Section II: Stencil Printer Challenges as it Relates to Cleaning
a. Aperture Fill & Release
The print material, which for this discussion is assumed to be a SMT solder paste, should be applied in a
quantity that fully wets the squeegee blade in a bead size that is at least enough for the paste to “roll”
during squeegee excursion, but not so large as to contact any part of the squeegee blade holder assembly.
An ideal amount of paste is considered to have a bead diameter that measures 50% to 75% of the exposed
squeegee blade height. The print speed and applied squeegee pressure should be set such that the
squeegee will wipe the stencil clean while also consistently achieving a rapid paste roll response. Sliding,
skidding, and/or hydroplaning paste bead roll will impede the opportunity for the paste to fill the stencil
apertures. The rolling paste bead will shear thin and experience a natural viscosity reduction via its
thixotropic material properties leading to enhanced solder particle mobility to fill stencil apertures under
squeegee pressure.
Filling apertures completely is an important prerequisite towards the next challenge of emptying all those
filled apertures in a consistently organized fashion to transfer individual paste deposits onto the printed
circuit board pads. For best printing results, both the aperture fill and release processes should achieve
containment the powder alloy and flux into tightly bound groups (i.e. solder paste deposits), resembling
the template of individual apertures from which they were created that do not spill into each other on the
printed circuit board. The “containment” principle is important as it relates to this IPC standard
document’s focus on stencil cleaning aspects. One penalty of a poorly set up print process is more
demand for stencil cleaning maintenance. Critical stencil design factors that influence printing results and
stencil cleaning requirements are described next.
i. Gasket
The term “gasket” is often used to describe the ability of stencil apertures to contain solder paste within
their individual boundaries during a fill and release printing cycle. More specifically, the aperture gasket
is the region of stencil foil around its perimeter that physically contacts the printed circuit board pad. The
illustration (Figure ?) identifies the stencil aperture gasket. An effective gasket essentially seals the
apertures off from each another, containing the solder paste inside them and preventing alloy powder and
flux from escaping.
The reality of a production printing process is that even good gaskets will degrade with continued
uninterrupted printing and the process will require a pause in order to clean off any paste material
accumulation interfering with the gasket. Designing for maximum gasket performance requires
understanding the design of the printed circuit board and sizing the stencil apertures such that the aligned
contact of the stencil to the printed circuit board achieves theoretically tightly sealed apertures. Solder
mask defined pads and non-solder mask defined pads may have significantly different aperture gasketing
levels comparing same the aperture sizes.
The topography of the circuit board can also severely hinder aperture gasketing potential. At the local
level, silk screen legend labels, solder mask thickness, surface traces, and via holes all contribute to
aperture gasketing degradation by reducing the surface flatness of the printed circuit board. At the global
level, board twist and warpage can also introduce planarity discrepancies that weaken aperture gasketing
potential. Similarly, stencil frame twist and bow can also upset aperture gasketing capability. Within the
printer, the uniformity of stencil to board contact is paramount for achieving well sealed apertures and can
be impaired by board and stencil handling components of the printer that do not achieve coplanarity
control. Full “on-contact” printing using zero print gap between printed circuit board and stencil is
recommended to achieve the optimum stencil aperture gasket condition.
ii. Area Ratio
As the separation sequence in the printer is initiated and the circuit board begins to decouple contact from
the stencil, the expectation is that the paste inside filled stencil apertures will drop out and remain
attached to the board. Luckily, the predictability of such a successful (or unsuccessful) outcome can be
predicted based on calculating Area Ratios using known physical stencil aperture dimensions.
Area Ratio is defined as the stencil aperture open area divided by the stencil aperture wall area and
represents a comparison of the attachment contact surface areas for solder paste residing in a stencil
aperture during a print stroke (see Figure ?). The IPC Standard 7525B describes this parameter in high
detail. The value of 0.66 is widely known to identify the threshold dividing high and low area ratios.
Low area ratio designs will tend to produce solder paste deposits with lower and more statistically
variable “transfer efficiency” compared to high area ratio designs.
Transfer efficiency describes the proportion of paste deposited on a circuit board pad relative to the total
aperture capacity from which the solder paste deposit originated. So a transfer efficiency of 75% means
that 25% of the paste still remains in the stencil aperture after separation. As electronic products continue
scaling to smaller and more portable form factors, the stencils used to print such circuit boards trend
toward lower area ratio designs. The consequence of this is reduced solder paste transfer efficiency and
more frequent and aggressive utilization of automatic under stencil cleaning maintenance routines to
maintain acceptable printing production yield.
iii. Interspacing
Designing stencils with sensibly large area ratio values is advantageous from a transfer efficiency
perspective, however, high I/O density applications will also have limitations on the maximum aperture
size. Using too large an aperture size can result in wet paste bridging defects. For today’s smallest pitch
components (0.3mm pitch) it is advised to maintain an interspacing between apertures that is no smaller
than the stencil thickness dimension. For coarsening pitch level, the minimum interspacing should scale
progressively wider, while also adhering to area ratio rules, to assure bridge free and high paste transfer
results. Under stencil wiping routines should be implemented to ensure bridged paste deposit defects to
not form at a programmed frequency interval that also accomplishes the minimum penalty on printing
cycle time.
iv. Design Guidelines (This Section is Blank Because Content Already Exists in Section a. i., a. ii., a. iii.)
b. Stencil Options
As stencil technology continues to evolve and improve, the user has a wider scope of stencil variations to
choose from. The most common stencil options are introduced next. Cleaning aspects will be reviewed.
i. Technology
The three fundamental stencil manufacturing technologies that exist today are Chemical Etch, Laser Cut,
and Electroform. Chemical Etch is a traditional method for creating stencil apertures that is not
commonly used today for that purpose. However, this technique is still commonplace for etching stencil
foils thinner to create step profiles necessary in applications that require multi-thickness foils. Stencil
designs that have step profiles located on the squeegee side (top) of the stencil are expected to accumulate
solder debris with use where squeegee contact is poor. As there is no known “on the printer” topside
stencil cleaning option, the stencil will need to be physically removed from the printing machine to
remove excessive paste residue accumulation. Stencil designs that have step profiles located on the
contact side (bottom) of the stencil may also be sensitive to solder residue buildup as well as more
frequent aperture clogging. Fortunately standard “on the printer” under stencil cleaning systems are
available to wipe off excess solder contamination and clear blocked apertures. However, the under stencil
cleaner wiping contact may not be optimal around the perimeter of bottom side step profiles as well inside
step down recessed cavities.
Laser cutting and electroforming are the two primary methods used today for creating apertures on
stencils. Laser cutting is the more popular option since it is more widely offered at typically lower cost
and faster turn to the customer compared to electroforming. Well maintained modern fiber laser cutting
machines are now capable to accurately cut tens of thousands of apertures per hour at high accuracy and
repeatability. Electroformed stencils utilize the plating process to build up the metal foil, atom by atom,
around a template of photo defined resist pillars. The fully plated foil will have aperture openings
remaining where resist pillars are removed. When comparing the physical aperture characteristics
between laser cut and electroformed stencils, there are a few differences to note.
i.
The aperture side walls from laser cut apertures will typically be more tapered and a coarser
texture compared to electroformed. The foil thickness and bottom side aperture perimeter
topography will be more variable for an electroformed stencil compared to a laser cut stencil.
While the tapered aperture wall cross section (compared to straight) has been reported to improve
paste transfer, data to support such claims is scarce. Similarly, the assertion of smoother
electroformed apertures improving solder paste transfer can also be challenged. The cleanability
differences (for either “on the printer” under stencil wiping systems or “off the printer” dedicated
cleaning machines) between laser cut and electroformed stencil technology is not a well
documented comparison. Therefore both stencil manufacturing techniques are considered to have
matched cleaning compatibilities.
ii. Foil Materials
Electroformed stencils are always nickel foils, but laser cut stencils can be either nickel or stainless steel
materials. Electroformed nickel will typically have a smoother aperture wall texture than both laser cut
nickel and laser cut stainless steel. The aperture wall texture differences between laser cut nickel and
laser cut stainless steel cannot be fairly generalized here, as laser tool operating variables and parameters
are considered to have stronger influence on aperture cut quality than material alone. In line with the
previous stencil technology discussion, there is not enough consistently reported data to recommend one
foil material type over another for cleaning compatibility. Notwithstanding, there is a new category of
stainless steel foils emerging, generically classified as “fine grain” materials. These foils comprise a
higher density microstructure that is stated to yield smoother laser cut aperture walls compared to
conventional coarser grain stainless steel materials. The alleged benefit of fine grain materials is
improved solder paste transfer efficiency, and with supporting data, it is assumed this should also
translate to reduced “on the printer” under stencil cleaning demand.
iii. Frames
There are many variations of stencil frame types available. The two common categories of frame types
are “mesh mounted” and “mesh less”. A mesh mount frame format consists of a stretched mesh material,
made of either polyester or stainless steel that is glued to a rigid outer frame. A thin metal stencil foil is
glued around its borders onto the center of the stretched mesh. The center of the mesh is then cut out after
the foil attachment glue is cured. Mesh less stencils are newer technology where individual loose foils
can be directly attached to a common rigid master frame. In these systems the mechanical attachment
process will generate foil tension by pulling perpendicularly outwards along the four foil edges. In
addition to these two frame categories frames are offered in different sizes, materials, and thickness.
From a stencil cleaning perspective it is important that the frame be straight and level. Twisted, bowed,
and warped frames will cause the stencil foil to follow this non planar profile, leading to greater risk of
poorly gasketed apertures on the printed circuit board substrate and increasing “on the printer” under
stencil cleaning demand. The manufacture of the stencil frame also influences stencil foil tension. High
tension stencil foils may potentially improve paste release uniformity characteristics via enhanced z-axis
rigidity that allows the foil to maintain a more stable position throughout the circuit board separation
process following the print stroke. Any stencil deformation during board separation can inflate the
possibility to produce variable solder paste volume deposits, necessitating more frequent use of “on the
printer” under stencil cleaning.
The preservation of foil tension through the life of the stencil is another factor that can be different
between mesh mount and mesh less frames. “Mesh mounted” foils are permanently fixed in tension, and
remain so while subjected to “off the stencil” dedicated washing machine cleaning cycles. Such wash
cycles can use chemistry and temperature that leads to eventual foil tension relaxation. On the contrary,
“mesh less” frame types only subject the foil to temporary tension while it is mounted during a printing
process. Foils in these systems typically require removal from the master frame while being cleaned in a
dedicated washing machine. The absence of glue and mesh in a “mesh less” foil makes these less
sensitive to experiencing natural tension loss. Frame size can be a concern if the squeegees required to
print the board cannot comfortably fit and move unobstructedly across the entire clamped board width.
The other caution with using small stencil frame sizes is to ensure both the board clamps and the under
stencil cleaner are appropriately sized to accommodate that frame.
iv. Image Position
It is recommended when possible to position the stencil aperture artwork centrally on the foil. This
ensures a balanced foil tension result is achieved across the aperture image, which can help paste release
uniformity. A centrally positioned aperture image also allows the widest printable margin, which can be
important for pastes requiring longer distances to generate “roll”. Similarly, the printing machine’s under
stencil cleaner will also be allowed to wipe across widest area surrounding stencil images using center
justified apertures. The multi-image stencil design strategy is the other option that is common in
production, whereby one stencil foil can have many aperture images for printing the same or different
substrates. Multi-image stencils only make sense with substrate designs small enough to fit on the foil
without overcrowding each other. The incentive to use multi-image stencils is to improve stencil
productivity, reduce overall stencil expenses, and lower stencil storage space. Pros and cons of multiimage stencil design strategy should be carefully considered.
v. Aperture Geometry
The shape of the stencil aperture typically matches the geometry of the circuit board pad and is slightly
undersized relative to it, in order to accomplish an effective “gasket” condition (Section i.). Varying the
stencil aperture geometry in any way that diminishes the opportunity to form a fully gasketed outcome is
discouraged. There is elevated risk to contaminate the bottom side of the stencil with solder particles and
flux using aperture designs that gasket to the substrate poorly. For example, use of square apertures
(instead of circle) on round pads may result in imperfect gasket conditions around the aperture corners.
Apertures with right angled corners, such as for QFPs, could be modified by rounding corners without
hindering the resultant gasket. The logic for rounding aperture corners is to lessen the opportunity for
trapping solder balls in apertures, and thereby reducing “on the printer” under stencil cleaning demand.
Although this theory lacks definitive industry proof, there appears no significant negative consequence to
round aperture corners.
vi. Fine Grain (This Section is Blank Because Content Already Exists in Section b. ii.)
vii. Nanocoatings
A new stencil improvement option, known generically as “nanocoatings”, offers significant benefits
towards reducing “on the printer” cleaning overheads. The primary functional purpose of a nanocoating
is to prevent flux (and paste) from contaminating the bottom of the stencil during a production print
process. Longer continuous printing with fewer “on the printer” under stencil cleaning interruptions is the
most widely reported benefit that has been widely and consistently published. Nanocoatings comprise of
a flux repelling chemistry that are very thinly applied to the contact side of the stencil (i.e. bottom side).
The prefix “nano” implies the coating is thin, but by no means does this accurately reflect a standard
coating thickness dimension.
Categories of nanocoatings include wipe on, plasma, and spray on, reflecting the methods of coating
application. Nanocoating chemistry is highly proprietary. “Wipe On” nanocoatings are customarily
transparent materials, making stencils that have such coatings on them difficult to distinguish from nonnanocoated stencils. A “wipe on” nanocoating is the only category where the stencil end user can apply,
or re-apply, their own coating. The thickness of a “wipe on” nanocoating is extremely thin, on the scale
of atoms thick. A permanent marker can be used to verify the presence or absence of a “wipe on”, or any
other category of nanocoating. The marker will draw a solid bold line on non-coated foil and a faint or
broken line on a coated foil.
“Plasma” nanocoatings are quite similar to the “wipe on” type in terms of the appearance and thickness of
the stencil coating, however, “plasma” coatings cannot be user applied and require specialized application
equipment. “Spray on” nanocoatings are the thickest form of nanocoating and can measure several
microns thick. The “spray on” coatings also cannot be applied by the user, but require specialized
equipment that only stencil manufacturers have. “Spray on” coatings are commonly tinted so that it is
very easy to identify which stencils are coated.
While all nanocoatings are designed to be permanent, chemical and physical abrasion from both “on the
printer” and “off the printer” washing cycles can degrade their potency over time. Nanocoating wear
characterization is awkward to monitor in production as tools available to do this are not widely known.
Dyne surface energy pens offer a convenient means to loosely validate nanocoating strength, stricter
quantification requires more time consuming techniques that involve measuring droplet contact angles.
Some nanocoating products are promoted for their aperture wall coating capability in addition to the
standard bottom side coating function, but this is very difficult to validate without physically cross
sectioning the apertures. While statements regarding solder paste transfer efficiency improvement
facilitated by nanocoatings may indeed be valid for specific situations, this benefit should not be the
expected universal outcome.
viii. Tension (This Section is Blank Because Content Already Exists in Section b. iii.)
ix. Step & Pocket Stencils (This Section is Blank Because Content Already Exists in Section b. i.
Chemical Etch discussion)
c. Application Specific Cleaning Challenges
The following sections describe component categories where unique “on the printer” stencil cleaning
concerns exist.
i. Area Array Components
Stencil apertures here are typically circle shape, but can also be square designs for straight pitch arrays.
Staggered array pad designs will not use square apertures, as bridged paste deposits will likely form
across diagonals if this design were to be used by mistake. If square type apertures are preferred to be
used in staggered array designs this aperture shape must be rotated through the angle that matches the
staggered pad array angle to maintain a proper spacing that minimizes bridging. (Show Figure)
Aperture area ratio and aperture gasketing effectiveness on the substrate pads will largely influence the
solder paste transfer success. “On the printer” under stencil wiping efficiency can be challenging for area
array components that are sensitive to aperture clogging (i.e. 0.4mm CSP and smaller). Vacuum under
stencil wiping system along with programmable solvent dispense options may be necessary to sustain an
automated process while maintaining production throughput expectations.
ii. Passive Components
Stencil apertures for passives can assume several different shapes from squares and rectangles to more
customized home plate and bow-tie that are designed to reduce the paste amount positioned under the
component body. Circle shape apertures are not commonly used for these components. Similar to the
area array component example, small passives (i.e. 01005s and smaller) can be arranged in high density
and present a challenge for “on the printer” under stencil systems to perform cleans well without
supplemental vacuum and solvent options.
iii. Bottom Termination Components
The perimeter I/O for these components will have either rectangle or round apertures. The center ground
pad on the substrate may have one large single aperture or what is more typical to find today is that the
ground pad is printed with a group of “window paned” apertures. The key factor to designing apertures
for the center ground pad is for the reflowed joint there to have minimal voiding. Similar to the former
component types described, the “on the printer” under stencil cleaner should be used to maintain printing
well formed individual solder paste deposits for both the perimeter pads and the ground pad apertures.
As ground pad apertures are supposed to be bridged together after reflow, it may be tempting to accept
printing such pads that result in wet paste bridges and continue without wiping the stencil. The
misguidedness here is that bridged ground pad deposits may block off vital venting channels for void
extraction and inhibit the ability for solder to form optimally designed joints. Window paned ground pad
aperture groups tend to have narrow webs between large apertures, which is fragile and over repeated
cleaning cycles can become stressed, bent, or broken. Gentle cleaning force should be applied to this
delicate area, whenever possible.
iv. Shield apertures
Apertures for shield traces are linked together by small webs to form a chain surrounding an enclosed
area. Individual shield apertures will be rectangles or trapezoids for straight conductive tracks and arc
shapes around curved tracks. Given the large aperture size designed for shields there is low risk to have
blocked apertures, however, bridging is certainly a more realistic defect outcome. Ultimately the paste
bricks deposited onto shield tracks will bridge together during reflow, so it may not be an obvious
concern if stencil printed bridging occurs between shield apertures. It is unwise to dismiss these defects
as they can rapidly accelerate along the aperture chain, or worse, trigger bridging defects to adjacent
component pads.
Basic dry wipe under stencil cleaner programs can be effective to manage shield aperture bridging.
Another concern with stencils that utilize shield apertures is the tendency for the foil area inside the
shields to lose tension. The delicate webs between shield apertures are essentially tension distributing
conduits where high stress concentrations exist. Sustained high quality printing within shields can
become challenging as tension degrades, requiring more frequent “on the printer” cleaning intervals,
which in itself can be causing further tension breakdown of those sensitive areas.
d. On Printer Cleaning Mitigation Strategies (NOTE: THE SECTION CATEGORIES ORIGINALLY
WRITTEN HERE (i.e. i. Hardware, ii. Software, iii. Vision, iv. Closed Loop, v. Housekeeping) HAVE
CHANGED
Modern stencil printing machines offer a variety of tools to deliver and maintain high quality output. A
description of the prevalent equipment options and strategies to accomplish this follows.
i. Clamping
Circuit boards must be secured in a locked position within the printer throughout the paste deposition
process. A loosely fixtured board cannot be reliably printed. Board clamps are built into the central part
of the conveyer assembly inside the printer and specifically designed for this purpose. The two main
types of board clamps are over top and side snugging systems. The over top clamp overlaps the board
edges slightly and presses them down against the conveyer. While the board will be very secure using
this type of clamp, it does cover the edges of the board and can obstruct printing pads located close to the
clamp borders. This clamp can also cause thin substrates to bow, which may degrade aperture gasketing
uniformity.
A snugger clamp type eliminates any contact on the top of the board, allowing the entire board surface
unobstructed contact against the stencil. The snugger clamp is designed to pinch the board from the sides
to keep it in place. Snugger clamps require height adjustment so that when the board is “snugged” the top
surface of the clamp will be level with the substrate. There are variations of the snugger clamp which
automatically heights adjust upon clamping. Similar to the over top clamp, snugger clamps can also cause
substrates to bow under excessive side pressure. Boards that to do not maintain planarity with the stencil
risk poorly gasketed apertures and degraded print quality.
New clamping systems are being offered that have integrated vacuum for aggressively pulling the stencil
down to elevate gasketing performance at the edges of the board.
ii. Tooling
While the role of clamping is to stabilize the position of the board by securing its leading and trailing
edges, tooling support offers a necessary complementary function by stabilizing the z-axis position of the
board throughout the printing cycle. As the circuit board experiences downward pressure by the squeegee
blade moving across it, under board tooling support counterbalances this force keeping the board rigid,
level, and appositionally stable. The most basic form of tooling consists of rigid posts that are manually
arranged on the tooling table. Advanced post type tooling systems incorporate automatic actuating pin
grids that conform to the substrate profile. Solid block tooling and custom machine routed block tooling
with integrated vacuum hold are other popular tooling options used. Circuit boards should not ever be
printed without using proper tooling.
iii. Alignment
Once the board is established in a fixed and secure positioned inside the printer (i.e. clamped and fully
supported with tooling), alignment fiducials on the circuit board and matching fiducials on the stencil are
located by the printing machine vision system. The printer will be capable to align fiducial pairs to
accomplish accurate x, y, and  registration between the stencil and the board. Accomplishing successful
fiducial alignment does not necessarily guarantee all pads and apertures will be perfectly registered.
Circuit board substrate manufacturing tolerance windows allow for some margin of stretch or shrink error
that can translate to offset positioned print deposits. The position of the print deposits can usually be
manually fine adjusted after a fiducial capture cycle if further alignment correction is required. This is
known as creating a “global offset” registration correction and such settings are applied to all boards
subsequently printed. Another alignment parameter that can usually be adjusted is the z-axis setting that
changes the contact pressure between the board and the stencil. Ideally the print contact height setting
should establish uniform board contact against the bottom of the stencil without raising the height of the
stencil foil. In reality, it is far better to apply a gentle positive contact pressure than the opposite scenario
of leaving any loose gaps between the stencil and board. Proper alignment and contact pressure of the
circuit board against the stencil will ensure stencil apertures “gasket” optimally to the pads.
iv. Standard Squeegees
The aperture filling process requires a squeegee blade to cleanly wipe a rolling bead of solder paste across
the stencil. A standard squeegee for SMT solder paste printing consists of a thin metal blade, usually
made of stainless steel that attaches to a holder assembly. The metal blade can be removable for
replacement when it gets worn or damaged. When attached to the holder, the metal blade is quite rigid.
Squeegee holders mount the blade at a fixed attack angle, commonly either 45° or 60°. The angled blade
assists with maintaining a “rolling” paste bead circulation and forcing the solder particles down into
stencil aperture holes. Squeegees usually include a hardware accessory that resemble clips or shields that
are affixed to the sides of the blade holder, known as deflectors. These function like a fence, keeping the
solder paste contained on the squeegee blade and preventing it from spilling off the sides of the blade
during a print stroke. Using these devices prolongs the number of print strokes before a solder paste
replenishment operation is required.
Another design feature of the squeegee is “overhang”, which is a measure of how much free squeegee
blade surface exists (i.e. the blade part that is not covered by the holder assembly). Large “overhang”
squeegee blades (i.e. ~15mm) are more flexible than small “overhang” blades which are quite stiff.
Automatic printing machines can apply specific operator programmed squeegee contact pressure. The
minimum squeegee pressure should accomplish a clean stencil wipe. Higher pressure will cause the
squeegee blade angle to bend. The combination of “overhang” and squeegee blade length will influence
the blade angle deflection sensitivity to the applied pressure. While there is no firm rule governing
maximum squeegee pressure, evidence of too much pressure is stencil coining and bending stress induced
squeegee blade creasing.
Squeegee speed is another programmable parameter that should be set to ensure the solder paste bead
achieves continuous rolling motion. Excess squeegee speed results in paste film coating the stencil or
paste bead skidding across the foil. Too low a print speed is more difficult to identify, but is also
characterized by poor paste bead roll response. Squeegee pressure and speed are inter-related variables,
where fast print speeds require higher applied squeegee pressure.
v. Specialized Squeegees
Unique squeegee blade types that are less common, but offer print process control and capability
improvements include multi-blade, vibrating blade, and enclosed printhead chamber systems. Further
details specific to these should be referenced with the vendors who offer them.
vi. Separation Speed
The final processing action in the stencil printing machine is to move the clamped circuit board away
from the stencil. The solder paste occupying stencil apertures should be extracted and end up sticking
onto the circuit board pads after completing separation. Automatic printers will have software
programmable separation speed levels. Tuning separation speed levels to achieve best paste release
performance requires optimization testing as there is no general industry consensus for blindly setting this
parameter.
vii. Inspection
Advanced automatic stencil printing machines may be equipped with tools to monitor and sustain print
process performance by inspecting for presence or absence of solder paste. On some printers, the vision
system that is used for fiducial capture can also be used as an inspection tool for discovering solder paste
print defects occurring on a freshly printed circuit board and stencil. This capability is usually known as
2D inspection. 3D inspection is uncommon inside the printer, but the technology is becoming
increasingly popular in dedicated machines placed downline the printer. 3D inspection offers a fast paced
highly detailed inspection characterization of the solder paste print deposits on a circuit board. 2D
inspection typically is not capable of providing quantitative characterization of individual solder paste
deposits, as featured by 3D inspection. Both inspection tools can stop the printer upon locating a failed
solder paste deposit on the circuit board. 2D inspection is the only type that can access view of the stencil
apertures for assessing blockage. 2D inspection can usually command the “on the printer” under stencil
cleaning system to turn on for removing unwanted solder debris off the stencil. Legacy 3D inspection
tools typically do not communicate instructions directly back to the printer, but newer models are now
available with closed loop feedback capability that can direct the printer to automatically adjust alignment
offsets and perform under stencil cleaning maintenance routines.
Another inspection option gaining popularity is active paste roll size monitoring. This purpose of this
feature is to instruct the operator when to top off the paste bead with fresh material by using strategically
located sensors to identify paste quantity. Advanced systems can even communicate with optionally
installed automatic paste dispensing hardware and command it directly to replenish paste without operator
intervention.
viii. Process Recovery
The printing process is vulnerable to natural deterioration caused by residual flux and solder powder that
gradually accumulates on the stencil over the course of several printing cycles. If left unchecked, this
stencil contamination can inhibit solder paste transfer performance and/or lead to excessive solder particle
transfer. Under stencil cleaning hardware is used to decontaminate the stencil and maintain this condition
during a production printing process. A conventional under stencil cleaner utilizes a fabric roll that
unwinds across a plenum assembly and rewinds onto a waste spool. The plenum may be configured with
routings to facilitate vacuum filtration, if so equipped. Additional design features may include wet
solvent dispense capability to moisten the fabric. The under stencil cleaner operates in dry wipe, wet
wipe, or vacuum wipe modes and is capable to contact only the bottom side contact area of the stencil.
Typical parameters for standard under stencil cleaner types include wiping frequency, wiping mode,
wiping speed, and fabric indexing distance. While the simplest utilization of the under stencil cleaner is
to dry wipe, continuous use of this mode over prolonged periods may leave sticky flux residue coating the
stencil foil. Intermittent use of a solvent dispense option to apply wetted wiping is effective to dissolving
flux coating buildup on the stencil. Wet and dry wiping alone are marginally effective to removing solder
debris lodged inside stencil apertures, but vacuum enabled wiping is quite useful to accomplish this task.
A further cause for low paste print volume defects relates to an insufficient amount of solder paste
available in the paste bead. If the paste bead is too small, the paste roll may not be adequate to fill the
aperture holes. As previously discussed active paste roll size monitoring can identify this condition. The
recovery action for an insufficient paste bead is to re-apply the solder paste manually, or if the printer is
configured with the automatic paste replenishment hardware, this can be utilized. The common paste
dispensing option is software controlled to re-apply paste on the stencil at programmable intervals. The
paste for this accessory is usually supplied in a tube cartridge format. The operator will usually have
software control of dispense position, speed, and pressure.
Another option that merits mention, but is still relatively uncommon, is on board solder paste microdispensing. A few of the printer brands offering this have a compact solder paste pump device that can be
programmed to dispense solder paste dots at specific locations on the circuit board, which can be useful
for applications requiring additional solder volume not achievable using uniform thickness stencils.