Guidelines for designing SMD part PCB footprint

[matrixofdynamism]

I am have created a footprint for an Altera CPLD in Eagle PCB. It has EQFP64 pin package with 0.4mm pitch with nominal pin width being 0.18mm. Details are found here https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/ds/pkgds.pdf#page=57

I know that the SMD pad must be bigger than the package pin. This way it shall be able to fit the maximum possible pin dimensions owing to manufacturing tolerance.

1) How big should the SMD pad dimensions be compared with the pin dimensions, is 20% bigger than nominal in both X and Y sufficient?
2) How much should the solder resistor area be bigger than the SMD pad size?
3) Are rounded pads preffered to square pads? Why?
4) Must the center of the SMD pad be where the package pin extrusion ends i.e at its tip?

5) In my case I have 0.1mm of distance between adjacent solder resist areas. Is this manufacturable? i.e can a 0.1mm of solder resist be printed?

 

[kabaleevisu]

Hi,
1.For PAD size use 1mmx0.2mm
2.solder resist should be 0.1524 mm (6 mils)
3.i could not understand your third point
4.yes,you should create center PAD
minimum solder resist to solder resist 3mil need to maintain

 

[senilicus]

Download PCB Librarie's "Library Expert Lite. It is free, can handle components with max 200 pins and generates landpatterns according to IPC-7351(B) or (C).
It can output in EAGLE format.

https://www.pcblibraries.com

 

[matrixofdynamism]

@kamableevisu The dimensions for the pin are 0.18 nominal with 0.23 maximum. Shouldn't one make the pad bigger than 0.23 which is the worst case dimensions? This is where the problem comes in. With the pad being so big, it is harder to get solder resist inbetween the pads.

By rounded pads I meant pads that are rounded from the end vs pads that have square ends.


@senilicus I shall do more research into this PCB library, came across it for the first time today. Looks very interesting. Makes sense that someone saw this market gap and created this tool for it.

 

[marce]

The solder resist and paste pad should be the same size as the pad for the pin.......
Work in mm not thousands of an inch the component is hard metric.....

 

[Mattylad]

As above, download the Library Expert Lite from PCBlibraries.com, enter your component details and you can build a part (footprint) and export it to Eagle.

I did not realize there is a 200 pin limit? I'll investigate that.

When I did it the pads were 1.84mm x 0.27mm and I used oblong shape because (AIUI) lead free solder does not wick into the corners so well any more so there is no point in having corners that may not be coated the same.

No oversize for solder resist (leave 1:1 and let manufacturer adjust to suit their processes).

 

[senilicus]

As above, download the Library Expert Lite from PCBlibraries.com, enter your component details and you can build a part (footprint) and export it to Eagle.

I did not realize there is a 200 pin limit? I'll investigate that.

.

You can enter the numbers for a +200 pin component, but you can not write it to a output file.

 

[matrixofdynamism]

OK, I downloaded the pcblibrary. It is very neat software I must say. I entered the dimensions for the part and then it generated an Eagle script file. I ran this file and it created the footprint. I noticed that while the maximum pin width is given as 0.23, the pcblibrary created an SMD pad of size 0.24.

Anyway, the fundamental issue still remains. Even in this case, the solder resist rectangles that were generated for the SMD pads were overlapping with the solder resist of the adjacent pads. I guess the last resort is to just contact a/the PCB manufacturer and ask them.

 

[marce]

Do the solder resist pads 1:1

 

[Mattylad]

FWIW there are plans to open up the Library Expert tool so that the List version does not have these restrictions, in the near future. That should be good news to many.