 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) |
|
|
immersion
precious metal plating |
|
|
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 The boards are then withdrawn from the solder bath while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.
|
|
|
|
| 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 |
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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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