Can you explain how the tool calculates the footprint pad size for BGA's?

 

Looking through IPC-7351B there are many explanations that seem to point to different sizes than the tool creates.

 

Also, most manufactures suggest the footprint pad size should match the interposer pad size and I have yet to see the calculators create an equal pad size.

 

Roy.

 

 

IPC uses different reduction values for various ball sizes and the Maximum Material Condition of the Land Variation. 

 

I have an issue with that chart from IPC - it doesn't take into account devices with less than 1.0mm pitch which (according to IPC) requires 10% reduction.

14.2.4  Attachment Site Planning The attachment site or land pattern geometry recommended for BGA devices is round with the diameter adjusted to meet contact pitch and size variation. The diameter of the land should be no larger than the diameter of the land at the package interface and is typically 20% smaller than the normal diameter specified for the ball contact for pitches greater than 1.0 mm and 10% smaller for pitches less than 1.0 mm. Refer to the manufacturer specification before finalizing land pattern array and geometry. 

Here are several things about BGA's that every PCB designer needs to know. 

1. Once the pin pitch is 0.50 or lower, the normal dog-bone via fanout cannot be used. 

2. The BGA Ball should collapse around the pad. 

Library Expert seems to standardize it for the Non Collapsing BGA at 0.06mm for 0.20 to 0.17 nominal ball diameter.

My question is what would be the Max - Min used for nom. balls that are not in the table for collapsing BGA.

Example a 0.26mm nominal ball diameter or a 0.33mm nominal ball diameter.

Would it be OK to set the Max- Min to a single value like 0.06mm for collapsing BGA balls of 0.30mm to 0.15mm nominal ball diameter?

Non-collapsing BGA's have larger pads to accommodate via-in-pad and establish an adequate annular ring. The BGA ball does not collapse around the edge of the pad like a collapsing ball does. 

Rather, the solder mask is usually 1:1 scale of the pad size or even a solder mask defined pad to allow solder over the pad edge. This technique helps secure the pad to the prepreg. 

When the pad size gets too small, the assembly might be compromised in drop tests where the solder joint does not break but the pad to prepreg becomes detached, creating an open connection and then failure. 

Make your min/max land variation decision with this information. 

Is the Nominal Land Diameter and Reduced Ball diameter from the above tables the same?

What will be the actual Land Diameter from the table to be used in our pad stack? Will it be the Nominal Land Diameter/Reduced Ball Diameter or the Finished Land Diameter?

I'd really appreciate your response.

Thank you very much in advanced.

 

 

Here is the BGA Pad Size Reduction table for Non-Collapsing Balls: