SMTAi Paper_Printing Small Aperture components Final Aug_2_2019.pdf - 第4页

The recommended seque nce is a vacuum/vacuum/dr y. The combined vacuum strokes eli minate any paste pulled from the aperture and left behi nd that was the result of the first pass vacuum. Sol vent should be used less fre…

variations in shape. Based on previous experiments we

determined that the 0.002” thick stencil had the best transfer

efficiency. When specifying a stencil thickness more often

we take in consideration the two ends of the spectrum for

paste requirements and find a compromise in-between. Most

applications so far using micro-component printing, have

had compatible component mixes where there was not a

significant difference in requirements. The frame size we

used was 29” x 29” (736 x 736mm), however, 23” x 23”

(584 x 584mm) stencil may be better suited based on

common average board size for 008004” (0201mm)

applications and stencil tensioning requirements outlined

below. It is recommended using a fine grain stainless steel

stencil that is laser cut. Electroform stencils have fallen out

of favor with reported issues such as variation on aperture

size and foil thickness and stretch being introduced to the

image. Recommended for this application is to use high

tensioned foils. Stencils have a range for tensioning

normally 28 – 40N/cm² (Newton/centimeter). Most stencil

tension falls into the lower 30-Newton range. Increasing the

tension into the upper 30-Newton range prevents stencil

drag. Stencil drag is when you are using a thin stencil foil

with a significant amount of aperture openings. The surface

tension of the paste that has now adhered to the PCB, pulls

at the stencil foil during release, resulting in lower paste

transfer efficiency. The higher tension results in a cleaner

more balanced release with no transfer issues. Nano-coating

is recommended as studies have proven it improves transfer

efficiency. Stencil manufactures have improved the

application methods for applying Nano-coating to stencils

that has improved its manufacturing life. However,

aggressive fluxes and repeated aggressive wipes will

eventually wear the coating off. Careful handling of thin

stencils should be emphasized as they are easily damage.

Special care should be used when storing and transferring to

and from the machine. Take care when handling blades over

the stencil in the machine as a dropped blade could ruin a

stencil quickly. Cleaning the stencil using ultrasonic

methods after printing is essential for continued stencil life.

Type 6 paste is difficult to clean and can become difficult if

not impossible to remove if not removed promptly after use.

Solder Paste:

The recommended solder paste for this aperture size is a

Type 6 powder size. The specification for Type 6 is a mesh

size of +635 mesh size with the ball size range of less than

20µ with an average of 10µ. Type 4 paste is the prevailing

powder size presently being used in the SMT market.

Significant improvements in powder size yields have eased

pricing for Type 4 and Type 5. However, Type 6 pricing has

remained constant where comparative pricing can be three

times the cost of the Type 4, they are presently using.

Squeegee speeds and release parameters are dictated by the

paste formulation and flux type. From the printing

prospective, Type 6 prints like any other paste, however

considerations of the requirements down steam need also be

considered. Matching the paste to the Pick and Place as well

as the requirement for using nitrogen during reflow must be

also considered when using Type 6 paste. As best practice

for a 0.005” x 0.006” (0.127 x 0.1524mm) aperture is a

Type 6 paste – experiments going forward need to be

performed to see if a hybrid Type 5.5 powder size or a Type

5 can be substituted for a Type 6.

PCB Support:

The consensus in printing is that tooling support is essential

to successful, repeatable print results. The aluminum tooling

plate is still the touchstone that all other forms of support

are tested against. Since most applications for micro-

component printing use PCBs 0.030” (0.762mm) or less, the

tooling in combination with vacuum assist to insure the

PCB is flat, level and supported will give the best results.

The plate should be designed so the PCB fits in a recessed

pocket with the PCB surface positioned above the tooling

surface. Support wings are also recommended to support the

squeegee outside of the print area to prevent long term

damage to the stencil. Recommended is a Venturi vacuum

system as the Hg (inches of mercury) produced by standard

vacuum systems may not be enough to flatten the PCB.

When implementing vacuum openings on the plate, take in

consideration the thickness of the PCB relative to the hole

size to prevent deflection or “dimpling” of the PCB surface.

Special attention needs to be focused on the leveling of the

bottom of the tooling plate fixture. This will be reflected on

how well the PCB gaskets to the stencil. Addressing how to

hold the PCB in place during the print process, vacuum is

the preferred method to insure a flat surface over top or side

clamping for PCBs thickness below 0.030” (0.508mm).

Wiping:

Wiping is the first defense against defects and can have both

a positive and negative impact on the process. Determining

the correct frequency, wipe sequence, compatible

chemistries and materials will impact repeatability and

eliminate potential defects. Micro sized apertures require

more frequent wiping where a simple experiment can

determine the starting point, however, over-wiping with

solvent can have the same negative effect as under-wiping.

The test involves printing a board and then drive the vision

camera under the stencil to inspect the apertures for any

paste squeeze out or clogged apertures. Note, that the

apertures will contain some paste that was not released

based on transfer efficiency and stencil quality. In most

cases this paste will be pushed out on the next print

sequence and will not require a wipe, please judge

accordingly. Continue this process of inspection until

defects are starting to form. Subtract 1 board from the total

and this can be your starting frequency. If a Solder Paste

Inspection machine is available, then based on results, this

can be used to determine the correct frequency of wipes.

The recommended sequence is a vacuum/vacuum/dry. The

combined vacuum strokes eliminate any paste pulled from

the aperture and left behind that was the result of the first

pass vacuum. Solvent should be used less frequently as the

purpose of this material is to address the flux that can build

up around the aperture opening. Recommended frequency

for a solvent wipe is every 4-6 wipe cycles. The

recommended solvent stroke sequence is a

solvent/vacuum/dry where a solvent application always

begins the sequence. Consult your paste manufacturer for

recommended solvents to ensure that the solvent used is

compatible with the paste flux. A quality paper should be

used as Type 6 paste can be difficult to clean, where

economy paper can have issues with retaining the solder

balls and contamination issues could result. [1]

The Printer:

The printer plays the major role in the success of printing

008004” (0201mm) components. It’s recommended prior to

printing micro apertures to make sure all preventive

maintenance and calibrations are up to date. The alignment

capability of the machine is vital for dealing with small

pads. Advancements in machine vision repeatability and

accuracy has kept pace with the introduction of micro-

components. However, if your machine was designed back

in the 1990’s, then it most likely will not have the accuracy

resolution to handle these component challenges. Testing

the machines vision alignment capability prior to developing

the process is recommended so that with the machine

verified, issues with alignment can be isolated and solved

more quickly. This can be done using a print verification

process, using embedded machine software that measures

paste deposits for accuracy and repeatability, the results will

determine if a vision calibration is warranted. Another key

calibration on the printer that is often overlooked is the table

leveling to the stencil rails. Since gasketing is paramount

when printing micro apertures, this calibration takes into

consideration the four corners of the table as it applies to the

stencil rails for proper seating between the PCB and stencil.

This calibration is overlooked as it was most likely done

when the machine was built by the manufacturer and never

addressed again after installation. One of the issues with

doing this calibration was the difficulty with the procedure

used. A feeler gage is employed to measure the distance

from the table to the bottom of the adjacent stencil rail. In

order to measure the four points, the gage is moved from

corner to corner repeatability to dial the distance to within

specifications. This process requires the machine to be down

significantly often taking hours to complete. A new tool

developed by MPM addresses this issue by adjusting all

four corners simultaneously. To date, specifications for table

to stencil leveling has been in the range of +/- 0.004”

(0.1016mm). However, studies have shown best results are

achieved when the specification is dropped to +/- 0.001”

(0.0254mm). To eliminate any tolerance issues between the

table and the tooling plate, the plate can be used as a

reference during this calibration. This specification can be

achieved using this calibration tool and has played a

contributing role in successfully printing micro apertures.

The time to complete this calibration has been reduced to

roughly 1 hour. Lastly, the printing machine should be

completely inspected for cleanliness and clean any paste

debris found. Root cause for many issues can be traced to

random paste deposits or residual paste that builds up over

time.

DESIGN OF EXPERIMENT:

Print Test for 008004 (0201mm)

Overview:

To demonstrate micro-component printing capability, with a

focus on the 008004” (0201mm) component using the new

SMTA miniaturization test vehicle. Using best practices

described above, examine the results to determine Cp, CpK,

Pp and PpK results. The goal is to achieve a process

capability, Cp, that is equal to or greater than 2.0 as well as

a CpK greater than 2.0 that demonstrates that the process in

within Six Sigma quality levels. The Pp and PpK numbers

will be examined to see how well the process is centered

with a goal of equal to or greater than 1.667. The test will

use the Edison platform to perform the printing using a

standard configuration. A description of the DOE and

details on the machine, materials and process used as well as

an examination of the results is as follows:

Design of Experiment:

The test consisted of printing a total of 24 PCBs with the

first 4 PCBs to be used as kneed boards to normalize the

process and get the solder paste to a working viscosity. A

wipe will be performed after each print to eliminate any

noise from the data. The remaining 20 PCBs will be

inspected by a Parmi SPI machine with the corresponding

data analyzed through a on board SPC package. Process

data for volume and height for the 008004” (0201mm)

components will then be gathered and displayed and studied

to determine the process capability of printing 008004”

(0201mm) components. All equipment used was recertified

prior to this test being performed.

Materials:

• Printed Circuit Board (PCB): The PCB used for

this experiment is the new SMTA miniaturization

test vehicle. (Figure 2) The board dimensions are

8.0” (203mm) in X and 5.5” (139.7mm) in Y with

a thickness of 0.062” (1.57mm). There are

approximately 400 pads per board with 200

positioned at 0 degrees and 200 pads positioned at

90 degrees. The 008004” (0201mm) pads are

0.005” x 0.006” (0.127 x 0.1524mm) with an air

gap between pads of 0.0047” (1.1938mm) and a

component pitch of 0.00126” (0.032mm)

• Stencil: 29” X 29” frame size, high tension, laser

cut with Nano-coating. Aperture sizes were one to

one matching the 0.005” x 0.006” (0.127 x

0.1524mm) pad size.

• Blades: 8” (220mm) stainless steel blades, at 55-

degree attack angle

• Paste: Type 6 - SAC305 No-clean flux

• Tooling: Dedicated work holder with vacuum –

custom made to SMTA PCB.

Figure 2: SMT Miniaturization Test Vehicle

Printing machine: the Edison platform was used for this

test. (Figure 3) The Edison was specifically designed for

small to medium sized boards and the printing of micro

sized components. The machine vision specifications of a

+/- 0.0003” (8µ) repeatability with a Cp of 2.0 @ six sigma

and a wet print repeatability of 0.0006” (15µ) with a Cp of

@ six sigma. The thin vision camera design reduces the

distance the z-axis must travel when loading or releasing a

PCB. The system uses a X/Y/Y alignment where the

alignment motors have been moved further apart for better

resolution. The Z-axis is tuned to the center of the board

where when the Z-axes is raised, the PCB and the whole the

rail assembly is decoupled from the table to eliminate any

stack up intolerances. This ensures that proper gasketing is

done and a clean release from the stencil. The stationary

wiper is positioned in the front of the machine and the

stencil is presented to the wiper via a shuttle eliminating any

contamination in the print chamber. The print head uses a

single load cell to monitor both squeegees to prevent print

direction variation. All calibrations were performed on this

machine and was validated using CeTaQ testing procedures.

Solder Paste Inspection: For solder paste inspection we

used the Pari Sigma X that was retrofitted with the new

high-resolution inspection head. Standard SPI inspection

heads do not have the resolution to handle micro component

paste deposits. The speed of the scan is slowed from a

100cm/sec to 60 cm/sec to facilitate the micro deposits. The

system uses a dual laser optical triangulation and has

specification of height accuracy of 2um with a height and

volume repeatability of 1%. A gage R+R test was performed

prior to performing testing.

Figure 3: MPM Edison Stencil Printer

Machine Parameters:

• Squeegee force: 14 lbs. (1.35 kg)

• Squeegee Speed: 1.5 in/sec (38mm/sec)

• Slow Release Distance/Speed: Distance = 0.100

(2.54mm) Speed = 0.100 (2.54mm)

• Wipe Frequency; Every PCB

• Wipe Sequence: Vacuum / Vacuum / Dry

• Board clamping: Vacuum and side snugging

Results:

Paste Volume and Height for Pads 0 degrees – refer to

(Figure 4) for volume measurements and (Figure 5) for

height measurements. The limits for volume were set to 50

% for the lower limit and the upper limit was set to 170%.

The distribution curve is centered and shows the average

paste volume to be 114.05%, the low end of the volume was

88.74% and the upper volume found was at 139.37. The Cp

was calculated at 2.37 and the CpK was calculated at 2.21.

The Pp was 2.37 and the PpK was 2.21. The height limits

were set to 50% for the lower limit and 150% for the upper

limit. The distribution curve is shifted towards the higher

end with an average height at 114% with a lower end at

99% and an upper reading of 129%. The Cp was recorded at

3.2 with a CpK of 2.32. The Pp was calculated at 3.28 and

PpK at 2.32.