro-etch of the copper surface. lated in industry. Often the variation on elements in a lead-free process and the
The panels are either lowered into a solder coatings is down to the time spent on visual appearance of the solder surface.
bath vertically or transported horizontally setting up the process for different designs. Panels should not be leveled twice due to
on a long inline process. As the panels The PCB industry could have done better the possible impact on copper dissolution
exit the solder, air knives squeegee off over the years even with vertical levelling on through hole plating.
the excess solder from the surface of the systems as has been demonstrated by sub- As a basic guide, the solderable life for a
pads or out of the through holes. Key to contract levelling services like CEMCO. correctly leveled board should easily exceed
successful levelling is making sure the sup- Contractor and the leading supplier of 12 months, minimum.
plier sets up his process to get the correct vertical and horizontal levelling systems (As a simple explanation of how solder-
coverage, and this will take time to get the know the process and have considerable able solder is, test boards produced for
best, consistent coverage. With users and experience and data to support users and the launch of a new wave solder design
suppliers working together, the process can suppliers alike. The levelling process over the “Omega” in 19 are still soldering well
the last 10 years has been demonstrating today!)
satisfactory coatings for lead-free in recent Nothing solders like solder, and its
trials on tin/copper/nickel boards for the performance is getting better all the time!!
SMART Group workshops and production
lines. references
The results in Figure 8 are XRF measure- • EM&T/SMART Group Glossary
ments taken from sample boards during “Jargon Buster”
a SMART Group hands on workshop • SMART Group Lead-Free Experience
conducted in 2006. The boards featured a Report
range of different pitches. • SMART Group training video “Guide
As a guide the following specification to PCB Solderable Finishes”
may be useful. It can used or modified • CEMCO Solder Levelling Systems
for solder levelled boards. Further inputs Guide
to improve these suggestions are always • Shipley Solder Finish Assembly
Figure 5. Solder mask residues.
welcomed by the author. Evaluation Project
If specified all exposed outer copper
surfaces shall be coated with solderable fin- Bob Willis is a process engineer providing
ish of tin/lead or lead-free on the surface engineering support in conventional and surface
of mounting pads, test points and plated mount assembly processes. He runs special
through hole. The coating should provide production features at exhibitions and offers his
a minimum of 12 months shelf life and seminars, workshops and PCB manufacture and
meet the solderability requirements of the assembly audits worldwide. These complement
IPC or IEC standards using a wetting bal- his own practical in-house hands on workshops.
ance. The coating thickness should average For further information go to
be between 10-15 µm on specified pads. www.ASKbobwillis.com.
Figure 6. Poor bonding.
Figure 7. Contamination on the copper bonding or
resin.
Figure 8. XRF measurements taken from sample boards featuring a range of different pitches.
6 – Global SMT & Packaging – September 2008 www.globalsmt.net
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