SMTAi Paper_Printing Small Aperture components Final Aug_2_2019.pdf - 第5页

• Stencil: 29 ” X 29 ” frame size, high tensio n, laser cut with Nano-coati ng. Aperture sizes were o ne to one matching the 0.00 5” x 0.006” (0.127 x 0.1524mm) pad size. • Blades: 8” (220 mm ) stainles s steel blades, a…

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.

Figure 4: Volume Results 0-Degrees 008004” (0201mm)

Figure 5: Height Results 0-Degrees 008004” (0201mm)

The results show that volume was treading slightly higher

but consistent. The Height was higher than desired but the

distribution is tighter, which was reflected in the higher Cp

number.

Paste Volume and Height for Pads 90 degrees – refer to

(Figure 6) for volume measurements and (Figure 7) 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 108.71% where the low end of the

volume was 82.31% and the upper volume was at 135.11%.

The Cp was calculated at 2.27 and the CpK was calculated

at 2.24. The Pp was 2.27 and the PpK was 2.28. The height

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

upper limit. The distribution curve is centered with an

average height at 105.84% with the lower end at 94% and

an upper reading of 117%. The Cp was recorded at 4.22

with a CpK of 3.81. The Pp was calculated at 4.31 and PpK

at 3.82.

Figure 6: Volume Results 90-Degrees 008004” (0201mm)

Figure 7: Height Results 90-Degrees 008004” (0201mm)

Figure 8: Overview of paste deposits 008004” (0201mm)

Figures 9: Individual Paste Deposits 008004” (0201mm)