Are Your PCB Data Unprepared? Print E-mail
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Written by Jeff Champa   
Monday, 12 July 2010 12:10

Changes to the design by non-designers usually result in unforeseen failures. 

A February article by Jack Olson and Mike Tucker titled “PCB Data Preparation” (http://pcdandf.com/cms/magazine/209/6996) spurred me to elaborate on a few items from the designer’s perspective.

It is true that often a designer does not want the fabricator to modify the PCB data file. We have a note similar to the one mentioned in the article:

DATA MAY NOT BE MODIFIED WITHOUT WRITTEN APPROVAL

I believe our version is a little more practical. We call it Note 23. We put it on every PCB print, always as Note 23. Note 23 reads as follows:

MODIFICATION TO COPPER WITHIN THE PCB OUTLINE IS NOT ALLOWED WITHOUT WRITTEN PERMISSION FROM MOREY ENGINEERING, EXCEPT WHERE NOTED OTHERWISE ON PRINT. MANUFACTUER MAY MAKE ADJUSTMENTS TO COMPENSATE FOR MANUFACTURING PROCESS, BUT THE FINAL PCB IS REQUIRED TO REFLECT  THE ASSOCIATED GERBER FILE DESIGN +/-0.001 IN. FOR ETCHED FEATURES WITHIN THE PCB OUTLINE.

This makes it clear to the fabricator that it can smooth geometry, perform edge compensation, etc., to make the fabrication process reliable. It is also clear that the designer requires the copper in the final product to match the Gerber files.

The real motivation behind Note 23 is simple: to send any given PCB design to more than one fabricator and get the same result. Most companies probably always fabricate the PCB for a given project with the same fabricator for consistency, but what happens if the fabricator goes out of business or is purchased by a competitor, or begins having quality issues? You may be forced to change your PCB fabricator, a messy business for many reasons. It’s even worse when boards from a different fabricator do not function the same way. If the fabricator follows the designer’s print and the PCB does not function properly, the responsibility lies with the designer for not having a complete and accurate description of their own design.

How can you verify you actually get what you asked for? It is true, as Olson and Tucker mention, that a designer should establish a good relationship with their PCB fabricator, which includes working to communicate one’s own needs and process, and to understand the fabricator’s needs and process. Note 23 is another step in the right direction. If the fabricator finds an error, the last thing you as the designer want them to do is to fix it without telling you. Given the chance to fix the problem, the designer will eliminate the possibility that another fabricator fixes the problem differently (resulting in a different final PCB) or, worse, fails to identify the problem and builds nonfunctional PCBs.

Another move is to work with the board fabricator to get it to provide individual layers of the PCB any time it changes the fabrication files. What would you do with individual layers? From experience, I can say that most of the time, if everything goes well, they will collect dust. If you do need them, however, you will not be able to get them after the fact.

Individual layers permit observation of differences in the copper in the PCB from different fabricators or from one design revision to the next. Let’s say the customer is having field failures and cannot determine the problem. It will go to its contract manufacturer for proof of the integrity of every component in their product.

Generally for the PCB, all the EMS firm can do is provide the first-article inspection (a piece of paper), and cross-section the PCB and verify the layer stackup, neither of which can reveal whether the copper on each layer inside the board was fabricated per the design. From a contract manufacturer’s perspective, the individual PCB layers can help the customer determine if the board was fabricated per its design.

Another big tradeoff is controlled impedance versus designed impedance. Controlled impedance is the practice of specifying specific traces on a PCB must be a certain impedance, and leaving it to the fabricator to adjust the design as necessary to get the specified result. (The adjustment typically includes trace widths and PCB material thicknesses.) Designed impedance is when the designer specifies the PCB stackup and trace widths to get the desired impedance.

As an engineer, controlled impedance can be scary. You have given control of critical PCB parameters to the fabricator. The fabricator could change the manufacturing parameters on the fly without notification based on its currently available material. The advantage is that you (could) receive a lower price, but at the cost of losing control over the design. This could be OK, provided the design is simple and can permit this variation. Complex designs usually mean less design headroom and lower tolerance for change.

Let’s say you have a design that incorporates several fine-pitch BGAs connected by an address/data bus that requires impedance A, and you have a few RF traces that require impedance B that route to antennae fabricated right on the PCB. The PCB complexity is now at six to eight layers, and the designer has the option to specify each “important” trace on the PCB and its impedance (controlled impedance), or specify the trace widths and PCB stackup (designed impedance). With controlled impedance, the fabricator would be responsible for the impedance and would measure it to verify and put the data on the first-article inspection. With designed impedance PCBs, the designer could ask the fabricator to measure the impedance as reference only, and include the data on the first-article inspection. The latter permits more design control with the same check and balance in place to ensure proper impedance has been realized.

The point here is that a good designer will take full responsibility for the design. No one, for any reason, should alter that design except the designer. The designer knows why everything is the way it is. Changing a design without the designer will usually result in unforeseen failures. Example: The PCB manufacturer removes nonfunctional pads around a via on innerlayers because they are electrically nonfunctional. Result: The structural integrity of the via is compromised when the end-product is subjected to stress testing. How would the fabricator know that the little bit of extra structural integrity of the via is needed for the product to pass HALT (highly accelerated life testing)?

We work on products every day that are successfully subjected to thermal shock rates of 50°C per minute temperature change, between -40° and +85°C, while experiencing random vibration levels of 30G. The best solution in this situation is Note 23. This forces the fabricator to consult with the designer. Feedback from the fabricator then gets incorporated directly into the design. This process makes the design better and hopefully minimizes any room for error. The designer must be in total control of the design.

It is great to have a fabricator that can provide its “secret sauce” to make the design work, but be ready to get locked into that supplier, because another fabricator’s recipe will be different. As a designer, is that a risk you want to take? Is that a risk you want to take with your product? Is that a risk you want to take with your business? 

Jeff Champa is department head, development engineering at Morey Corp. (moreycorp.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 12 July 2010 12:44
 

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