Screaming Circuits: October 2008

Silk Screen Marking

When we select our board house, most of us carefully consider space & trace, cost, turn-times and things like that. It's important to not forget about their capabilities regarding the silk screen layer. A great board can easily be made mediocre by poor quality silk screen. A Mediocre board can be made much worse. Ambiguous marking can make either difficult to use.

Yucky_brd_i2c This board here has silk screen printed right on the pads. Mis-registration like this reduces the solder surface area and can prevent a good connection. See a little more detail in this post. The board house has a service add-on to clip the silk screen off of the pads which would eliminate this problem.

Then, the ambiguity factor can be a problem with assembly too. Sometimes when parts are very close Yucky_brd_d2_3 and the board house can't make fine line silk screening, we can't tell which reference designator goes with which part. And on polarized parts, especially diodes, the polarity markings can be ambiguous as described in this post. Diodes, especially barrier diodes can be a real problem with markation. A simple (+) or (-) just isn't clear enough.

Duane Benson
weiße Zeug auf Silber Zeug ist schlecht

Parts Substitution Issues

A while back, we surveyed a number of you folks about various things that are important to you. One of the key concerns we heard about was a fear of an unapproved part substitution on a turn-key order. As a rule, when we are buying parts for a turn-key order, we do not make any substitutions without your authorization. We know this is important to you and we follow those wishes.

Schottky_top A related issue to that occurs on kitted jobs. We do sometimes get in parts that don't match the bill of materials exactly. On the one hand, we do know that on kitted jobs, you sent us the parts and it's your design. On the other hand, we also know that it's not difficult to accidentally grab the wrong bag of parts, especially if you have someone else kitting up the job for you.

We also know that there are plenty of times where there isn't any effective difference between a 10K, 1/4w resistor and a 12K, 1/8w resistor or between a 40V diode and a 36V diode. Of course, we also know that there are plenty of designs where those slight variations can be the difference between a 100pxthree_ic_circuit_chips great design and a non-functional pile of junk. And there are plenty of cases where a difference in suffix can indicate the same part but the wrong package.

If you know for certain that the substitution won't matter, the best thing to do is give us an updated bill of materials. The second best thing is to tell us in the order process that it's okay - like we describe in this post.

If we aren't sure what to do and have to call you, it can delay your job and we don't want that. We want to get your assembled boards to you when you want them.

Duane Benson
That's not me. It's my evil twin

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