Particle contaminants in electronics manufacturing

Pasi Tamminen

Purpose

• To get more understanding of the most critical processes being sensitive to particle contaminants in electronics manufacturing.

• Analyse volume and type of typical larger particle contaminants in high volume manufacturing environment

• Design particle prevention principles and methods suitable for high volume electronics manufacturing.

Outline

• Motivation• Typical component and final assembly processes• Particle critical processes• Example case: Particle analyses in high volume

manufacturing• Particle size distribution• Typical particles found in the area

• Source of particles• Prevention principles• Conclusion

Motivation

• Particle contaminants may cause both functional and visual quality challenges.

• Product parts may have different particle critical surfaces due to many technologies used with the parts.

• Large scale electronics manufacturing has novel challenges with particle control due to high material flow and large area under control. Clean room environment may not be easy to implement.

• Sources of particles due to high material flow may be obscure and needs to be analysed in order to focus corrective actions.

Component and final assembly processes

• PWB assembly processes consist of the next typical processes:• Printed Wiring board (PWB) unloading & conveying• Paste Printing• Component placement• Reflow• Testing• PWB separation• Other product specific processes

• (ACF joint, dispensing, inspection, …)

Component and final assembly processes

• Final Assembly has typically some of the next processes:• Testing and/or Programming• Mechanics assembly

• Covers, RF shields, antenna, keyboard,…

• Electro mechanic parts assembly• Display, camera, loudspeakers, microphones, sensors,…

• Other processes• Screwing• Welding• Gluing• …

Component and final assembly processes

• Summary of component assembly processes• PWB assembly has typically always the same process

phases.• Most of the processes are made inside of an equipment cabin.• Particle critical surfaces on PWB are visible or without any

cover a few minutes during component assembly.• Main challenges with PWB assembly are related to functional

defects with joints.

SMT and final assembly processes

• Summary of Final Assembly processes• Final assembly varies a lot depending on the complexity of the

product. • Most of the processes are uncovered.• Final assembly parts are visible from a few minutes up to tens

of minutes during assembly.• Main challenges with final assembly are with visual defects.

Particle critical processes

Processes being critical to particle contaminants• Functional defects:

• Joints: paste, ACF, …• Tester pins

Visual /Optical Defects:

• Displays• Cameras• Covers• Other visible surfaces

Area [mm^2] Amount / product Typical area [mm^2/product]Test pads 0.12 7 1ACF joint 0.09 30 3Camera lens 16 2 32Paste joint 0.2 1000 200Display 1500 2 3000Cover 7500 2 15000Note: Example numbers

Particle critical processesCritical size of particles• More than 30 µm size particles with ACF joints and more than 100

µm size particles with solder joints may cause functional defects. • Large surfaces (display and cover) can have visual defects if the

particle size is more than 30µm.

Amount of particles• It is not easy to give a value for the maximum number of particles

in air for assembly processes. This depends on e.g. • type of the contamination,• sources of the particles,• exposure time,• assembly processes• and product technologies.

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125 250 500 1000 2000 4000 8000 16000Number of particles in air / m3

Num

ber o

f par

ticle

on

pads

/PW

B

(10

min

utes

& 1

.92*

10E-

4 m

2 ar

ea) 0.01

0.020.040.080.16

Particle critical processes

0

20

40

60

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125 250 500 1000 2000 4000 8000 16000

Number of particles in air / m3

Num

ber

of p

artic

le o

n di

spla

y/P

WB

(10

min

utes

&28E

-4 m

2 ar

ea)

0.010.020.040.080.16

Particle speed m/s

Particle speed m/s

• Particles deposition on surface can be estimated if target area and amount of particles in air is known.

• The examples beside are calculated for solder joints and for the display area.

Particle analyses in high volume manufacturing

• Target of the analyses were to locate major sources of particle contaminants and estimate volume of particles with the size of 10 µm or more in high volume manufacturing.

• There were also material storage, recycling and logistic operations at the same area.

• Contaminants were analysed with HIAC/ROYCO 5230, MetOne 237 B and 3113 particle counters.

• PIMEX video and particle counters were used to analyse potential process phases releasing particles.

• Tape samples and optical microscopy analyses were used to classify larger particles.

Particle analyses in high volume manufacturing

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10000000

0.3 0.5 1 2 3 5 10 25Particle size [um]

Part

icle

s / m

3

Final assemblyPlacement

• Air particle counter measurements revealed much more large particles in final assembly than in PWB assembly area.

• This result was also supported by optical tape sample analyses.

One example of the measured particle distributions

Particle analyses in high volume manufacturing

1000um

500um

• Tape samples were used to analyse particles on the surfaces.

• Most of the particles were fibres and size varied from a few micro meters up to several millimeters.

• Both conductive metal and paper or PWB based dielectric particles were found.

1000um

Particle analyses in high volume manufacturing

• Summary of the particle analyses• Probability to get challenges with electrical joints were found

to be low when compared to possibility to have visual challenges.

• Main corrective actions should be targeted to final assembly area.

• The main sources of large particles had to be analysed and prevention methods designed.

Particle sources

• Sources of particles will vary due to different manufacturing principles and product materials.

• Component assembly process does not have many major sources of large particles.

• PWB separation may produce a lot of large particles. Some of these may attach onto the surface of the PWB.

• Component tape handling may produce particles if paper based carrier tape is used.

• Final assembly may have several major sources of large particles, but this is strongly depending on packaging materialsand product parts.

Particle sources

• The main source of particles in this study was found to be the packaging materials.

• Amount of packages handled in the area were such a high that all other typical sources of particles, like personnel, subassembly or facility, were found to be in minority.

• There were more particles close to the packaging area, but packaging material based fibres were identified also with tape sample analyses taken close to the testing, final assembly and PWB assembly area.

• Packaging materials are mostly made of cardboard and forming and handling of these releases fibres.

Particle sources• Process phases releasing particles were analysed also with an air

particle counter and PIMEX software. This software combines particle counter results and video.

• The figure below shows an example of the results. In this process phase effect of an ionisation was measured. Ionisator blew significant amount of particles into the air and this can be seen as a high peak at the moment of 405 second.

Prevention principles• Final assembly phases producing particles should be separated

from the processes having high requirements with cleanness, but this would require major changes in process layout.

• Most of the particle prevention activities in the facility under this study should be targeted to packaging material handling operations or to the packaging materials itself.

• Particles could be prevented by reducing particles peeling off from cardboard boxes. Packages should have durable edges and solid surface to reduce peeling.

• Package format could be modified so that shearing edges of the cardboard will not be subjected to rubbing during handling.

• Plastic based materials could be also used with those areas where particle prevention has more importance.

Prevention principles• Some of the particle contaminants can be prevented by having

better material cleanness control.

• Dielectric trays should be changed to dissipative and all trays should be cleaned between usages. This would prevent most of the particles in the process due to electrostatic attraction.

• Subassembly parts should be also packed and transported into theassembly area with materials not releasing particles.

• Assembly operation modifications have to be used as a part of particle prevention.

• For example, ionisation phase should be modified so that it will not disturb normal air flow in the area and blow particles into environment.

• Overpressure could be used locally in case of particle critical process.

Conclusion• Particle contaminants causing functional or visual defects with consumer

electronics assembly operations were studied.

• There are particle critical processes both in component and final assembly phases. However, more challenges can be expected with particle sizes more than 30 µm with final assembly processes.

• The main source of particles in this study was found to be packaging materials made of cardboard. However, this result is valid only in the facility under this study and different situation can be found if other manufacturing layout or processes are used.

• In high volume electronics manufacturing typical particle sources can be in minority due to cumulative effect of high material flow. Therefore, more focused prevention methods should be used and analyses are needed to bring these out.