 The
base metal conductor used in the fabrication of printed circuit boards is
copper. Although copper is an excellent conductor of heat and electricity,
it is also a very active metal that quickly oxidizes in the presence of
air and water. If a copper surface is not coated or treated with a protective
agent, the exposed area rapidly becomes unsolderable using conventional
assembly processes. For this reason, all printed circuit boards use some
form of a surface finish on the exposed pads to which electronic components
will be soldered. |
| Current manufacturing processes typically
also require circuit traces to be protected with a masking material, called
soldermask. The soldermask is relieved only where a subsequent operation,
such as soldering of electrical components, requires electrical access to
the circuitry. The relieved areas, which are not covered with soldermask,
need to be protected with some form of a surface finish. |
| The role of the surface finish is to coat
the copper pads and exposed traces in order to protect them between the
time the board is manufactured and when it is assembled. By protecting the
copper from oxidation, the surface finish ensures that the board can be
soldered successfully later during the assembly process. |
| The three most prevalent surface finish
processes are: |
| |
 |
hot air solder level (HASL) |
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|
immersion precious metal plating |
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|
organic surface protectant (OSP) coating. |
 |
| Hot Air Solder Level |
| The hot air solder level (HASL) process
entails the application of tin/lead solder to exposed copper. The solder
and exposed copper form an intermetallic chemical bond that protects the
copper from oxidation. |
| To prepare circuit boards for solder coating,
the boards are first processed through a flux containing amine hydro-bromide
flux activators in a polyglycol carrier. A solder bath is prepared using
a tin/lead alloy, normally 63 percent tin and 37 percent lead, heated to
500°C. In a vertical process, the “fluxed” boards are immersed in the solder
bath. The solder coats the areas not covered by soldermask. The boards are
then withdrawn from the solder bath while hot air knives remove or level
the excess solder. |
| The resulting solder thickness can vary
due to gravity, surface tension, and the geometry of the circuit board pads.
The use of a vertical process results in some droop or meniscus of solder
on the bottom side of the pads. This meniscus can be a problem if the board
requires high density chip placement (pitch spacing below 20 mil).
|
| HASL is still the most common coating
applied to protect copper pads and exposed traces. It provides good solderability
and excellent shelf-life for the circuit board. |
| There has been much discussion in recent
years about banning the use of tin/lead coatings. However, at this time
regulatory agencies have granted specific exclusions from lead-free restrictions
for most high-reliability applications due to the lack of a proven alternative.
For this reason, HASL continues to be used far more extensively than the
coating alternatives described below. |
|
| Immersion Precious
Metal Plating |
| The immersion process uses ion displacement
reactions to plate the circuit board surface. When the surface metal finish
(nickel/gold, silver or tin) has been deposited, the source of electrons
is used up and the process is complete. The process is self-limiting because
the copper forms an intermetallic layer that inhibit the immersion reaction.
|
| Immersion coatings have become popular
as circuit densities have increased and the pitch of surface mount technology
(SMT) components has decreased. A flat attachment pad is paramount in achieving
a reliable solder joint with fine pitch parts. Although the solderability
of each coating is different, all immersion coatings provide a very flat
attachment surface. |
| The electroless nickel / immersion
gold (ENIG) finish is the most expensive and also the most solderable
over the widest range of conditions. This coating ensures minimal long-term
degradation of solderability prior to assembly and excellent immunity to
corrosion from environmental exposure in the field. The nickel/gold coating
ranges from 3 to 10 µin in thickness and costs about twice as much as HASL.
|
| Silver is the next most costly
metal finish and is only slightly less solderable than nickel/gold. Deposited
to a thickness of 5 µin, the cost of silver is only about one and a half
times the cost of HASL. |
| Immersion tin has gained popularity
because its cost is favorable compared to the cost of HASL. However, the
long-term solderability of immersion tin is questionable and highly dependent
on the process controls of the fabricator. The plating is a tin oxide formed
from stannous fluoborate in an acid suspension. About 50 µin thick, tin
plating costs about 1.3 times as much as HASL. |
|
| OSP Coating |
| The process for applying an organic surface
protectant (OSP) coating does not require electron exchanges since the circuit
board is coated upon submersion in a chemical bath. A nitrogen-bearing organic
compound allows adhesion to the exposed metal surfaces and is not absorbed
by the laminate or soldermask. Although coating adhesion levels vary according
to the type of organic compound, the process is self-limiting and results
in a typical coating thickness of .5 µin. Organic coatings are equivalent
in cost to HASL and provide relatively flat pad topography. However, these
coatings break down during thermal cycles in assembly and are not recommended
for double-sided circuit boards (boards with SMT components on both sides).
Furthermore, these coatings do not hold up very well to long-term storage.
Boards with an organic coating must be kept sealed in a stable environment
and used very shortly after application of the coating. |
| Mr. Tech Dweeb
Tech Tip |
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Although customer specifications
vary, typical coating thickness are shown in the table below.
| Coating |
Thickness
(microinches) |
| HASL |
200 to 300 µin |
| ENIG |
100 to 250 µin of nickel
3 to 10 µin of gold |
| Tab nickel/gold* |
100 µin of electroplated
nickel
30 to 50 µin of electroplated gold |
| Silver |
8 to 20 µin |
| Immersion tin |
30 to 70 µin |
| Organic surface
protectant |
0.4 to 0.6 µin |
|
* Tab nickel/gold plating is applied
using an electroplate process that results in a harder and
thicker coating than the ENIG process. Tab nickel/gold is
used only for areas of a circuit board that will be inserted
into a connector.
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| The fabrication industry developed the
immersion process to address the anticipated environmental mandate to eliminate
lead, even in alloy form (tin/lead is an alloy and has no free lead), from
the manufacturing process. However, exclusions given to military hardware,
implanted medical devices and critical automotive systems have delayed the
lead ban until an unspecified future date. As a result, the need for flat
pad surfaces, rather than the need to eliminate lead, has become the driving
force behind the development of alternative coatings. |
| Mr. Tech Dweeb
Tech Tip |
.jpg) |
Cost comparisons of various coating
options are difficult since two of the metals (gold and silver)
are sold by the troy ounce. However, for a typical board with
approximately 15 percent exposed soldering surfaces the following
table provides a good comparison, using HASL as the baseline.
| Coating |
Cost Factor |
| HASL |
1.00
|
| ENIG |
2.00
|
| Silver |
1.36
|
| Immersion tin |
1.30
|
| Organic surface protectant |
1.05
|
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