Tight Control Limits - Tape & Reel Process, the process of packaging ICs

A

ajm723

#1
Hi to all.

Need expert advice on how to go about implementing SPC effectively for Tape & Reel Process, the process of packaging ICs by placing units on carrier tape then sealed by cover tape. Currently we monitor process by doing peel-back force testing (PBFT) on a 15cm sealed sample once every 12hrs shift. Sample is loaded on a PBF Tester w/c measures the sealing force every mm as the cover is pulled from the carrier tape. Therefore for a 15cm sample, we obtain 150 individual measurements. We tried plotting data in Xbar-S chart with s/s=150 using SPC software. The control limits generated were so tight resulting to almost all datapoints out-of-control on both xbar & s charts. It's impossible to implement the controls else we always end up w/ machine downtime. Something must be wrong. Kindly enlighten us. Thx.
 
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D

Darius

#2
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

First, wellcome to the cove ajm723.

once every 12hrs shift... we obtain 150 individual measurements ... The control limits generated were so tight resulting to almost all datapoints out-of-control on both xbar & s charts.
IMHO Tight control limits almost allways means wrong charting, try "individuals with moving range chart" (I-MR), this may be your ticket, this kind of chart is recommended for data with too much autocorrelation (altho autocorrelation factor is recommended for such case) and continuos processes, if that don't solve your problem add autocorrelation factor to individuals and moving range chart.:cool:
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#3
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

Are you able to share the data so that we may take a look ourselves and offer some ideas?
 
A

ajm723

#4
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

Dear all,

Really appreciate your support & concern.

We do thought of X-Mr but then realized we could be missing the w/in subgroup/sample variations. We think that this is critical for this type of process (tape sealing), that every segment (or mm) of the tape should be checked for sealing strength.

Pls. see attached file for one of our machines, contains the raw data, plus the Xbar-S chart generated (1) using tables, then (2) using std. dev. for calculating control limits. You'll notice the tight control limits generated using either method for both xbar and s charts. I also included the histogram.

Thx again.
 

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Tim Folkerts

Super Moderator
#5
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

I haven't had time to look closely at the data, but I see two things.

  1. The limits on the S-bar chart are quite different for the two methods. In particular, the mean values are not the same. I'm not sure what method you are using for the two graphs, but I would expect the mean to be the same no matter how it is done.
  2. The process probably really is out of control. In other words, some of the variation is not simply due to random luck, but some samples really do have a different mean and/or standard deviation than the others.
If no one else lools at the data soon, I may be able to look a little more over the weekend....


Tim F
 

Miner

Forum Moderator
Staff member
Admin
#6
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

When in doubt, always plot the data in time sequence. See the attached file.

Your data raises many issues:
  1. Rational subgrouping; your subgroups consistently include trends and outliers. Your subgroups should be small enough that no trends or outliers can occur on a regular basis.
  2. Reducing the subgroup size to 2 or 3 provides more realistic limits but shows an out of control process in both charts
  3. This makes the short term trends very obvious, as well as other modes at ~ 35 and 45.
  4. Have you performed an MSA on this measurement device? If not, perform an MSA to rule out the effect of the gage.
 

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Statistical Steven

Statistician
Staff member
Super Moderator
#7
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

A subgroup size of 150 units is too large. Any control with that size subgroup will have very narrow limits. This is because the A2 value is a function of sample size, so basically you are expecting every lot to have almost the same mean. You can try to take the mean of every 10 readings doing a control chart on the 15 means instead of the 150 individuals. This will give a little wider limit while still being rational and capturing the within and between variability.
 
D

Darius

#8
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

The trends on the data are clear, too much autocorrelation, cyclical process (2 of them, one a 150 points, the other each 16 points), the better aproach may be to get rid of trend (if part of the process- then in a mathematical way), then to use wichever tool is wanted. I-MR could do nice control limits, but as Xbar-S, without the trend consideration no usefull info could be achived.:nopity:
 

reynald

Quite Involved in Discussions
#9
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

ajm,
As already pointed out, aside from being autocorrelated,your sub -grouping is very large.
Using the data in your x-bar-s_chart_using_Stdev worksheet, notice the ff:

Subgrouping 150
3-sigma value in your chart 1.9344
sigma value in your chart 0.6448
estimated sigma by your chart 7.897154931
Actual sigma of your data 7.8972


Note that sigma value used by most softwares in Control charts is calculated by:

Stdev/sqrt of sub-group size

using a subgroup size of 150 gives you 0.6448 as sigma and CLimits 56.2088 and 52.3400. Very small approximations. As already suggested you can use I-MR. Or plot series in using as max and min scales the natural 3-sigma limits as shown in your histogram. With any of these the autocorrelation should be evident.
 

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A

ajm723

#10
Re: Control Limits So Tight - Tape & Reel Process, the process of packaging ICs

Dear all,

In trying to control-chart the tape & reel process, we are actually attempting to hit 2 birds w/ 1 stone -- (1) capture the flaws on every tape we sampled, & (2) the usual monitoring of process variation over time. In fact, the semicon industry std. for peel back force testing is supposed to be 10 measurements/mm w/c translates to 1500 measurements for a 15cm sealed tape sample. We tried to compromise (actually, sacrificing better chances of capturing flaw) by reducing to just 1/mm hence we came up w/ s/s=150.
Are we taking the right strategy?

B.t.w, just want to note that process specs. is 20 to 120 g. Low seal strength may result to delamination, too high.. you end up w/ tearing. 20 to 120 is too much that we actually want to limit to w/in 40 to 80. Ironically, the xbar-s charts are giving us around 52 to 57.

Hope this gives good lead to your brilliant minds and help us come up w/ the appropriate solution/strategy to our problem.

Best rgrds.
 
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