SECTION II: STENCIL PRINTING
I.
Introduction to Stencil Printing: The goal in stencil printing is to place an exact amount of material in
a precise location. The materials might be solder paste, adhesive, flux, or thick film materials. The
stencil is the primary deposition controlling function in the printing process. Although squeegee
pressure can affect quantity of material as we’ll see later, the stencil is the control mechanism for
material deposition.
a. Surface Mount Trends
i. High Density Interconnect - Definition - Printed circuit board with a higher
wiring density per unit area than conventional printed circuit boards (PCB). They
have finer lines and spaces (≤ 100 μm), smaller vias (<150 μm) and capture pads
(<400 μm), and higher connection pad density (>20 pads/cm2) than employed in
conventional PCB technology.
ii. The closer the components become to each other on a PCB the more difficult it is
to fixture. This can lead to improper support and hold down. Boards that bend or
move during a print stroke can suffer gasketing and misalignment issues ending
up in increased residues on the bottom side of the stencil. Increasing the wipe
frequency may help decrease defects, but the root cause should be addressed.
Custom recessed vacuum tooling plates or pallets may help in addressing this
issue.
iii. Miniaturization - As components continue to decrease in size, the apertures
needed to deposit the solder paste also shrinks. This pushes the limits of the
apertures area ratio’s and will require even thinner stencils. In the case of the
008001(Metric 0201) the specification calls for a 100X130µ aperture on a 50µ
stencil. This will cut the solder volume for the other components on the assembly
in half making a full release of solder paste from the aperture extremely
important. Thinner stencils also have a higher risk for solder paste bleed through.
This is where the paste leaks between the aperture and pad edge leading to paste
on the bottom of the stencil and possible a wet bridge of paste between fine pitch
pads. Stencil cleanliness, will therefore, become even more important.
0.25 x 0.125 mm Ferrite Bead
(.008 x .004”)
0.3x0.15mm Chip Resistor
(0.012 x 0.006”)
.3mm Pitch BGA
Stencil
Area Ratios
Aperture Size
Stencil Thickness (mil)
Component Discription
mm
mil
2.0
2.5
3.0
3.5
08004
Chip
.100 X.130 4 X 5 0.556 0.444 0.370 0.317
03015
Chip
.100 X.150 4 X 6 0.600 0.480 0.400 0.343
.3mm Pitch CVBGA368 .180 X .180 7 X 7 0.875 0.700 0.583 0.500
008004 Ferrite bead in the tip
of a 25 gauge needle
.4mm pitch BGA
.3mm pitch BGA
b. Factors affecting the cleanability of the Solder Paste ~ Greg, the current draft speaks to
this in different sections. I suggest you write your thoughts and we integrate that into the
document.
i. Solder Paste Flux Composition
Effects of cleaning rosin based solder pastes with water based solvents
ii. Print Process
1. Cycle time
a. Screen printer cycle times are affected greatly by wipe frequncey
and the stencil cleaning profile. Many printers on the market
allow the operator to create a custom wipe sequence.
It is very important to be careful when adjusting parameters like
print stroke, print speed and release speed. They will have little
impact on cycle time and can affect performance of the solder
paste. The main parameters that will increase cycle time are
frequency, quantity and type of wiper pass.
2. Gasketing – The gap between the stencil and board should be zero.
Verify this by loading PCB to print height, use a flashlight, focus closely
inside an aperture and look for movement as you gently push down on
the stencil surface. Little to no movement should be seen
3. Labels- can cause gasketing issues by lifting the stencil off the board.
This can be remedied by back etching the stencil to receive the label
4. Mask thickness
5. Over etched pads
6. Worn/coined stencil
7. Worn damaged blades
8. Silk screen deposition
iii. Stencil Type
1. Frameless
2. Intrusive
3. Step stencils
4. Stencil coatings Nano Coatings
5. PH of fluxes and solvents
6. Nickel Plating
iv. Aperture area ratio is calculated as the area of the aperture’s PCB-side opening
divided by the area of the aperture walls, and is an indicator of the relative
adhesive forces on the solder paste deposit during separation from the stencil. As
area ratios decrease, so does the amount of paste transferred. Due to advances in
solder paste, stencls and printing equipment, area ratios down to 0.50 are now
possible if.
v. Stencil coatings
vi. Understencil Cleaning
1. Wiper paper material
2. Wiper maintenance
c. Why Clean SMT Stencils ~ Listed below is the content within the current draft. Please
review and add or adjust as you see fit.
i. Transfering flux to the back side of the stencil
ii. Solder Paste Transfer Efficiency
iii. SMT Defects
1. Shorts
2. Opens
3. Head on pillow/Non wet open
4. Tombstoning
5. Poor Solder Joints
6. Solder balls
7. Graping/non-coalescence
8. Powder Satellites
iv. Stencil cleaning do’s and don’ts
1. Using brushes to clean stencils is non-repeatable and will damage fine
feature apertures
2. Place fiducials outside the print area as wiper can push paste into them
leading to Poor fiducial recognition.
3. Flux build up on stencil causing boards to stick to stencil or machine
errors
4. Manual cleaning inside the can damage, especially the vision system.
1.5. Common Production Problems: Printing or dispensing of solder paste is a crucial step in production
of SMT PWA’s. This operation is the source of a large fraction of the in-process defects that occur in the
manufacturing cycle. The stencils used must be clean and free of solder paste residue to avoid
misprinting/misapplying solder paste deposits to PWA’s. Additionally, the wetness and thixotropic
property of the paste being applied, flatness of the PWA, the positioning of the stencil with respect to the
PWA pads, the squeegee pressure, stencil aperture opening in relation to the PWA pad dimensions, type
of stencil technology, etc. can all result in solder paste being left on the stencil.
For the reasons stated above, the bottom of the stencil requires inspection, monitoring and wiping
throughout the process. Printers provide various methods to wipe the stencil bottom. The understencil
wiping following the printing process is only as good as the materials that go into it. If a bad print occurs,
nothing downstream will correct these errors. For printers that do not provide understencil wiping,
manual methods must be employed.
Failure to implement a process that removes residue from stencil apertures contributes to board
processing problems. Factors outside the cleaning process may also contribute to process issues. Figure 19 identify issues that contribute to stencil printing problems.
Figure 1: Cleaning Stencil Bottom: Failure to underwipe or clean the bottom side of the stencil
contributes to inconsistencies in solder paste deposition.
Figure 2A
Figure 2B
Figure 2A and 2B: Uncleaned Stencil Apertures: Solder balls attached to the walls of the stencil aperture
over time. Failure to clean the stencil after solder paste printing could potentially cause open solder joints,
bridging, and solder ball formation on the subsequent PWAs.
Figure 3: Solder paste build up on the bottom side of the stencil can result in excess paste. Bridging
defects occur from pump-out of solder paste and build up of solder paste on the bottom side of the
stencil.1 (SMTA Printing 101)
Figure 4: Solder paste build-up on the aperture side walls can result in lack of solder paste on fine pitch
pads. The defect leads to opens and solder joint weakness.1 (SMTA Printing 101)
Figure 5: Solder paste inconsistencies can lead to tomb-stoning defects. When solder paste is thicker on
an adjoining pad, the component may lift during reflow. The defect results in an open with the component
lifting on the pad that was printed with higher volumns of solder paste.1 (SMTA Printing 101)
Figure 6: Pads deficient of solder paste leads to poor and weak solder joints.1 (SMTA Printing 101)
Figure 7: Conversely, excess solder paste can also lead to poor solder joints as well as excess flux under
bottom terminations.1 (SMTA Printing 101)
Figure 8: Excess solder paste on the bottom side of the stencil can also lead to solder balls and flux
bleeding away from the component pads. Solder balls can lead to shorts while excess flux residues
can compromise the integrity of no-clean, which can eventually lead to electrochemical migration.1
(SMTA Printing 101)
II.
Figure 9: Defective stencils over time can lead to solder spheres migrating away from the
solder pad. The result leads to solder balls reflowed onto the solder mask.1 (SMTA Printing 101)
Figure 10: Poorly wetted solder joints can result from pad oxidation, ineffective reflow profile, dried out
solder paste, or a poor flux package within the solder paste. Each of these deficiences can result from lack
of control over the stencil printing process.(SMTA Printing 101)
Figure 11: Failure to Place the Stencil on a Level Plane: The stencil must sit on a level plane that is flat
with well-identified points and lines. The stencil should contact the board at the solder mask height. Poor
quality hot air leveling or excessive legend ink thickness can lead to poor gasketing, which will allow
solder paste to flow beyond the aperture opening on the bottom of the stencil resulting in poor print
definition.
III.
Figure 12: Variations in Solder Mask Thickness: Variations in solder mask thickness contributes to
inconsistencies in solder paste deposition. This condition is similar to the board coplanarity problem and
will lead to poor definition and increase the need for stencil wiping.
.
Figure 13: Inconsistent Component Pad Width: Component pad width must be consistent. Variations
contribute to inconsistencies in solder paste deposition. If the pad is smaller than the aperture, paste will
flow beyond the pad and if soldermask is not present, the possibility of bridging between the pads will
occur.
IV.
V.
Figure 14: Poor Alignment: Poor alignment contributes to inconsistencies in solder paste deposition.
Frequent contamination of the stencil bottom leads to poor definition on subsequent prints and increased
frequency of the stencil bottom cleaning to maintain acceptable print quality.
Action Steps:
1. Greg Wade of Indium volunteered to be the Lead Person for Collecting information to
write Section I
2. Time Line for collecting information ~ By February 1, 2015
3. Set up a Live Meeting to reveiw with committee
4. Mike Bixenman to take this information and draft Section I prior to APEX 2015