Forward
I began learning about printed circuit boards (PCB’s) when I was a recent hire at Enthone and while I knew much about functional and decorative plating I knew very little about electronics. As I began to probe into how our product lines served the electronics market I found a plethora of quality information dedicated to specific products but little encompassing everything and explaining to a newcomer how the various processes interacted, how our products fit these processes, and what competitive advantages Enthone products offered. To get this information I went straight to our tech service reps and customers who were troubleshooting issues and optimizing processes. Going to the center of activity was able to create this guide to fill in the missing information. Hopefully with this guide, anyone can understand the basics of printed circuit manufacturing and how our company supports this business.
Special thanks to Patrice Dumas, Ken Mckeown and Rudi Reetz
for their invaluable input and time, without which this guide would not be
possible.
Intro
A printed circuit board is an apparatus used to connect and
structurally support discrete components in a variety of electronics applications.
PCB’s are thin, planar rectangles constructed using multiple steps in a
manufacturing facility or “fab”. First, sheets known as inner layers are
laminated together to form the basic structure of the PCB. The resulting
laminate is further processed to make the outer surface conductive so that
components may be connected electrically from the outside. Finally, a coating
is applied to the board to protect it during transport to another facility,
where discrete electronic components will be assembled on the board.
I/L Core
Inner layers are the building blocks of PCB’s and are transformed
from thin sheets known as cores – copper clad epoxy resins – into an array of
copper pathways laminated together to form the circuit board. On average each
circuit board contains between 4-20 inner layers while some in special
applications have over 40. Depending on the application there are a variety of core
types to choose from and differ depending on the type of resin and copper foil
used.
Copper foil comes in two categories, Reverse Treated Foil
(RTF) and Hyper Very Low Profile (HVLP). RTF has a matte surface which is desirable
for bonding between inner layers whereas HVLP cores have a smooth surface
resulting in high signal integrity. The difference between these copper foils arises
from the manufacturing process where copper is electroplated over a stainless
steel drum (see figure a.).
Often in PCB fabrication a tradeoff is made between
manufacturability and performance criteria. Rough, thin surfaces are easiest to
work with because roughness creates greater bond strength in lamination and
thin surfaces require less processing. However, signal integrity is compromised
with rough surfaces. Additionally, applications requiring high current require
thicker copper surfaces just like a pipe required to carry large volumes of
water must have a large diameter. Thickness is measured in ounces per square
foot and the most common sizes are 0.5, 1.0, 1.5, and 2.0.
The epoxy/resin material is also chosen according to tradeoffs
between performance requirements and manufacturing considerations. Softer
laminates have higher bond strength, are easier to drill, and have a greater
receptivity to plating. These laminates have a low Tg, or glass transition
temperature where the material can be warped due to high temperature. To
prevent warping at higher temperature (such as when lead-free solder must be
used), harder laminates are chosen but create issues with adhesion and plating.
It is common to see laminates in lead-free applications with Tg values of
180-210 F. Figure b highlight the step change in thermal expansion that occurs at the Tg.
I/L Imaging
In 1968 the DuPont Company commercialized the first dry film
photoresist which would fundamentally change the way circuit boards were
manufactured. This new process involved applying a dry film mask to the laminate,
photo-imaging the laminate, and stripping away the unexposed area to allow the
selective removal of copper. This process consists of pre-clean, dry film
laminate, and imaging.
The purpose of the pre-clean is to expose active copper while
removing contaminants and the barrier coating. The barrier coating is used to
prevent oxidation of the copper foil after manufacturing, during transportation,
and in storage and can be any type of anti-tarnish ranging from a benzotriazole
to a chromate. ENTHONE PC-7078 is a
best-in-class product used in the pre-clean step. The PC 7078 is a tri-acid
blend which effectively removes barrier coatings and contaminants without
etching the copper. Most competitive products use only 2 acids which tend to
etch the copper.
Once bare copper is exposed it is ready to accept the dry
film. The cores are placed into a machine which rolls dry film laminate (DFL) onto
either side of the core. It then goes to imaging where the dry film is
polymerized by photo-imaging to create the pathways for electrons on the
circuit board. There are two options for imaging the dry film. The dry film can
be polymerized using laser technology or it can be photo-imaged using a template,
or piece of “artwork”. Each method has advantages and disadvantages.
Photo-imaging allows collimated light to pass through the artwork and be
exposed to the dry film. However, artwork may differ from the CAD
specifications for many reasons. Over time the integrity of the artwork
degrades naturally due to light exposure and must be replaced periodically. It
can also be scratched or contaminated with dust, rendering slightly different
variations of the images. On the other hand, a laser exactly replicates the CAD
drawing each time and has no shelf life. The drawback to laser technology is
the lower productivity and the initial cost of investment in equipment. Figure c. shows how light passes through the artwork to polymerize the dry film.
DES
DES stands for Develop, Etch, Strip. This process is used to
remove the monomer dry film, etch away the exposed copper, and strip the
remaining resist to uncover the functional copper. This can be seen in figure
d.
The developer is a 1g/L sodium carbonate solution used for
removing the dry film. The removal rate is determined by pH and over time the
accumulation of dry film in the solution drops the pH. The bath should be
maintained between pH: 11.0 – 10.6. Once the pH drops below 10.6 the removal
rate becomes prohibitive and the bath must be replaced.
After the dry film is removed bare copper is exposed and must
be stripped, or “etched” (etch is the proper term in order not to confuse with
other stripping steps) down to the underlying resin/epoxy. A commodity blend of
hydrochloric acid and an oxidizer is used to make a cupric chloride solution which
attacks the exposed copper. The oxidizer maintains a positive ORP and is used
to drive the formation of cupric chloride CuCl2 from copper (I)
chloride CuCl. Two oxidizers may be used. Hydrogen peroxide is the most
economical in terms of material costs but is dangerous since it is susceptible
to runaway exothermic reactions. Sodium Chlorate is another option which is
more expensive but easier to control and to analyze (it could still exotherm
but it is less likely than H2O2). As the bath ages the
concentration of sodium increases and raises the specific gravity. This is the
parameter most easily used to determine the bath age and control the solution.
The etch-speed is determined by the concentration of copper and the ORP. The
etching of the copper doesn’t occur evenly however. As can be seen in figure
e,
the profile of copper is parabolic at the interface between the
resist and the exposed laminate. This geometry is known as a “foot” and is
nearly impossible to avoid. The shape of the foot is controlled by adjusting
the etch rate of the solution as well as the contact time.
The final step in DES is stripping the polymerized dry film
known as the resist. The main strippers are the ENTHONE RS-1609, ENTHONE PC-4052,
and the ENTHONE PC-4025. The 1609
and the 4052 are aqueous based products where the 4052 is used for more
tenacious resists. The 4025 is a solvent
based product and is the strongest of the three. Proper selection depends on
performance, wastewater treatment implications and preference.
Alternative Oxide
The final step in producing inner layers is treating the
copper to promote bond strength in inner layer lamination. The main idea is to create
surface roughness and generate a higher coefficient of friction to support
adhesion achieved by a controlled microetch (see figure f).
After the
microetch (alternative oxide) it is desirable to apply a barrier film to the
copper to prevent corrosion during the Automated Optical Inspection (AOI). The
process is: Alkaline Clean/Microetch | DI Rinse | Dry | Alternative Oxide
(copper microetch) | DI Rinse with anti-tarnish | Dry.
AlphaPREP PC 7096
is used during the Clean/Microetch step to initiate the alternative oxide. AlphaPREP PC 7030 follows and is a full
microetch. This product is a proprietary mix of organics, sulfuric acid, and
peroxide designed to attack copper and create roughness. It has many advantages
over competitors. It has no pre-dip step and can withstand copper loading of
about 45 g/L, four times more than come competitive processes (case studies at certain
customers have shown this to be true despite claims to high loading on the competitive
TDS). The etch rate in this step is extremely sensitive to chlorides as can be
seen in figure g.
For the chemistry to work properly, the board must be
free of chlorides and as dry as possible.
This requires using a DI rinse since chlorides are abundant in city
water. A quick way to determine whether a solution is contaminated with
chlorides is to put 5 drops of silver nitrate into a sample and see if a
precipitate forms. If the water turns cloudy it indicates the formation of
silver chloride and confirms the contamination. Drying the board after the DI
rinse helps to ensure that the concentration of solution at the surface
interface is not diluted and that etching occurs at the desired rate.
Once the copper has been etched it must be neutralized and
then made passive in a subsequent rinse. ENTEK
CU-56 should be used in the second to last rinse as an anti-tarnish. This
product is sensitive to staining so should not be put in the last rinse.
To control the alternative oxide bath a variety of methods
are used. The final color of the copper results from the surface geometry and
etch pattern. The concentration of organics and chlorides are the main drivers
to changes in the appearance as they affect the etch pattern. Specific gravity
can be measured to indicate bath loading (copper is heavy) and the chloride
concentration can be measured with an ion selective electrode. Peel tests are
conducted on test copper panels bonded to pre-preg to verify the performance of
the bath at a given time.
Drilling
Drilling holes in the circuit board has three purposes: to
create vias which connect inner layer conductive pathways, to create vias for
connecting the O/L and I/L conductive pathways and to produce mounting surfaces
for various components to be added to the O/L in assembly. It can occur
multiple times throughout the manufacturing process.
Inner Layer
Lamination
Once the inner-layers have been produced they must be
laminated and held together with additional resins known as pre-preg. Pre-preg
is a partially cured thermoset resin which has just enough plasticity to fill
the gaps between copper and the base laminate material (see crossection figure
h).
Once heated in the
lamination oven it melts, fills in any holes between the inner layers, and then
fully cures into a sturdy structure. Inner layers and pre-preg are laminated in
a vacuum to minimize air bubbles which could cause structural or electrical
defects. Failures arise from contamination between interfaces, poor bond
strength from the alternative oxide, and misalignment of the inner layers.
Electroless Copper
The outer layer is formed by electrolyzing the entire outer
surface and vias with electroless copper (similar to how the inner layer cores
came clad with copper foil). There are 7 steps to plating electroless copper:
Sweller, Permanganate, Neutralizer, Microetch, Pre-activator, Activator, and Electroless
Copper, in addition to rinsing and drying steps. All surfaces must be cleaned
and active to accept electroless copper, difficult considering a function of
the epoxy/resin material is to be inert, and for this the electroless copper
takes many steps. The most prominent contaminant to be cleaned is known as
“smear”, and occurs during the drilling step when the drill smears epoxy/resin
through the vias. It is critical to remove the smear so that “voids” or plating
failures in the vias are not created. This is done in the step known as the
sweller. The sweller is a solvent or alkaline based chemistry designed to
prepare the resin for processing by penetrating the material and softening the
surface. Once the resin has been prepared it goes into a permanganate (MnO4)
bath which dissolves the smear around the vias. Permanganate is very toxic to
electroless copper as it is a strong oxidizer and so the board must go through
a series of neutralizers and rinses to convert the remaining MnO4 to
MnO2. A persulfate based microetch is then used to create roughness
on the resin before going into a two stage activator. In reality, only a 1
stage activator is required but for economic reasons many processes use a two
stage step. The activator is made up of proprietary salts and palladium is the
key ingredient. In order to reduce expensive consumption of the palladium the
step is divided into two identical activators with the exception that palladium
is missing from the first, or pre-activator. This way, when the board enters
the palladium activator, the chemistry on the surface does not dilute the
chemistry and only additions of palladium are necessary. When the board exits
the activator it is coated with a palladium-tin colloid which is the basis for
the electroless copper. The rest of the process is simple; the board enters
electroless copper and an auto-catalytic deposition of copper occurs. The board
is rinsed and dried, and exits the process covered with a matte copper finish.
O/L Imaging
Imaging the outer layer is nearly identical to imaging the
inner layers. It still consists of a preclean, dry film, image, and develop but
a new consideration arises. After the image is developed the board will undergo
further electroplating of copper and subsequently tin or a tin/lead alloy.
Therefore, the dry film must be at minimum just as tall as the tallest plating
which will be deposited in subsequent steps. Failure to do so results in “trapping”
the dry film (see figure i).
Electrolytic Plate
After imaging and developing over the electroless copper
layer the next step is to build up the exposed copper layer in an economical/fast
way. Although some processes will use electroless copper to continue to build
thickness this is a slow and expensive process. Therefore, it is more common to
electroplate copper over the existing electroless copper. Once the appropriate
thickness is achieved, a tin or tin/lead mix is plated after the copper to serve
as a resist in later steps and to protect the copper from oxidation before the
SES step (boards can sit around for a while before electrical testing).
SES
SES stands for Strip, Etch, Strip and refers to stripping
the resist, etching the exposed copper, and then stripping the tin or tin-lead
plating to expose the functional copper surfaces. The resist strippers in this
line are the same as used in the DES lines, the RS 1609, PC 4052, and
the PC 4025 are the main candidates.
The etching step is different than the typical DES line, however. Rather than
using a cupric chloride etch, many SES lines use an alkaline ammonia etch. This
etch is too aggressive for use on inner layers since it can attack dry film but
outer layers tend to be more robust. The pH of the etchant controls the etch
rate (thus the profile of the foot. Tight control of the pH will ensure a
constant etch rate. Specific gravity can be used to approximate the ammonia
consumption. After the
exposed copper has been removed only the laminate and the tin/tin-lead plating
are visible. The last stripping step is used to remove the tin/tin-lead without
etching the copper. The preferred product to use is the ENTHONE PC-1111. The PC-1111 works on plated and immersion tin as
well as lead based solder. This is useful for processes requiring flexibility.
Once only the copper and the laminate material remains the board is ready to be
solder masked and treated in the final process.
Solder Mask
Solder mask is used to coat the board with a barrier layer, inhibit
oxidation and prevent solder bridges from forming between closely located pads.
The application of solder mask requires a light acid clean to deoxidize the
copper followed by a pumice scrub. As it is important that no undesired
electrical contacts are formed the board must be cleaned with non-conductive
materials. Pumice is used because it is a non-conductive, inexpensive, fine,
and light abrasive that is removed easily after scrubbing. The disadvantage of
this scrub is that the vias are approaching smaller diameters that can trap
pumice particles, but this step can be replaced by an acid microetch. After the
board is clean a mesh is spread over the surface and solder mask is dripped
through the mesh. Different meshes vary in their fineness and alter the
thickness of the solder mask. A higher mesh count rejects more liquids and
generates a thinner film, while a lower mesh count creates a thicker film
designed for heavier duty applications. The board is then dried and an imageable
soldermask is applied over the entire board. Before being photo-imaged the
solder mask is partially cured to a “tack dry” finish in an oven, typically
150F for 75 minutes. The solder mask is then photo imaged, developed and cured
in an oven. Afterwards, a final coating will be applied to protect the
solderable surfaces during transport and storage.
Final Finishes
The purpose of final finishes is to provide a solderable surface
for electrical components after fabrication while protecting the copper during
transport and storage. Enthone is involved with three areas of final finishes:
Organic Solderability Preservatives (OSP), Immersion Tin, and Immersion Silver.
Other final finishes include Electroless Nickel/Immersion Gold (ENIG),
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), and Hot Air
Solder Levelling (HASL).
OSP
OSP is a final finish typically used in higher volume applications. It selectively deposits on the board to create a
barrier coating. This coating will then be displaced in soldering during
assembly.
OSP is applied through a process requiring a cleaner, microetch,
pre-coat, and immersion OSP. The enthone products used are the ENTHONE CE-2220, ENTEK PRECOAT PC-1030, and the ENTEK
PLUS HT. The CE-2220 functions as both a
cleaner and a microetch, thereby reducing process steps. Next, PC-1030 deposits
a seed layer followed by the OSP which replaces the seed and
deposits a eutectic organic layer. Because the OSP contains delicate organic
molecules it should not be applied in a spray application (which would break
the organic chains) and is applied using a flood immersion. The board is then
rinsed and dried completely to avoid re-dissolving the OSP.
Immersion Tin
Immersion tin is a final finish that unlike OSP which is
displaced in soldering is used as solder during assembly. A deposition reaction
driven by differences in electric potential deposits tin over exposed copper
pads. Immersion tin requires three steps: Clean, Microetch, and Immersion Tin
and uses the ENTHONE PC-7096, ENPLATE AD-485, and either the ORMECON CSN 7001 or the ORMECON CSN 7004
In order to choose between the 7004 and the 7001 one must
consider the differences specific to the application. The 7004 is a two
component sulfuric acid based system which has superior rinsing and limited
attack on soldermask while the 7001 is a 1 component MSA based product meeting
the minimum performance standard for most applications. Being a 1 component
system means it is simpler to operate than the 7004 and is thus the preferred immersion
tin product if it can meet the customer’s requirements.
Running an immersion tin bath requires special precautions. Copper
is simultaneously dissolved into the bath as tin is deposited on the surface which
produces two undesirable phenomena: intermetallic contamination and copper
loading.
Intermetallic contamination is the migration of substrate
metal through the plating and occurs as the copper dissolves and then
redeposits with the tin. This effect is less apparent as plating thicknesses
increase. Initially this can be represented by a continuous relationship. Over time thermal expansion produces specific alloys inducing step
changes in the copper concentration profile. Pure tin, Cu6Sn5,
Cu3Sn, and pure copper are the main species. This phenomenon is not exclusive to immersion tin but
it is particularly prominent and requires building additional thickness to
avoid issues.
Copper loading is another issue that must be dealt with in
immersion tin baths to ensure optimum performance. Not all the copper dissolved
off of the board redeposits with tin; this generates a gradual increase in
copper loading over time. The maximum allowable copper in the bath is around
8g/L before the deposition rate of tin is adversely affected. To remove copper
a bath is put into a holding tank and chilled to 55 F. This drops the copper
without pulling out additives. The bath must then be filtered and analyzed,
making appropriate additions according to the TDS. Once the parameters are
brought back into range the bath is ready for use once again.
Immersion Silver
Immersion silver is similar to immersion tin and includes a
cleaner, microetch, and plating step. The enthone products for immersion silver
are carried under the AlphaSTAR name and include the AlphaStar 100 Cleaner, the AlphaSTAR
200 Microetch, and the AlphaSTAR 300
immersion bath. Compared with the immersion tin, silver exhibits far less
intermetallic contamination and can thus be plated with much lower thickness.
In contrast to OSP, which is displaced in assembly and
immersion tin which becomes part of the solder, the immersion silver final
finish can be directly soldered to.
Product List
-
ENTHONE PC-7078
o Tri-acid
cleaner for copper to remove barrier coatings, oxides, and light contaminants
-
ENTHONE RS-1609
o Aqueous resist stripper for I/L and O/L
-
ENTHONE PC-4052
o High performance aqueous resist stripper for I/L and O/L
-
ENTHONE PC-4025
o High Performance solvent based resist stripper for maximum performance on I/L and O/L
-
AlphaPREP 7096
o Alkaline
cleaner/microetch for use prior to alternative oxide, immersion tin, and
immersion silver
-
AlphaPREP 7030
o Alternative
Oxide which provides high copper loading (long bath life) and high bond
strength between inner-layers in the lamination step
-
ENTEK CU-56
o Anti-tarnish
used in rinses to protect copper from oxidation
-
ENTHONE CE-2220
o Cleaner/Etch
(hence CE) combo used prior to precoat in the ENTEK OSP process
-
ENTEK PRECOAT PC-1030
o Pre-coat
used to deposit a seed layer before OSP
-
ENTEK PLUS HT
o Organic solderability preservative
-
ORMECON CSN 7001
o Single
component MSA based immersion tin bath
-
ORMECON CSN 7004
o Two
component sulfuric acid based immersion tin bath
-
AlphaSTAR 100
o Cleaner
prior to immersion silver
-
AlphaSTAR 200
o Microetch
prior to immersion silver
-
AlphaSTAR 300
o Immersion
silver bath
Figures
-
A. Copper electroplating process with matte and shiny
areas displayed
-
B. Tg of laminates, show coefficient of thermal
expansion
-
C. Collimated light exposing on a dry film
-
D. Develop, Etch, Strip Cross section
-
E. Foot left behind by copper etch
-
F. Before and after diagram of copper in alternative
oxide step
-
G. Alternative oxide etch rate vs. [Cl]
-
H. Cross section of lamination inner layer w/ pre-preg
-
I. Trapping Dry film with overplate on O/L
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