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November 2001

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Subject:
From:
"<Peter George Duncan>" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 28 Nov 2001 12:25:36 +0800
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                    Yu-Hung Shiau
                    <yshiau@ATPUS        To:     [log in to unmask]
                    A.COM>               cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                    Sent by:             Aero/ST Group)
                    TechNet              Subject:     [TN] IPC SM-782 on-line calculator
                    <[log in to unmask]
                    ORG>


                    11/27/01
                    06:53 AM
                    Please
                    respond to
                    "TechNet
                    E-Mail
                    Forum."







Land Pattern calculation is still an artform, not a science and depends on
a number of parameters such as board material, component package (often
manufacturer-specific), assembly process to be used (Wave or CR, etc), P&P
machines being used to populate the boards and your own design rules.

I raised this issue on TechNet a few weeks ago, especially since I was
getting different answers for land pattern sizes depending on which method
I used ( component data sheets, SM-782 or calculators). IPC stated that
they are working to address these issues. However, to answer your specific
questions, see comments below, PLUS the 4 main factors to consider are the
board manufacturing tolerence, the component placement tolerence and the
tolerence of the component itself, as well as the component's actual
lead:board contact area and component dimensions.

If you have any experience of determining land patterns that work, use that
background to judge the "reasonableness" of the calculated results. For
example, the larger allowance between heel and toe has to go to the end
where max stress is going to be experienced. Usually this is the end where
the lead goes on to join the component body, if you follow.

Hi,
Does anyone know how to use the on-line SM-782 calculator? There are few
fields I don't know what numbers should I plug in.

1-1. Man. Allowance- P Place Accuracy **FIND OUT FROM YOUR ASSEMBLY PEOPLE
WHAT THE PLACEMENT ACCURACY IS OF THE PICK AND PLACE MACHINES THEY USE AND
PLUG THAT FIGURE IN.**
1-2. Man. Allowance- J/T Toe Min. ** FROM YOU FAB HOUSE, OBTAIN THE PAD
POSITIONAL TOLERENCE THEY ARE WORKING TO WHEN FABRICATING THE BOARDS AND
PLUG THAT FIGURE IN.**

2-1. Solder Joint Design Goal- J/T Toe Min **IN THE ABSENCE OF COMPONENT
DATA SHEET INFORMATION OR SPECIFIC DATA FOR THE PACKAGE FROM SM-782, MIN.
TOE ALLOWANCE CAN BE TAKEN FROM TABLE 3.4 (NEAR THE FRONT) OF SM-782. I
DON'T BELIEVE IT'S ACCURATE FOR ALL PURPOSES, BUT IT'S THE BEST AVAILABLE
IN THE ABSENCE OF ANY OTHER INFO.**
2-1. Solder Joint Design Goal- J/H Heel Min ** SAME AS FOR TOE-TAKE FROM
THE SPEC TABLE**
2-1. Solder Joint Design Goal- J/S Side Min ** SIDE ALLOWANCE DEPENDS ON
THE COMPONENT PITCH AND LAND SPACING REQUIREMENTS OF THE DESIGN. FIND A
COMPROMISE BETWEEN YOUR DESIGN RULES AND THE REQUIREMENTS OF THE COMPONENT.
OFTEN THE SIDE ALLOWANCE IS ZERO BECAUSE THERE ISN'T ROOM TO HAVE THE PAD
ANY WIDER THAN THE COMPONENT LEAD, BUT ARE ALSO NOT MUCH LARGER IN MANY
CASES THAN ABOUT 0.02MM. I THINK A SIDE ALLOWANCE IS ALSO GIVEN IN THE SPEC
TABLE **

**IN SUMMARY, YOU HAVE YOU USE YOUR OWN BEST KNOWLEDGE/JUDGEMENT TO OBTAIN
THE FINAL PATTERN. DON'T MAKE THEM TOO LONG AT EITHER END, OR TOO SHORT
EITHER, BUT DETERMINING WHAT IS TOO LONG AND WHAT IS TOO SHORT COMES DOWN
TO EXPERIENCE, TRIAL AND ERROR.**

I have lost my hard copy of my IPC SM-782 standard for a while. **I DON'T
HAVE A COPY OF THE SPEC TO HAND EITHER, OR I WOULD BE ABLE TO REPRODUCE THE
TABLE FOR YOU - IT'S NOT VERY LARGE.**

By the way, which category I should go for the resistor array (resistor
network) while I use the SM-782 calculator?  **SORRY, CAN'T HELP YOU HERE**

Thank you all for any feedback.
YH Shiau

Best of luck

Peter Duncan

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