HIGH-SPEED VISUALIZATION OF DEFECTS IN SMT, JANUARY 2016

High-Speed Visualization of Defects Formation

during Robotic Assembly of Electronic Components

at Parker Hannifin Canada

1/20/2016

Final Project Report

Red River College: Oyedele Ola, P.Eng. (Author)

Sarah Hodgson, C.E.T.

Parker Hannifin Canada: Jeremie Robin

HIGH-SPEED VISUALIZATION OF DEFECTS IN SMT, JANUARY 2016

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Executive Summary

This report provides an overview of the work titled “High-Speed Visualization of Defects

Formation during Robotic Assembly of Electronic Components at Parker Hannifin Canada”, a

project funded through the NSERC Engage Grant for Colleges. The problem addressed is the

determination of the root cause of assembly defects, including missing parts, misorientation of

parts and turning of parts upside down, which are observed during final quality inspection/testing

of electronic circuit boards. These defects are usually undetected by automatic inspection

techniques, such as laser measurement and infrared inspection built into the production process.

Red River College’s experience and expertise in high speed imaging was leveraged by Parker

Hannifin to study and determine the root cause of the assembly defects. The picking and placing

actions of specialized pneumatic picking heads integrated with high-speed robots were

monitored by high-speed imaging. The pneumatic heads usually place hundreds of parts on

boards in a matter of seconds. Several experiments involving in-production testing and off-

production simulated testing were conducted. The approach used in the study included the

selection of optimum illumination option for the high-speed imaging situation, systematic

acquisition of imaging data and post processing of imaging data. The direction of the project was

mainly driven by existing production data on the frequency of occurrence of defects and the

hardware and parts involved. The problem was eventually subjected to a “5 WHY” approach of

root cause analysis.

The results of this work showed that the root cause of the assembly defects in Parker Hannifin’s

Surface Mount Technology (SMT) is the flipping and misorientation of parts in the pockets of

unnoticeably faulty twin-tape feeders. The parts are eventually picked with defects and,

unfortunately, some of defects make it to the board because the automated laser inspection is

unable to discern certain degrees and/or types of misorientation. In order to address these

problems, it is suggested that (a) an offline feeder testing program, such as in this work, should

be conducted to isolate problematic feeders (then investigate and fix, or scrap the feeder), (b) the

feeders should be arranged in a queue for use as opposed to random usage to ensure that all

feeders wear and / or age concurrently, and (c) the lifetime of feeders should be standardized.

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Table of Content

TABLE OF CONTENT ............................................................................................................... II

LIST OF FIGURES .................................................................................................................... III

1.0 INTRODUCTION................................................................................................................... 1

2.0 OBJECTIVE ........................................................................................................................... 2

3.0 EQUIPMENT, APPROACH AND PROJECT CONSTRAINTS ...................................... 2

4.0 RESULTS AND DISCUSSION ............................................................................................. 4

4.1 HIGH-SPEED IMAGING OF PICKING AND PLACEMENT OF PARTS .............................................. 4

4.2 ANALYSIS OF EXISTING PRODUCTION DATA – VARIABLES CAUSING DEFECTS ...................... 4

4.3 OBSERVATION OF THE ASSEMBLY DEFECTS ........................................................................... 7

4.4 THE ROOT CAUSE OF THE ASSEMBLY DEFECTS ................................................................... 12

5.0 CONCLUSION ..................................................................................................................... 17

6.0 RECOMMENDATIONS ...................................................................................................... 18

7.0 REFERENCES ...................................................................................................................... 18

HIGH-SPEED VISUALIZATION OF DEFECTS IN SMT, JANUARY 2016

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List of Figures

Figure 1: Frames extracted from high-speed imaging, showing (a) a part picked up from the

feeder (b) a part placed on the board. ............................................................................................. 4

Figure 2: An SOT23 part with a lead on one side and two leads on the other side. The lead

attaches to the solder paste on the board to make electrical contact. .............................................. 5

Figure 3: Side view of a twin tape feeder (TTF) ............................................................................ 5

Figure 4: The problem, 5-WHYs and recommended actions. ........................................................ 6

Figure 5: A frame extracted from high-speed imaging showing a picked up SOT23 part with the

single leading pointing outward and located at the lower part of the chip ..................................... 7

Figure 6: Frames extracted from high-speed imaging showing (a) (b) tombstone defects (c) 90

degrees misorientation of SOT23 parts ........................................................................................... 8

Figure 7: A frame extracted from high-speed imaging showing an SOT23 part that was picked

upside down. ................................................................................................................................... 9

Figure 8: A frame extracted from high-speed imaging showing a tombstone defect. .................... 9

Figure 9: Frames extracted from high-speed imaging showing (a) an SOT23 part with 180

degrees misorientation (b) a part turned upside down (c) a part with 90 degrees misorientation.

The parts in (a) and (b) were successfully placed on the board. ................................................... 11

Figure 10: Tracked locations of defects on the boards. Parts picked from lane 2 of TTF. ........... 11

Figure 11: Frames extracted from high-speed imaging of an SOT23 part that was dislodged from

the feeder. t is the instantaneous time. .......................................................................................... 12

Figure 12: Frames extracted at various times, t from high-speed imaging showing an SOT23 part

advanced in a properly working twin tape feeder. ........................................................................ 13

Figure 13: Frames extracted at various times, t from high-speed imaging showing how a

tombstone defect occurred in a faulty twin tape feeder. ............................................................... 14

Figure 14: A photo showing the tombstone defect that was observed in Figure 12. .................... 14

Figure 15: Frames extracted at various times, t from high-speed imaging showing how an SOT23

part was flipped upside down in a faulty twin tape feeder. .......................................................... 15

Figure 16: A photo showing the upside down SOT23 part that was observed in Figure 14. ....... 15

Figure 17: Frames extracted at various times, t from high-speed imaging showing how an SOT23

part that was misoriented to an angle close to 180 degrees in a faulty twin tape feeder. ............. 16

Figure 18: A photo showing the misoriented part that was observed in Figure 16. ..................... 16

Figure 19: Frames extracted at various times, t from high-speed imaging showing how an SOT23

part was completed dislodged from a faulty twin tape feeder. ..................................................... 17

HIGH-SPEED VISUALIZATION OF DEFECTS IN SMT, JANUARY 2016

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1.0 Introduction

The purpose of high-speed imaging is to provide insight into fast-changing, microscopic time

scale phenomena that elude visual understanding [1]. High-speed imaging is currently being used

in several applications including, fluid motion as in particle image velocimetry [2, 3], study of

explosions [4], car crash testing [5], visualization of welding processes [6, 7], etc. Particularly,

the Technology Access Centre (TAC) research team at Red River College (RRC) introduced

high-speed imaging into robotic welding at the College’s Centre for Aerospace Technology and

Training (CATT) in 2014 to troubleshoot different welding problems for process improvement

purposes. At the April 2015 applied research lunch and learn event, RRC invited several industry

participants, including Parker Hannifin, to benefit from the showcase of the TAC’s research on

the role of high-speed imaging in robotic welding. Although RRC’s high speed imaging system

was originally acquired to address issues in robotic welding, the TAC team seeks to extend this

capability beyond welding to non-welding applications. This project with Parker Hannifin was

developed on this effort.

During electronic assembly at Parker Hannifin, a robotic head equipped with a pneumatic pick-

up tool and operated at a very high speed is used to pick electronic components from feeders and

place over designated locations on the circuit boards. In certain instances, the parts are missing,

turned upside-down or misoriented at the destination, leading to lengthy rework or outright

scrapping of the parts. The occurrence of these defects, in many instances, requires temporary

shut-down of assembly operations in order to determine the faulty robotic station. Several circuit

boards may be affected at the same time. This problem is not limited to one type of assembly

equipment only. Members of the manufacturing team have attempted to solve this problem by

looking into production data and using a trial-by-error approach, where certain hardware, such as

feeders, are being excluded from use. However, the actual root cause of the problem was

unknown. Therefore Parker Hannifin and RRC have partnered to use high-speed visualization to

study the “pick and place” operation of the robotic heads (using RRC’s Olympus iSpeed TR

Imaging System).

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2.0 Objective

The objective of this project was to use high-speed visualization to determine the root cause of

robotic assembly defects of electronic components at Parker Hannifin Canada to support the

company’s process improvement effort. The project was aimed at providing an opportunity for

Parker Hannifin to design and implement a strategy for reducing the defects, thereby mitigating

the requirement for lengthy rework and / or the cost of scrap. Through this project, RRC’s TAC

team aimed to extend their knowledge of lighting for high-speed imaging to a non-welding

application. This should be useful for future applied research engagements with industry.

3.0 Equipment, Approach and Project Constraints

The Olympus iSpeed TR High-Sensitivity Monochrome System with a top speed of 10,000 fps

and 2.16 µs global shutter, specifically designed for R&D purposes, was used in this work. The

camera is equipped with an electronically controlled i-focus capability, Tokina AT-X M100 AF

PRO D Lens, and an i-speed control suite of software. The camera was operated via a controller

display unit at its maximum resolution of 1280×1024 and speeds ranging from 1000 to 2000 fps.

The illumination source for high-speed imaging was the commercial BE-CHB200W-D50 LED

supplied by Back To Earth Energy, Inc., Alberta, Canada. The LED light was remotely

controlled at different illumination levels and is capable of delivering up to 24,000 lumens

maximum. In order to reduce the beam angle from 50 degrees to a smaller angle, a custom beam

modifier was built by the project team. Aluminum sheets were used as reflectors in the modifier,

which was effective in constricting the beam to achieve higher intensities. Both the camera and

light were mounted on tripods equipped with suitable ball heads for angles manipulation.

The approach used to achieve the objective of this work includes, (a) the optimization of high-

speed camera and LED settings, (b) systematic acquisition of high-speed imaging data, (d) post-

processing and analysis of high-speed imaging results, and (e) a “5-WHY” approach of root

cause analysis.

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The following table contains the projects constraints identified before and during the project. The

approaches used to address the constraints are provided in the table.

Table 1: Project Constraints.

Constraint / Risk Approach

1 The illumination source. The team could

not use lasers due to safety issues. Also,

the use of backlighting was not feasible.

A 24,000 Lumen LED was used. A beam

modifier, built by the team, was used to

constrict the beam to achieve high

intensities.

2 Limit of camera’s resolution. The parts

under investigation are very small, about 3

by 2 by 0.5 mm.

The highest camera resolution was used in

each case. Speeds limited to 2000 fps max,

and were sufficient.

3 Access to viewing SMT equipment. AX5

was very difficult to access.

We gained access to about 30% of the

machine length from the side.

4 Possible interference with inspection

lighting.

There were issues but the LED was not

focused directly on the inspection light.

5 Knowledge of the specific time of the

defect opportunity. The probability of

defects capture in production was very

low.

Off-production simulated processes were

used to achieve the results.

6 Equipment availability constrains. The team negotiated with production staff.

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4.0 Results and Discussion

4.1 High-speed Imaging of Picking and Placement of Parts

Figures 1a and 1b show the picking of a part from a single lane feeder and the placing of a part

on a circuit board, respectively. These frames were extracted from the source videos and were

used as part of the baseline data for the project.

Figure 1: Frames extracted from high-speed imaging, showing (a) a part picked up from the

feeder (b) a part placed on the board.

4.2 Analysis of Existing Production Data – Variables Causing Defects

There are a large number of variables involved in the Surface Mount Technology (SMT). These

variables include the equipment, type and quantity of defects, product family / part type, feeder

type, feeder lane, feeder location, feeder slot, placement location on the board and operator.

Therefore, the existing production statistical data were analyzed in order to understand the

variables with the highest probability of causing defects. A spike in the number of defects was

observed for the months of August and September, 2015. Data were extracted for both months

and the following key observations were made:

Circuit

Board

Pick-up

Tool Part

Pick-up

Tool

Part

Feeder

a

b

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The common defect types are misorientation, misalignment, tombstone (parts sitting on

sides other than the flat wide side), missing parts and parts turned upside down.

SOT23 parts are the number one overall source of defects in the SMT. A picture of an

SOT23 part, alongside its dimensions, is shown in Figure 2. There are two leads

(electrical connectors) on one side and one on the other side.

Picking from lane 2 of twin tape feeders (TTF) accounted for 53% and 72% of defects

associated with SOT23-type of parts in the months of August and September,

respectively. Figure 3 shows the side view of a TTF.

Figure 2: An SOT23 part with a lead on one side and two leads on the other side. The lead

attaches to the solder paste on the board to make electrical contact.

Figure 3: Side view of a twin tape feeder (TTF)

(http://www.smtnet.com/mart/index.cfm?fuseaction=view_item&item_id=96418)

Parts are picked from

this location

Reel containing

parts

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These observations were used to design experiments with focus on the picking and placing of

SOT23 parts. Also, this analysis answered the first “WHY” in the 5-WHY approach. The 5-

WHY diagram, summarizing the results of the project, is presented in Figure 4. The reason for

rework is the occurrence (and spike in the number) of defects, where SOT23 parts and twin tape

feeders are mostly involved. Although the problem appeared somewhat simplified by narrowing

down the study to SOT23 part and twin tape feeders, the real cause of the problem remained

unknown.

Figure 4: The problem, 5-WHYs and recommended actions.

WHY?

SOT23 parts are found missing, misoriented or

turned up-side down at the time of final inspection.

A spike in the number of defects was noticed at final

inspection. The boards require rework/scrapping.

The parts were either successfully placed in the

wrong orientations or are missing because they fell

away after being placed.

The parts were placed with defects because they

were picked with defects. The feeder presented the

part to the pick head in the wrong orientation.

A fault in a problematic twin tape feeder caused the

part to jerk and become unstable during forward

advancement of the part into the pick-up position.

The part flips inside the feeder’s pocket, becoming

misoriented or turned up-side down. Unfortunately

the automatic laser and infrared light could not

detect upside down parts or certain misorientations.

Recommended Actions:

1. Conduct an offline feeder testing program to isolate faulty feeders. Investigate and fix, or scrap feeders.

2. Put feeders in a queue for use rather than random usage to ensure that all feeders wear and / or age concurrently.

This is useful if the fault was caused by wear and tear.

3. Standardize the lifetime of feeders and scrap those that may be old enough to cause problems

The Problem: Sporadic defects are observed during final inspection of electronic circuit boards. The root cause of

these defects calls for concern because such defects are expected to be captured by automatic inspection techniques

such as laser measurement and infrared inspection built into the production process.

What is wrong with the circuit boards?

WHY?

WHY?

WHY?

WHY?

(Check

actions

below)

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4.3 Observation of the Assembly Defects

The occurrence of the defects was captured by high-speed imaging for the first time. Twin tape

feeders that were randomly isolated from production during a spike in the number of defects

were used in the experiments. Over 500 parts were picked from single lane and twin tape feeders

and were placed on the board. In the first set of experiments, the AX5 machine was set up to

make 3 attempts at pick-up in the case of a missing part or a defect. The correct orientation of the

part with respect to the camera is such that the single lead points toward the Camera and is

located at the lower part of the SOT23 part as shown in Figure 5. During the pick and place

processes, a tombstone defect was captured for the first time (Figure 6a). The part was picked on

the side having one lead. The two leads on the other side are seen facing downward. Since the

machine was set up to make 3 attempts, this tombstone part was rejected and a second attempt

was made at picking. The second attempt was also another tombstone (Figure 6b). The part was

picked on the side with two leads. The single lead on the other side was facing downward with

the part misoriented 90 degrees. The part was rejected again and a third attempt was made. The

third attempt was also a defect. As seen in the picture (Figure 6c), the leads are on the right and

the left sides, suggesting that the part was misoriented 90 degrees. The machine eventually

displayed an error message and stopped abruptly. These observations confirm that the parts were

picked with the defects. However, none of the defects made it onto the board as the parts were all

rejected.

Figure 5: A frame extracted from high-speed imaging showing a picked up SOT23 part with the

single leading pointing outward and located at the lower part of the chip

Part with lead

pointing outward

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Figure 6: Frames extracted from high-speed imaging showing (a) (b) tombstone defects (c) 90

degrees misorientation of SOT23 parts

Also, in the first set of experiments, a part was observed to have been picked upside down

(Figure 7). The parts picked before this defect were picked correctly. At this point, it could not

be confirmed whether the part picked up side down was placed on the board or not. If it was not

placed (rejected), there would have been no error message displayed by the machine because the

system would have made another attempt at picking. The next part after this defect was actually

observed to have been picked correctly. Therefore the machine set up was changed in the second

set of experiments such that there will be an error message and the machine will stop after every

a

c

b

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rejection without a second chance / attempt at picking. This allowed the team to investigate the

possibility of a part being successfully placed on the board with a defect. Figure 8 shows another

tombstone type of defect that was observed after changing the machine settings. The part was

rejected, an error message was displayed and the machine stopped, as expected. The laser

inspection did not allow the part to pass on to placement on the board.

Several picking processes were studied with the system set to stop when a rejection occurred.

Results from several high-speed imaging data showed that certain misoriented and upside down

parts were picked and successfully placed on the board without any machine error message or

rejection. Figure 9a was extracted from the video of a part with 180 degrees misorientation. The

side with two leads was seen facing the camera rather than the side with one lead. This part was

Figure 7: A frame extracted from high-speed imaging showing an SOT23 part that was picked

upside down.

Figure 8: A frame extracted from high-speed imaging showing a tombstone defect.

Upside down part with

lead pointing upward

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picked in the wrong orientation and was successfully placed on the board. Similarly, a part that

was picked upside down was observed to have been placed on the board without rejection during

inspection (Figure 9b). However, a part with 90 degrees misorientation was rejected (Figure 9c).

It was also observed that parts with misorientations other than 180 degrees were rejected. These

results showed that the laser inspection process could not detect the position of the leads. It does

not matter whether the part was picked upside down or misoriented 180 degrees, the laser

inspection process still assumed the correct part dimensions and orientation. Therefore the parts

are placed with the defects. Also, if the parts are misoriented 180 degrees, the parts would have

been rotated from the expected position on the board, with the two leads on the side of the board

with the solder paste for one lead and the one lead on the side of the board with solder paste for

two leads. In this case, there will be no contact between the lead and the solder paste. The parts

would then be susceptible to falling off the board. A similar argument is valid in the case of parts

turned upside down. This could have been responsible for the missing parts on the board. Also

importantly, parts picked from lane 2 of the twin tape feeders accounted for all the defects on the

board (Figure 10).

Although the observations reported so far answer why the SOT23 type parts are found missing,

misoriented or turned up-side down at the time of final inspection (the second and third WHYs –

Figure 4), the reason why the parts are picked with the defects remained obscure. The results

showed that the feeder presented the part to the pick head in the wrong orientation. It is unlikely

that the parts supplied (parts in the reel) to the feeder had defects. The probability of the parts

being loaded in the feeder with defects is almost non-existent. Answering why the parts are

presented wrongly by the feeder (the fourth WHY), despite the fact that the part reels are defect-

free, is key to achieving the aims of this work. Fortunately, results from the second set of

experiments provided a clue to finding the root cause of the problems. Figure 11 shows extracted

data from high-speed imaging for a part that was observed to have been displaced from the reel

and flying out of the feeder. This implies that there was some agitation (or jerking) of the part

when it was being advanced forward for pick-up in the feeder, such that the agitation force was

enough to dislodge the part from the feeder. It is intuitive to conclude that a force that could

cause such dislodgement would be capable of flipping that part to different orientations during

advancement and exposure for pick-up. Therefore, an experiment was set up to visualize the

advancement of the reel in the feeder when the part is being presented for pick-up. In order to

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Figure 9: Frames extracted from high-speed imaging showing (a) an SOT23 part with 180

degrees misorientation (b) a part turned upside down (c) a part with 90 degrees misorientation.

The parts in (a) and (b) were successfully placed on the board.

Figure 10: Tracked locations of defects on the boards. Parts picked from lane 2 of TTF.

a

c

b

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Figure 11: Frames extracted from high-speed imaging of an SOT23 part that was dislodged

from the feeder. t is the instantaneous time.

view the inner part of the feeder, an offline automatic feeder testing equipment was used. The

results are discussed as follows.

4.4 The Root Cause of the Assembly Defects

In this part of the work, an offline (stand-alone) automatic feeder testing equipment was used to

test the operation of both properly working and faulty twin tape feeders in order to understand

the motion of the parts when they are advanced forward for pick up in the feeder. A tweezer was

suddenly pointed into and retracted from the opening containing the part. New parts were

advanced when the tweezer tip was sensed by the feeder, similar to the actual production process

where the pick head triggers the advancement. High-speed imaging was carried out for the

advancement of only one part at a time, where each part is manually removed from the feeder

after the experiment.

t = t1 t = t1 + 26 ms

t = t1 + 39 ms t = t1 + 59 ms

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Figure 12 shows frames extracted at various times from high-speed imaging of the operation of a

properly working twin tape feeder. The part advanced very fast, but gently, without any

noticeable agitation or jerking. However, part advancement in the faulty twin tape feeder

exhibited a different behavior. All the defects that were noticed in the previous sets of

experiments were observed during this testing. Figure 13 shows frames extracted at various times

from high-speed imaging of the operation of a faulty twin tape feeder, showing that the part was

indeed jerked, flipped and misoriented to the tombstone position. A picture of the part, as flipped

in the feeder, was taken from the vertical direction (above the part) and presented in Figure 14,

showing the misoriented part. In a similar manner, a part was completely turned upside down

(Figures 15 and 16), while another part was misoriented at an angle close to 180 degrees (Figures

17 and 18). In another experiment (Figure 19), a part was observed to have been completely

dislodged from the feeder.

Figure 12: Frames extracted at various times, t from high-speed imaging showing an SOT23

part advanced in a properly working twin tape feeder.

t = t1 t = t1 + 16.5 ms

t = t1 + 33 ms t = t1 + 50 ms

Tweezer Part Advancing

Part Advancing Part Fully

Advanced

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Figure 13: Frames extracted at various times, t from high-speed imaging showing how a

tombstone defect occurred in a faulty twin tape feeder.

Figure 14: A photo showing the tombstone defect that was observed in Figure 12.

Part Advancing

Part Flipping

Part Flipping Tombstone

t = t1 t = t1 + 21.5 ms

t = t1 + 43 ms t = t1 + 65 ms

Tombstone

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Figure 15: Frames extracted at various times, t from high-speed imaging showing how an

SOT23 part was flipped upside down in a faulty twin tape feeder.

Figure 16: A photo showing the upside down SOT23 part that was observed in Figure 14.

t = t1 t = t1 + 40 ms

t = t1 + 80 ms t = t1 + 120 ms

Part AdvancingPart Flipping

Part Flipping Part Upside Down

Part Upside

Down

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Figure 17: Frames extracted at various times, t from high-speed imaging showing how an

SOT23 part that was misoriented to an angle close to 180 degrees in a faulty twin tape feeder.

Figure 18: A photo showing the misoriented part that was observed in Figure 16.

t = t1 t = t1 + 25 ms

t = t1 + 50 ms t = t1 + 75 ms

Part AdvancingPart Flipping

Part Flipping

About 180 Degrees

Misoriented Part

Misoriented Close

to 180 Degrees

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Figure 19: Frames extracted at various times, t from high-speed imaging showing how an

SOT23 part was completed dislodged from a faulty twin tape feeder.

5.0 Conclusion

The root cause of the assembly defects in Parker Hannifin’s Surface Mount Technology (SMT)

is the flipping and misorientation of parts in the pockets of unnoticeably faulty twin-tape feeders.

The parts are eventually picked with defects and, unfortunately, some of defects make it to the

board because the automated laser inspection is unable to discern certain degrees and/or types of

misorientation.

t = t1 t = t1 + 15 ms

t = t1 + 30 ms t = t1 + 45 ms

Part Advancing Part Flipping

Part Out of

Feeder Part Flying Freely

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6.0 Recommendations

The following actions are recommended to address the root cause of the defects:

1. Conduct an offline feeder testing program to isolate feeders that may be problematic.

Parker Hannifin can investigate the fault and fix the problem, or consider the replacement

of the affected feeders.

2. Put feeders in a queue for use rather than random usage to ensure that all feeders wear

and/or age concurrently. This will be useful if the fault relates to wear and tear of feeders.

3. Standardize the lifetime of feeders and scrap those that may be old enough to cause

problems.

7.0 References

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3. Goji T. Etoh, K. Takehara, Y. Takano, 2001. High-speed image capturing for PIV, DLR-

Mitteilung, No 3, pp.845-856, September 17, 2001 - September 19, 2001, Deutschen

Forschungsanstalt fur Luft-und Raumfahrt.

4. Marcia A. Cooper, Phillip L. Reu, Timothy J. Miller, 2010. Observations in explosive

systems with high-speed digital image correlation, 14th International Detonation

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5. Robert Godding, 1990. System for the photogrammetric evaluation of digitized high-

speed images in the automobile industry, Proceedings of SPIE - The International Society

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6. M.J. Zhang, G.Y. Chen, Y. Zhou, S.C. Li, H. Deng, 2013. Observation of spatter

formation mechanisms in high-power fiber laser welding of thick plate, Applied Surface

Science Vol. 280, pp.868– 875.

7. Y. Ogawa, 2011. High speed imaging technique Part 1 – high speed imaging of arc

welding phenomena, Science and Technology of Welding and Joining, Vol. 16, pp.33-43