Integrated Circuits Cycle Time at Test
Operation Estimation
for Production Planning
using Computer Simulation
By
Jitrayut Junnapart
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction(1): Assembly Process
• General Process of IC manufacturing:
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction(2): Electrical Test Process
• All products must be 100% tested before
deliver to customers
• Electrical Test process is shown below:
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction(3): Electrical Test Time
• Test Time depends on:
– Test Rate at main flow (in Units Per Hour)
– First Pass Yield (0 to 1 for 0% to 100%)
– Test Rate at rescreen flow (in UPH)
To P.15
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction(4): An Example
• Suppose one production lot of:
–
–
–
–
3,000 units
Main Flow Test Rate
= 1,000 UPH (Units Per Hour)
Rescreen Flow Test Rate = 300 UPH
First Pass Yield
= 90%
• Calculate Cycle Time:
Main Flow Test Time
Rescreen Flow Test Time
Total Test Time
= 3,000 / 1,000
= 3 hours
= [ 3,000 * (1 - 0.9) ] / 300
= 1 hour
= 3 hours + 1 hour
= 4 hours
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Introduction(5): Why interesting?
• Electrical test time is very important
because it dominates overall IC
manufacturing cycle time
• There are methods to calculate cycle
time:
– Use average values (of UPHs, FPY)
– Apply simulation method
– Others
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(1)
• This project applies Monte Carlo
Simulation based on historical data
of:
– Main flow test rate
– Rescreen flow test rate
– First pass yield
to simulate for total cycle time of
one production lot
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(2)
• Monte Carlo simulation uses random
number to “pick” one value of data
as a representative for further
calculation
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(3): Example
• Suppose we want to simulate main flow
test time
• And partial of historical data of main
flow test time (in UPH) looks like:
100
130
150
160
140
120
160
110
120
150
100
160
160
130
110
140
110
150
170
150
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(4): Step 1
• Rearrange the numbers into frequency
and probability table:
UPH
Frequency
Probability
100
1
0.0122
110
3
0.0366
120
3
0.0366
130
5
0.0610
140
7
0.0854
150
11
0.1341
160
14
0.1707
170
20
0.2439
180
18
0.2195
Total
=
82
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(5): Step 2
• Construct cumulative probability table
from probability table:
UPH
Frequency Cum. Prob.
100
1
0.0122
110
3
0.0488
120
3
0.0854
130
5
0.1463
140
7
0.2317
150
11
0.3659
160
14
0.5366
170
20
0.7805
180
18
1.0000
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(6): Step 3
• Set up probability intervals for ranges
of UPH:
UPH
Probability Range
100
0.0000 - 0.0122
110
0.0123 - 0.0366
120
0.0367 - 0.0854
130
0.0855 - 0.1463
140
0.1464 - 0.2317
150
0.2318 - 0.3659
160
0.3660 - 0.5366
170
0.5367 - 0.7805
180
0.7806 - 1.0000
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(7): Step 4
• Generate a random number and map
onto the appropriate interval to find a
simulated value of main flow UPH
UPH
Probability Range
100
0.0000 - 0.0122
110
0.0123 - 0.0366
120
0.0367 - 0.0854
130
0.0855 - 0.1463
140
0.1464 - 0.2317
150
0.2318 - 0.3659
160
0.3660 - 0.5366
170
0.5367 - 0.7805
180
0.7806 - 1.0000
Random No.
generated is
0.5934
Simulated UPH
= 160
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(8): Step 5
• Repeat step 1-4 but for:
– First Pass Yield and
– Rescreen Flow UPH
• Use equation in slide 5 to calculate
Simulated Total Test Time
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(9)
• BUT real test rate (UPH) data is
continuous number (appendix C,p79)
• We need to group them first into
ranges
• Then count number of data that are in
each range
• And find the center value of the
group as a representative number of
the group
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Literature Review(10)
• Number of group can be chosen
• Groups shall cover every data
–See appendix D, p.86 for detail
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development (1)
• Problem Definition:
– Electrical test time is the main dominator of
overall production cycle time in back-end IC
manufacturing
– Cycle time affects shipping commitment to
customers
– Using average test time is not effective
because the numbers vary over wide range
Production control people wants a tool to
forecast electrical test time of the IC
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development (2)
• Objectives:
– To develop computer software that processes historical
test rate data (in Units Per Hour - UPH) at main flow and
rescreen flow, and first pass yield (retest rate-related)
from production and simulate for probability to finish
testing a production lot with various allocated time.
– The computer software must be separated into main
program and auxiliary data files. The main program will
work with various environments without modification.
– The developed simulation program code must work on
both Personal Computer (PC) and larger computer
running on UNIX operating system.
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development (3)
• Scope and limitations of the project:
– It is considered that main flow test rate, rescreen flow
test rate, and first pass yield being input of this project
are available beforehand. They are stored in separated
text files which, in fact, easy to develop the system
further.
– This project gives calculation output as a text file for
easy to use in the future. Example of this is putting into
Hypertext Markup Language (HTML) and post on
facility’s Intranet.
– The software in this project will calculate for success
probability to finish electrical test process based on a
certain size of production lot. User can later use linear
relationship to calculate for other size.
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development (4)
• Project Time Line:
– The project consists of 9 major tasks
– Total time taken is 7.5 months
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Development (5)
• Feasibility Study:
– Technical » Feasible
• The champion has knowledge of simulation and
programming language
– Economic » Feasible
• The project does not require special expenses
– Operational » Feasible
• The project does not interfere the normal work
practice of concerning people
– Legal » Feasible
• Software used in development is legal and all data
is legally used
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (1)
• Concerns
1) Output must be easy for user to understand
2) Test time depends on lot size, and it is not
feasible to provide all lot sizes output 1,2
3) User should have control over some
3
essential simulation parameters
4
4) Change in source code is undesirable
Need modifications
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (2)
• To solve concerns #1 and #2
To concerns
1) An allocated time is set
2) The simulated test time for a certain lot size is
calculated and compared to a allocated time. If test
time is shorter than allocated time, the result is
possible. This 'success number' is counted.
3) Repeat step 2 for many times and calculate success
chance from success times and total simulation times
and report this chance to user.
4) Increase the allocated time by delta defined
5) Repeat step 2 to 4 for certain rounds defined
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (3)
• To solve concerns #3
To concerns
Store simulation parameters in file para.txt
–
–
–
–
Number of types of IC to perform simulation
Number of divisions to group historical data
Lot size that simulation is based on
Number of repetitive simulation for one allowable
time
– Start point of allowable time
– Increment of allowable time
– Steps of increment of allowable time
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (4)
• To solve concerns #4
To concerns
All input data are stored in separated files:
1)
2)
3)
4)
Historical files of Main flow UPH (.mnf)
Historical data of Rescreen flow UPH (.rsf)
Historical data of First pass yield (.rrf)
List of IC types file (progname.lst)
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (5): Files
567.0652174
580.6871609
547.6097561
594.5454545
514.270073
503.5632184
554.0727273
554.9477352
•
Main flow UPH file
icname.mnf
415.5066079
309.3377483
415.5066079
452.0338983
392.7272727
368.125
376.8181818
385.9550562
•
•
Rescreen flow UPH file
icname.rsf
List of icname file – progname.txt
First pass yield file
icname.rrf
1
5
5
5
1000
100
1
0.5
11
fh368fa
•
70.51945563
70.98281016
59.51362908
75.28669725
82.22269534
83.70235106
86.00118133
81.10755321
•
Parameters file – para.txt
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (6)
Open file “progname.lst” and para.txt that stores
test program names to simulate and read all the
names and parameters
End
Processed for every test program?
Yes
No
1.Open file that stores UPH Main flow, and
read all main flow UPH for next program
2. Count number of data
3. Divide into divisions
4. Calculate for accumulated probability
5. Repeat step 1-4 but for rescreen rate and
UPH of rescreen flow
1.Simulate total time used to test the production lot
2.Count for number of success that the time used
is less than allocated time
3. Repeat 1 and 2 and calculate success probability
4.Record result in output file
5.Increase allocated time
Simulation Method
Allocated time > limit?
No
Yes
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Implementation (7)
• Programming
– Coding is done in ANSI C language
– Length of the program ~900 lines
– Special statements and commands are
avoided for the program can run both
on PC and UNIX based computer
system
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result (1)
• PC based result:
1) Screen output
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result (2)
• PC based result:
2) File output (result.txt)
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result (3)
Compiled
program
• Unix
Based
result
Control parameters input
Printed out for monitoring
Screen
output
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result (4)
• Unix
Based
result
Output file
Generated
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Simulation Result (5)
• Unix
Based
result
bkkeis{pemg}34: vi result.txt
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Evaluation
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Evaluation (1)
• Verification of Objectives:
– Output is easy for non-engineering people to
understand  Yes
– Control parameters used in simulation can be
input to program through external file  Yes
– Various simulations can be performed without
any changes to main program  Yes
– The source program can be compiled to work on
both PC and UNIX based computer  Yes
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Project Evaluation (2)
• Verification of Result:
– The simulated result is verified back to
historical data
Times (Hrs)
2.00
2.50
3.00
Success
22
56
64
Total lots
66
66
66
% Success (real data) % Success (simulated)
33.33
30
84.85
81
96.97
92
– The result is 3-5% deviation
– This is very good for simulation
(probabilistic work)
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Conclusions
• Simulation technique can generate result that
gives adequate accuracy (3-5% error in this
work). The result can be transform in the way
that it is easy to understand by end user.
• The planner (user) can assign allowable time to
test production lot based on his/her confidence
level that the lot will finish in time
• In this particular work, essential parameters in
simulation can be changed easily in control files
without modification to the main program
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart
Random Number Generation
• Confirm the even distribution by JMP
1,000 numbers
10,000 numbers
Cycle Time at IC Electrical Test Operation Simulation by Jitrayut Junnapart