What the resaoning behind smaller heels for SOP fine pitch parts?

[coreya]

Professor Tom,

Great web site and I love all the land patterns.

My contract manufacturer is balking at the heel dimension I get when using the landpattern calculator for SOP50X600-40M, JEDEC MO-154C-BB. The calculator always gives .2mm heels for parts <.625m pitch. They would like something closer to .5mm. Based on my reading of other threads the new IPC-SM-782 will reflect this smaller heel change. What is the science behind this .2mm heel dimension? Is there any reading material on the subject?

Are there any indexes that relate JEDEC outlines to land patterns on this website?

Thanks

Corey

[Tom]

Corey,

You must be using the "LEAST" formulas to get a 0.2mm Heel. The heel dimension is actually much larger than what the numbers reveal. First, the component manufacturers try to build their parts using the NOMINAL data. When calculating the land pattern, we use the LEAD SPAN (lead tip to tip) Nominal but the TERMINAL Length Maximum. So when you get nominally built components, there is actually more heel then what's originally calculated.

Here is the Heel formula for the three tiers for SO Gullwing parts:

Least = 0.2mm
Nominal = 0.35mm
Maximum = 0.5mm

Summary: When using the Least Library, you only get a 0.2mm Heel when the components are delivered in the Maximum Material Condition Tolerance.

We are currently working on a JEDEC to PCB Standards cross-reference document.

[coreya]

Professor Tom,

This is a great forum with quick answers. I'll tell all my friends.

I was using the landpattern calculator set to maximum since the handbook only contains nominal landpatterns. Changing the tier didn't seem to have an effect on the heel dimension. Only changing the pitch had an effect. I have attached the spreadsheet that I used. Let me know what I am doing wrong.

Thanks

Corey

[Tom]

Corey,

First, always make sure you are using the latest land pattern calculator.
http://www.pcbstandards.com/downloads/Metric%20Environment/Calculators/Land%20Pattern/

When I use the calculator for SOIC Gullwing parts and I toggle the Use Environment from L to N & M, it changes the value in the Heel value to:
Least = 0.2mm
Nominal = 0.35mm
Maximum = 0.5mm

The Toe value to:
Least = 0.2mm
Nominal = 0.5mm
Maximum = 0.8mm

The Courtyard Excess value to:
Least = 0.05mm
Nominal = 0.25mm
Maximum = 0.5mm

It also changes the Y Pad Size and the C Pad Center dimensions.

The basic fundamental concept for calculating the Heel and Toe is to have the Pad Length accomodate the various tolerances between different manufacturers. Most parts have a second source. The EE engineer could list a specific component manufacturer on the Bill of Materials and the assembly house will always use the alternate if that's what they have in stock.

[coreya]

Professor Tom,

I downloaded the calculator on 9/5. It works just like you said for pitch > .625. Try changing the pitch to .5mm and see what you get. If you look at the formula for E15(HEEL)=if(C21(PITCH)>0.625,R19,S19). It appears that S19=0.20 for all the use environments. My question is why?

Thanks

Corey

[Tom]

Corey,

You are absolutely correct. And the calculator is correct also. For any pitch SOIC or QFP less than 0.625mm (which only includes the 0.5mm and 0.4mm pitch part families) the heel is 0.2mm. This is data that IPC gave us two years ago. I can understand why. The pitch is so fine and the Heel is under the lead which makes the solder weld difficult to see. If you made the Heel size larger, you could experience Solder Bridging and not be able to visually see it.

The 0.2mm Heel is only when the Lead Terminal Length comes in a Maximum Material Condition and they normally come in the Nominal Material Condition. When you install a Nominal Lead, the Heel is actually 0.4mm - 0.5mm, but we compensate for the maximum condition just in case.

We're releasing an updated calculator today. It only affects the Molded Chip Family.