PCB Surface
Treatments
Definition of surface finish
A surface finish can be defined as a coating,
g, either metallic or organic
g
in
nature, which is applied to a PCB in order to assure solderability of the
metal underneath after various time of storage / conditions.
paste or wave
Most of the surface treatment dissolves into the solder p
solder during the soldering process and the solder joint is forming between
the solder and the copper.
p
is ENIG / Immersion Gold where the solder dissolves the
One exception
thin layer of gold and forms a joint with the underlying nickel phosphorous
alloy.
Definition of surface finish
There are actually only two different types of surface finishes for PCB.
Below is the most common on the market
market.
 Organic
 Metals
•
•
•
•
•
•
•
•
OSP
HASL (leaded and lead-free)
Immersion Sn
Immersion Ag
ENIG
ENEPIG
ASIG
ENEG
Definition of surface finish
OSP is more like a lacquer,
q
that p
prohibits oxygen
yg attacking
g the copper
pp
underneath.
All other are coating are metals and they may be applied using one of two
different methods = either electroless or immersion.
There is a big
g difference between these two p
processes for metallic
coatings and it is important to be aware of these differences as we discuss
finishes.
Definition of surface finish
Electroless or Autocatalytic systems,
both work in the same fashion in that
they use an reducing agent inside the
bath itself.
This means that
Thi
th t the
th metal
t l thickness
thi k
increases during the whole period that
the PCB is in contact with the solution.
Electroless process
Ni2+
eReducing
Agent
Ni2+
Ni2+
Ni2+
Ni Ni Ni Ni
 Nickel in ENIG
 Silver
Sil
iin ASIG or ESM100
 Gold in Electroless gold
Reducing
Agent
e-
Ni2+
e-
e-
Ni2+
Cu
Ni Cu
Ni Cu
Ni Cu
Ni Cu
Ni
Cu Cu Cu Cu Cu
Cu Cu Cu Cu Cu
Definition of surface finish
Immersion systems, This process uses a
chemical displacement reaction to
deposit a metal layer onto the exposed
metal surface of the PCB. The base metal
donates the electrons that reduce the
positively charged metal ions present in
solution.
Immersion layer will continue to grow,
however as the thickness of deposit
increases the rate of deposition falls
increases,
falls.
Therefore the process is self-limiting.
 Tin in Immersion Tin
 Silver in Immersion Ag
 Gold in ENIG
ENIG Gold bath
+
Au
Ni3+
+
Au
Ni3+
+
Au
Ni3+
+
Au
Ni3+
Au
Ni Au
Ni Au
Ni Au
Ni Au
Ni
Ni Ni Ni Ni Ni
Cu
Ni Cu
Ni Cu
Ni Cu
Ni Cu
Ni
Cu Cu Cu Cu Cu
Cu Cu Cu Cu Cu
+
Au
Ni3+
General recommendation Handling
g/
Storage condition / Time
A. Handling Recommendations:
It is recommended that gloves are used for
handling panels/circuits during all assembly
processes. Or
O att the
th VERY least,
l
t handle
h dl the
th
boards without touching the surfaces. Salts/acids
from fingerprints will have a negative affect on the
solderability.
solderability
B. Storage condition and time
The storage environment should not to exceed
30C and 75% RH (except immersion tin). Boards
should be stored in original vacuum packaging to
g the surface, and the board.
limit air accessing
General recommendation Baking
g
C. Baking Recommendations:
Th purpose with
The
ith baking
b ki iis tto reduce
d
th
the risk
i k off out-gassing,
t
i
measling
li
or delamination, by eliminating moisture or solvents within the laminate
structure/board prior to soldering.
Temperature and time of baking is to be determined on an individual
basis.
The time between baking and solderability testing should be kept to a
minimum (not more than 24 hours) in order to prevent re-absorption of
moisture into the laminate structure.
Baking should be kept to a minimum, adhering to the production
procedure, to prevent excessive oxidation and intermetallic growth.
General recommendation Baking
g
D. Baking Recommendations given by NCAB
Baking can be advantageous and may be employed before any type of
soldering operation. The recommended conditions are 120oC for 2 hours
Oven conditions
Baking needs to take place in clean oven to prevent any form of
contamination during the baking process. The boards should also be
placed in the oven in such a way that the air can circulate freely around the
boards during the baking time
time.
All baking can be considered as advanced ageing and therefore may affect
the solderability negatively. As such the time and temperature referenced
above must be seen only
y as recommendations – the customer must take
responsibility to approve processes.
http://www.ncab-pcb.com/pub/1332/Baking%20Flex_rigida%20kort%202.pdf
HASL ((Hot Air Solder Level)) Lead-Free
Typical Thickness: 1-40µm
ADVANTAGES
DISADVANTAGES
+ “Nothing Solders Like Solder”
- Huge Co-Planarity Difference
+ Easily Applied
- Not Suited for High Aspect Ratios
+ Long Industry Experience
- Not Suited for < 20 Mil pitch SMT and BGA
+ Easily Reworked
- PWB Dimensional Stability Issues
+ Multiple Thermal Excursions
- Bridging Problems on Fine Pitch
Assemblies
+ Good Bond Strength
+ Long Shelf Life
+ Easy Visual Inspection
+ Low Cost
- Inconsistent Coating Thicknesses
- High Process Temperature 260-270 deg C
- Not Suitable For HDI Products
Short p
process description
p
HASL (Leaded and Lead-Free)
micro-etch—overflow rinsing—clean water rinsing—blowing— flux
coating—hot air solder leveling—cooling—hot water rinsing—soft
brush scrubbing
scrubbing—DI
DI water rinsing—strong
rinsing strong air blowing—hot
blowing hot air drying
Storage
g condition / time / handling
g
HASL (Leaded and Lead-Free)
HASL is the most robust of all treatments if applied under correct
conditions, however there are still reasons to be careful:
A. Handling
g Recommendations
Whilst the surface treatment is rather robust, general recommendations
should be followed.
B. Storage condition and time
General recommendations must be followed.
If nothing else is specified,
specified IPC6012 sets the storage time according to
JST-STD-003 Category 2 in other words a coating durability up to 6
months, NCAB recommends the category is set to JST-STD-003
Category 3 (6 months+)
Baking
g / Temporary
p
y masking
g
HASL (Leaded and Lead-Free)
C. Baking.
C
B ki
HASL can withstand baking with maintains its solderability well as long
as the deposit is not too thin. See general recommendations for baking
recommendations.
recommendations
D. Temporary masking / Peelable mask
Since the HASL treatment is a rather robust treatment and not so
sensible to contamination, nearly all types of maskings (such as
Peelable mask, Kapton tape etc.) are acceptable.
Available standards and test methods
HASL (Leaded and Lead-Free)
IPC-6012
IPC
6012
“Coverage & solderable”. The coating durability category shall be
specified on the master drawing according to J-STD-003, if the category is
not specified the category shall be set to class 2
2.
J-STD-003
There are 5+5
5 5 difference methods described as t0 how to test the
solderability of the PCB. The two listed below are the most common if
nothing else is specified
A – Edge Dip Test
(3sec dwell time)
C – Solder Float Test (5sec floating time)
Leaded solder - Sn60/Pb40 - 235±5° C
Lead-free
f
solder - SAC305
S C
- 255±5°° C
Design aspects
HASL (Leaded and Lead-Free)
There are not so many design concerns for this surface treatment.
A. Small pitches QFP < 0.50mm & BGA < 0.80mm
On very small pitch the co-planarity will become an issue and it will be
difficult to assemble due to surplus of solder on the tiny SMD features.
B. HFFR4 & High Tg FR4 coated with lead-free HASL.
We have, from NCAB point of view, seen a problem with increased
cases off measling
li and
d copper peeling
li on hi
high
h copper thi
thicknesses
k
(≥3Oz) and the possible root cause is low peel strength of the foil for
such materials, also the CTE difference between copper and substrate
and finally the temperature shock as a result of the HASL process itself
itself.
Design aspects
HASL (Leaded and Lead-Free)
There is not so much design concerns about this surface treatment.
C. Solder balls.
Holes plugged from only one side (partially plugged) with soldermask
can entrap solder that the air-knives cannot remove from the surface
(f
(force
it into
i t the
th partially
ti ll plugged
l
dh
hole).
l ) Th
These solder
ld b
balls
ll may b
become
dislodged and re-deposit onto the surface during the reflow operations
D. Edge plating on thick boards
D
We have experienced issues on thicker boards with edge plating, where
the edge plating has been found to become loose after the lead-free
HASL process.
Quality aspects
HASL (Leaded and Lead-Free)
With lead-free
ead ee solder
so de ,qua
,qualityy control
co o is
s more
o e ccritical
ca that
a with traditional
ad o a
SnPb HASL.
A. Control the alloy.
It is critical to have good control over the alloy so all elements are within
limits, the new Pb-F alloys dissolve more copper from the boards.
B. U
B
Use good
d fl
flux.
Many of the low cost fluxes struggle with the heat and may perform badly.
C. Maintenance on machinery
C
The Pb-F HASL’s are more challenging to wet on the surface and are also
much more easily to ‘blow off’ during processing so that thin layers provide
insufficient pure tin and limits storage time and solder cycles
OSP (Organic Solderability
Preservative)
Typical Thickness: 0.15-0.30µm
ADVANTAGES
DISADVANTAGES
+ Flat, Coplanar pads
- Difficult to Inspect
+ Reworkable (at PCB Fabricator)
- Question Remains Over Reliability of
+ Doesn’t
D
’t Affect
Aff t Finished
Fi i h d Hole
H l Si
Size
E
Exposed
dC
Copper Aft
After Assembly
A
bl
+ Short, Easy Process
- Limited Thermal Cycles
+ Low Cost
- Can not be Reworked at EMS/OEM; Sensitive
to Solvent Used for Misprint Cleaning
+ Good Soldermask Integrity
+ Environmental friendly
+ Clean process
- Limited Shelf life
- Easy to scratch
Short process description
 OSP (Organic Solderability Preservative)
OSP is thin layer of either Benzimidazole or imidazole lacquer
lacquer.
Degrease—overflow rinsing—micro-etch—overflow DI rinsing-OSP—DI rinsing—strong air blowing--hot air drying
Storage
g condition / time / handling
g
OSP (Organic Solderability Preservative)
Since OSP is a rather thin organic and sensitive deposit
deposit, these
recommendations should be followed.
A. Handling Recommendations:
Since the deposited layer is so thin and soft, it is
important to handle the boards with care.
B. S
B
Storage condition
di i
and
d time
i
General recommendation needs to be followed.
If nothing else is specified, IPC6012 sets the
storage time according to JST-STD-003
JST STD 003 Category
2 in other words a coating durability up to 6 months,
NCAB recommends the category is set to JSTSTD-003 Category 3 (6 months+)
Baking / Temporary masking / Others
OSP (Organic Solderability Preservative)
C. Baking.
Baking with OSP will have a negative effect on the solderability. So the
customer
custo
e must
ust e
evaluate
a uate the
t e baking
ba g p
process.
ocess
D. Temporary masking / Peelable mask
It is not recommended to use peelable mask on OSP treatment since the
chemicals inside these masking material will/can a negative effect on the
solderability.
E. Cl
E
Cleaning
i
off Misprinted
Mi
i t d Solder
S ld Paste
P t
The OSP coating is soluble to varying degrees in
most solvents, acidic materials. Any solvent used to
clean solder paste will dissolve some OSP
E. Process time
The time between first and last soldering should be
as short as possible, preferable within 8-12 hours.
Available standards and test methods
OSP (Organic
(O
i Solderability
S ld
bilit Preservative)
P
ti )
We have same lack of a available standards as for HASL
IPC-6012
“Coverage & solderable”. The coating durability category shall be
specified on the master drawing according to J-STD-003, if the category is
not specified
ifi d the
h category shall
h ll be
b set to class
l
2
2.
J-STD-003
There are 5+5 difference methods described as to how to test the
solderability of the PCB. The two listed below are the most common if
nothing else is specified
A – Edge Dip Test
(3sec dwell time)
C – Solder Float Test (5sec floating time)
Leaded solder - Sn60/Pb40 - 235±5° C
Lead-free solder - SAC305 - 255±5° C
Design aspects
OSP (Organic Solderability Preservative)
There are some design concerns about OSP as a surface treatment.
A. ICT test points
It can become on issue that there is problems for test pins to penetrate the
rather thin but hard layer
y of OSP and get
g connection with the test p
points. But
there have been a lot of studies about this and the problem can be
overcome with correct test pins and pressure.
B. Single sided plugged holes close to SMD pads.
There is a big risk the chemistry get trapped in these hole and contaminate
the surface.
C. Multiple solder operations.
Even if OSP can handle multiple solder operations, the surface finish have it
limitations and solderability can become on issue
issue.
Immersion Sn ((Immersion Tin))
Typical Thickness: 1.00-1.20μm
ADVANTAGES
DISADVANTAGES
+ Soldering direct to copper
- Handling Concerns
+ Good for Fine Pitch Product
- Contains Thiourea, Which Is
+ Good Solderability
Carcinogenic
+ Planar Surface
- Difficult To Rework
+ Eliminates Nickel
- Growth
G
th off Intermetallic
I t
t lli Concerns
C
+ Mid Expensive
- Whiskers concerns
+ Popular for press fit / backplanes - Aggressive
gg
against
g
soldermask
Short p
process description
p
 Immersion Tin
degrease overflow rinsing—micro-etch—overflow
degrease—overflow
rinsing micro etch overflow DI rinsing—pre-dip—
rinsing pre dip
immersion Sn—post-dip—alkali rinsing—double DI rinsing—hot air
rinsing—drying board
Storage condition / time / handling
Immersion Tin
Immersion Tin is excellent surface treatment, but also rather sensitive
so extra concern need to be addressed at the handling
A. Handling Recommendations:
It is
i important
i
t t that
th t gloves
l
are used
d ffor allll assembly
bl steps.
t
B. Washing of boards.
No washing is recommended either prior to / between any soldering step
step.
C. Storage condition and time
The storage environment should not to exceed 25C
25 C and 50% RH.
Baking
g / Temporary
p
y masking
g
Immersion Tin
Immersion Tin is excellent surface treatment
treatment, but also rather sensible
so extra concern need to be addressed at the handling
D. Baking.
Baking before soldering will have a negative
effect on the solderability and also ‘consumes’
storage time since baking will trigger the growth of
th IMC between
the
b t
the
th tin
ti and
d copper – reducing
d i
usable tin.
E. Temporary masking / Peel able mask
E
It is not recommended to use peelable mask on
Immersion Tin, since it is an immersion surface
with a porous structure,
structure all masking can easily
lead to contamination.
Available standards and test methods
Immersion Tin
There are available
Th
il bl standards
t d d that
th t define
d fi both
b th th
the thi
thickness,
k
appearance and performance of immersion tin.
A. IPC
IPC-6012
6012
“Solderable”. IPC-6012 has not been updated yet with the reference to
IPC-4554.
B. IPC-4554
A complete spec for the finishes with clear demand on the thickness,
performance and appearance. For solderability test it refers to J-STD-003
C. J-STD-003
Defines how the solderability test should be performed.
Available standards and test methods
Immersion Tin IPC-4554
Key point from the standard
standard.
A. Thickness
Minimum 1µm measured with XRF
Min 0.4µm of usable tin for Category 1 boards
Min 0.5µm
0 5µm of usable tin for Category 2 boards
Min 0.6µm of usable tin for Category 3 boards
B. Visual
Uniform plating and complete coverage of surface
to be plated.
Key
y points
p
with Immersion Tin
Immersion Tin
Thickness is the number one factor when it comes to being able to
provide a good soldering result. The finish always requires at least
0.2µm fresh tin on top of the IMC layer.
Topography of IMC after aging
Key
y points
p
with Immersion Tin
Immersion Tin
The graph shows how much fresh tin remains after 3 reflow cycles and
also after accelerated aging.
Quality aspects
Immersion Tin
As mentioned the thickness in number one to have a good working
finish that can withstand multiple
p soldering
g operation.
p
A. Control the IMC layer.
It is important for the supplier, to have control over
there process so the IMC layer is as thin as possibly
when they dispatch the boards.
B. Use good chemistry.
There are many bad vendors of immersion tin
chemistry, so it wise to approve famous brands
(Atotech, Enthone etc)
C. Specify the thickness or refer to IPC
C
IPC-4554
4554
Since the immersion tin is the most aggressive of all
available finishes, many Asian factories only deposit
around 0.7-0.8µm as standard because some
soldermask can not withstand any thicker deposit
deposit,
due to undercut problem (see right hand image)
Design
g aspects
p
Immersion Tin
Th
There
are some design
d i concerns about
b t this
thi surface
f
ttreatment.
t
t
A. Contamination
Since this treatment is very sensitive
to contamination, holes plugged from
one side (partially plugged) are not
recommended. See right
g hand image.
g
Also holes very close to SMD pads
are not recommended, since the
plating solution will be trapped inside
and can contaminate and destroy the
solderability.
Design
g aspects
p
Immersion Tin
Th
There
are some design
d i concerns about
b t this
thi surface
f
ttreatment.
t
t
B. Soldermask bridges between SMD
pads.
pads
Since this treatment is very aggressive to
the soldermask, there is need for larger
soldermask bridges. Normally we can
produce 3-4mil bridges. But with
immersion tin 5mil is the minimum
required.
DFM Soldermask openings I Sn
There is an risk that via holes
close to SMD pads will become
partially exposed during
production. This will lead to two
issues:
1.Solder paste ‘escaping’ into
the via hole during soldering.
2. Chemistry becomes trapped
i id the
inside
h via
i h
hole
l d
during
i
production and it can lead to
corrosion and contaminations
problems.
problems
Immersion Ag (Immersion Silver)
Typical Thickness: 0.05-0.40μm
0 05-0 40μm
ADVANTAGES
DISADVANTAGES
+ Good for Fine Pitch Product
- Some systems cannot throw Into
+ Planar Surface
+ Inexpensive
+ Short, Easy Process Cycle
+ Eliminates Nickel
+ Doesn’t
D
’t Aff
Affectt Hole
H l Size
Si
+ Medium Shelf-Life
+ Can be reworked/Re-Applied by
the PCB Fabricator
µvias with aspect ratios > 1:1
- Tarnishing must be controlled
Short p
process description
p
 Immersion Ag (Immersion Silver)
degrease—overflow rinsing—micro-etch—overflow DI rinsing—pre-dip—
immersion Ag—DI rinsing—strong air blowing--hot air drying
Storage
g condition / time / handling
g
Immersion Ag (Immersion Silver)
Immersion silver, is an immersion finish and also rather sensitive to
contamination. Silver is also sensitive to sulphur and chlorides
A. Handling Recommendations:
It is important that gloves is used for all
assembly steps.
B. Process time
Immersion silver finish also contains OSP
inside that works to prohibit tarnishing, and
since this is consumed during
g the first reflow
cycle, it is therefore important to keep the
cycle time as short as possible, 8-12 hours.
To prevent or limit tarnishing
Storage
g condition / time / handling
g
Immersion Ag (Immersion Silver)
C. Storage condition and time
C
1. Immersion silver boards should be packaged
as soon as possible, to prevent exposure to
chlorides and sulfides in the air.
2. Use sulfur free, pH neutral paper to wrap
stacks and then plastic wrap. Storage should
be in sealed bags to eliminate direct contact
with air.
3. Adhesive tape / labels, stamps, markers and
rubber bands are forbidden on silver boards.
4 If the
4.
th original
i i l package
k
iis opened
d and
d nott allll
of the boards consumed at the EMS/OEM
during the build, they should be re-wrapped as
soon as possible
possible.
Storage
g condition / time / handling
g
Immersion Ag (Immersion Silver)
C. Storage condition and time
General recommendation needs to be followed.
If nothing else is specified, IPC6012 sets the storage time according
to JST-STD-003 Category 2 in other words a coating durability up
to 6 months, NCAB recommends the category is set to JST-STD003 Category 3 (6 months+)
Baking / Temporary masking
Immersion Ag (Immersion Silver)
Immersion Silver is sensitive treatment and it recommended to be
careful when it comes to baking.
D. Baking.
Baking always is recommended before soldering
soldering, but on silver boards
this can have a negative effect on the solderability. As stated, within the
immersion silver formulation there is also a mix of OSP to prohibit
tarnishing.
g So the EMS/OEM customer must approve
pp
any
y baking
gp
process
so solderability is not destroyed. NCAB have carried out practical tests
and can achieve good solderability after baking – but care must be taken
here.
E. Temporary masking / Peel able mask
It is not recommended to use peelable mask on Immersion silver product
as allll masking
ki can easily
il llead
d tto contamination.
t i ti
Available standards and test methods
I
Immersion
i Silver
Sil
There are available standard that define both the thickness, appearance
and performance of immersion silver
A. IPC-6012
“Solderable”. The IPC-6012 amendment 1 is referring to spec IPC-4553 for
th performance
the
f
off immersion
i
i silver
il
B. IPC-4553
A complete
p
specification
p
with clear demands on thickness,, p
performance and
appearance. For solderability test it refers to J-STD-003
IMMERSION SILVER DEPOSIT
The minimum thickness shall be 0
0.12
12 μm with a maximum of 0
0.4um
4um
C. J-STD-003
Defines how the solderability test should be performed.
Design
g aspects
p
Immersion Silver
There are some design aspects that should be considered for silver
silver.
A. Contamination
As with immersion tin, the treatment is
very sensitive / susceptible to
contamination. Via holes plugged
from one side (partially plugged) are
not recommended. Also via holes very
close to SMD pads are not
recommended, since the plating
solution may become ‘trapped’
‘
inside
the hole and potentially contaminate
or destroy the solderability.
ENIG (Electroless Nickel/Immersion Gold)
Typical Thickness: 3-6µm
3 6µm Ni / 0.050 05
0.125μm Au
ADVANTAGES
DISADVANTAGES
+ Planar Surface
- Expensive
+ Consistent Thicknesses
- Black Pad Issues on BGA
+ Multiple Thermal Cycles
- Waste Treatment of Nickel
+ Long Shelf Life
- Cannot be Reworked at PCB Fabricator
+ Solders Easily
- Not Optimal for Higher Speed Signals
+ Good for Fine Pitch Product
- Complex process requires good control
+ Al Wire-Bondable
- Not soldering direct onto surface copper
Short process description
 Electroless Nickel Immersion Gold
degrease—double rinsing—micro etching—overflow rinsing—DI rinsing—
pre-dip—Pd activation—double DI rinsing—immersion Nickel—double DI
rinsing—immersion Au—double DI rinsing—drying board
Storage
g condition / time / handling
g
Electroless Nickel Immersion Gold
Electroless Nickel Immersion Gold is an excellent surface treatment
and also rather robust against treatment/handling
A. Handling
g Recommendations:
The surface treatment is rather robust, but general
recommendations should always be followed.
B. Storage condition and time
General recommendation needs to be followed.
If nothing
thi else
l is
i specified,
ifi d IPC6012 sets
t the
th storage
t
time
ti
according
di
to JST-STD-003 Category 2 in other words a coating durability up
to 6 months, NCAB recommends the category is set to JST-STD003 Category 3 (6 months+)
Baking
g / Temporary
p
y masking
g
Electroless Nickel Immersion Gold
C. Baking.
ENIG can withstand baking whilst maintaining solderability. See
general recommendation for details.
D. Temporary masking / Peelable mask
Since the ENIG treatment is a rather robust treatment and not so
sensitive/susceptible
iti /
tibl tto contamination,
t i ti
nearly
l allll ttypes off maskings
ki
are acceptable - such as Peelable soldermask, Kapton tape, etc.
Available standards and test methods
Electroless Nickel Immersion Gold
There are available standard that define both the thickness
thickness,
appearance and performance of Electroless Nickel Immersion Gold
A. IPC-6012
R f tto IPC-4552
Refer
IPC 4552 for
f the
th demands.
d
d Th
The coating
ti d
durability
bilit category
t
shall
h ll
be specified on the master drawing according to J-STD-003, if the
category is not specified the category shall be set to class 2.
B. IPC 4552
A complete spec for the finishes with clear demand on the thickness,
performance and appearance. For solderability test it refers to J
J-STDSTD
003
C. J-STD-003
Defines how the solderability test should be performed and judged.
Available standards and test methods
Electroless Nickel Immersion Gold IPC-4552
K point
Key
i t ffrom the
th standard:
t d d
A. Thickness
Minimum 2,54µm
2 54µm Nickel (100µ
(100µ”)) measured
with XRF
Minimum 0,05µm Gold (2µ”) measured with
XRF
B. Visual
Uniform plating
g and complete coverage
g of
surface to be plated.
Design aspects
Electroless Nickel Immersion Gold
There are rather few design concerns associated wit ENIG, however:
A. Soldermask defined BGA pad
Soldermask defined BGA pads should be avoided, due to the risk of brittle joint and also
the risk of black pads since the chemistry has less possibility to be rinsed, especially on
smaller BGA pads
pads. See graphic
graphic.
B. Single sided plugged via holes.
As with most finishes
finishes, via holes plugged from one side
(partially plugged) are not recommended. Also holes very
close to SMD pads are not recommended, since the
plating solution can become ‘trapped’ inside and may
contaminate or reduce the solderability
solderability.
PCB
C. Soldermask bridges between SMD pads.
As with immersion tin, this treatment is aggressive towards the soldermask, therefore
larger soldermask bridges may be necessary with some factories
factories.
Quality aspects
El
Electroless
l
Nickel
Ni k l Immersion
I
i Gold
G ld
There are two quality aspects that are worth highlighting:
A. Black Pad.
This is the result of a lack of balance within the ENIG plating chemistry.
In principle it is either caused by a too aggressive immersion gold deposition
process OR an overly active nickel surface. Whatever the cause, the result is overetching of the nickel (uncontrolled gold immersion reaction) which leads to an
enrichment of phosphorus in the upper most Ni layer
layer. Since the gold immersion
reaction makes the Ni atoms go into solution it leaves the P atoms on the surface.
B. Brittle fractures.
Recent studies have shown that brittle solder-joints may form on an ENIG surface
even if there is no black pad defect. The brittle fractures occurs in the Inter Metallic
Compound (IMC) that is formed when soldering against Ni.