Engineering Spotlight: Are we on the same page? - Design Vs. Manufacturing

By Mark Thompson | Published on January 30, 2010 - 10:00 AM PST

In This month’s column I will talk about tolerances and drawing callouts and the questions that arise in fabrication due to vague or incomplete notes on drawings or read me files. In a fabrication environment we frequently say we deal with anyone from a customer that provides detailed 14 page drawings all the way to the proverbial “ball point pen on a cocktail napkin”. This means we see drawing notes that can sometimes be incomplete or misleading. In this column, I will give examples of both complete and relevant drawing notes those that are less complete requiring clarification from the customer.

Let’s talk about drawing or “read me“ notes.

The overall part thickness should be expressed as the desired thickness measured between two points including tolerances. Example: .062 +/-10% metal –to –metal including surface finish this tells us the part will be .062 thick NOMINAL with an allowed variance of +/-.0062 measured over the finished plated metal and surface finish. If the drawing or “read me” merely says, “.062 “the fabricator must establish whether or not this is a MAX thickness or a tolerance is associated. In addition, the note says nothing about what distance is to be .062. Is it .062 glass to glass (over material) or is it from metal to metal? Does this include plate up and surface finish or is it an overall dielectric with a tolerance such as plus or minus ten percent?

Thickness callouts for single or double sided orders are even more critical. As a fabricator, we are able to control the thickness of the multi-layer by using different combinations of pre-pregs/cores. If a customer calls out a single or double sided job as “.008 “is this the core dielectric or is this an overall dielectric? If the .008 is the core dielectric callout on a 2 ounce finished part the final thickness would be closer to .013. If the “.008” callout pertains to the overall finished thickness, we would need to start on .004 core to finish at approximately .009 (after plate, surface finish and mask).

Again, notes pertaining to overall thickness should ideally describe what the overall thickness is to be, what points this is measured FROM , as in .062 +/-10% metal – to metal, or over glass or overall including plate up and surface finish.

Another example of drawing callouts that can require clarification is for copper callouts.

If the drawing or read me file expresses the copper merely as "2 ounce” this again leaves the fabricator with questions, Is the “2 ounce” a starting or finished copper callout? If the part is a multilayer board, Is this 2 ounce inners and 2 ounce FINISHED outers? Which would mean a fabricator would start on 1 ounce copper clad outers and plate to 2 ounce finish.

IPC dictates that there be a minimum of 8/10’s of a mil of copper in the barrels of holes for conductivity concerns. Fabricators typically plate up in whole ounce increments to ensure this minimum requirement Is met, so a part started on half ounce copper foil would finish at 1.5 ounce after plate.

If a Note merely reads,“1 ounce finish “ the fabricator must call the Customer to clarify their intentions as starting on half ounce foil and plating to 1 ounce does not meet IPC Min’s for hole wall thickness. A good copper callout note on a drawing or read me should indicate a FINISHED copper for both inner and outer layers and well as some reference to minimum copper in holes. Something like, “Material : Copper clad cores and pre-pregs per IPC 4101/26, 83, 98. Copper weight 1 ounce on INTERNAL layers and ½ ounce plated to 1 ½ ounce FINISH on EXTERNAL layers unless otherwise specified on stack up.” – This tells the fabricator a few things at once,

#1) which materials he is allowed to use ( 4101/26,83,98 )
#2) what the INNER layer and OUTER layer starting copper will be including a FINISHED copper callout for the OUTER layers.

For the Holes, a good note could read something like, “All Holes to meet IPC 6012B class 2 annular ring. Plated hole wall thickness to be .001 . 8/10ths of a mil absolute MINIMUM.“ This gives us criteria for acceptability and a tolerance for hole wall plating.

Additional notes in reference to Holes could address things like allowed tolerance for True drill position, “All holes shall be located within .08 mm diameter of true position.”- For instance.

Or whether or not certain hole sizes such as VIAS require conductive or non conductive filling, “ All .008 Via’s to be filled with conductive material , the Final surface shall be flat with no dimples or protrusions.” This is a typical note for VIA-IN-PAD applications. Where the VIAS are drilled, filled with the conductive material, then PLANARIZED (Made flat- so that no dimples or protrusions exist on the surface mount.) If done right, there will be no evidence that a via even exists in the surfac mount.

And any holes that may require solder-mask plugging or tenting. “All Via’s to be filled and covered with solder-mask.” It is Important to remember that the absence of such notes on a fairly complicated board can result in a call or clarification e-mail from the fabricator that can delay the fabrication of the parts.

And any holes that may require solder-mask plugging or tenting. “All Via’s to be filled and covered with solder-mask.” It is Important to remember that the absence of such notes on a fairly complicated board can result in a call or clarification e-mail from the fabricator that can delay the fabrication of the parts.

One that we frequently need to clarify is where specific dielectrics are shown on a stack up detail on a drawing. In conjunction with this notes on the drawing refer to the acceptable types of materials that can be used. Many times the customer will also show a Dk that all Impedances are based upon. Many times this Dk number does not match what the fabricator knows to be the real effective Dk of that particular sub –section based on the allowed materials and customer specified dielectric.

As you know from previous columns, a general rule of thumb is that the thinner the dielectric of Pre preg , the higher the resin content, the higher the resin content , the lower the Dk. If, a Dk is listed for a .004 dielectric sub section as 4.5, we know that to achieve this dielectric, we must use thin pregs which can be as low as 3.3 for fr-4/406 derivatives and 3.87 for the thinnest pregs on Nelco N-4000-29 (a common allowed material for RoHS Compliance and the elevated temperatures at assembly due to alternative surface finishes). Ultimately, this can result in the need for either altering the specified dielectrics or re-sizing Impedance lines (less desirable). In conclusion, It is Important to make sure any drawing notes address the customers reliability concerns in a manner that is clear to the fabricator minimizing any time lost due to note (Or absence of note) clarifications.

Thank you for your time.
Please feel free to call me with any questions or comments at:

(425) 823-7000 extension 239
or E-Mail me at MarkT@Prototron.com