LMC Courtyards too small?

[John Wagstaff]

I am running into a problem and would like to get opinions from Tom and others on this.

I am using 0402 chip components in many of my designs (very small board sizes). I typically place the parts on a .05mm grid and in the design rules under "body to body" I use .05. This forces me to keep a .05mm space between courtyards. This approach has worked well in the past.

The question I have is, does anyone use zero clearance between courtyards? See attached file for an image of two 0402 resistors in this configuration.

If placing these components courtyard to courtyard... then the space between lands is .1mm. Some board houses can handle this, some can not. Even if the board house can fab the board, there is a bigger problem... assembly. The distance between the actual components is also .1mm. (based on IPC definition of a nominal 0402 resistor) Now the problem is that typical pick and place equipment has a tolerance of +/- .1mm. So there could be a scenario where R1 is placed offset in Y axis (toward R2) by .1mm and R2 is offset in Y axis (toward R1) - end result is interferance of the parts.

If the pick and place equipment had better accuracy then this wouldn't be an issue. I've been looking at Fuji equipment and contacting different assy shops about this. The assy shops said that they would like 10mil clearance between comps. Is 10mil realistic? Not sure... what do you think?

[Tom]

John,

The courtyards for the LMC library are perfect. Your application of the use of them is incorrect. Courtyards are never supposed to touch each other. You are supposed to set a "Body to Body" clearance in Setup/Design Rules/Default/Clearances of at least 0.05mm minimum. Place parts on a 1mm, 0.5mm or 0.1mm grid. You adjust the Body to Body clearance according to your assembly shops tolerance rules.

Always leave a gap between the courtyards and always run Tools/Verify Design/Clearance/Setup/Body to Body/OK/OK. You should have NO ERRORS FOUND when running a "Body to Body" DRC Clearance check.

All this information is defined in the document "Creating a CAD Library.doc".

[ltrakal]

As tom says, I personally use 0.1mm grid for placing my parts but never left less than 0.4 mm between components. Why? Because that's the minimun clearance between parts my assembly guy can rework. You have to think about that too.

Laszlo

[Tom]

Here is the excerpt right out of the "Creating a CAD Library" document. Page 26.
The grid placement courtyard is the area required to place land patterns and their respective components in adjacent proximity without electro/mechanical interference with each other. They should be designed as the component clearances and never intended to touch each other. There should be a “Manufacturing Machine” clearance (air space) between the courtyards. The size of that clearance should be defined as a “Design Rule” for “Body to Body” DRC clearance checks. This DRC check will insure that all components meet your manufacturing tolerances up front.

[Claudia]

John

For our Cell and Mobile Phone I am using for a few years 0.3mm space between components. We never had a problem with our assembly manufactory to build boards.
When you have space problems I recommend first to check if it is possible to go smaller with traces, vias, etc before you try to change the assembly rules for manufactory.
In the last few years I never changed the assembly rules. If I had to do it, I did it always with prior consultation and only where I really needed, but most time I just change the traces, space, and vias. I am now using microvias and 0.10mm traces, don’t mean you have to go with microvia it is not really cheap.

Claudia

[jdeking]

I'm doing military designs now, so I don't usually have the luxury of such tiny clearances. Still, too often I run across overkill in both directions; unnecessarily large clearances or frighteningly tight clearances. Fortunately for all involved I look up the numbers now and then and am not always proven right. Gotta stay humble.

When I decide each individual "effective" courtyard - because at design time it will vary, as you know, depending on "who is standing next to who" - some of the things I take into consideration are: the initial assembly process (hand vs. machine), rework (it does happen), voltage and amperage (when necessary), solder bridge potential ... so many things that seem like second nature now, many learned from long evenings discussing assembly issues with my ex-wife. She worked in assembly, post-solder and QA at my first design job. I still talk to the people in the prototype assembly lab on a regular basis and take my breaks with people from the assembly line. You learn a lot that way. It also teaches you humility sometimes, when they are complaining about that board that you were so proud of two months ago ...