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I'm reading this DAC datasheet. The last page (p. 32) gives certain of the chip's dimensions, some in units of "BSC SQ".

enter image description here

What does BSC SQ mean? Is it standard to express sizes in BSC SQ?

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  • \$\begingroup\$ There is currently a high rep battle royal over your accept rep. \$\endgroup\$ – Kortuk Aug 11 '12 at 12:48
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    \$\begingroup\$ @Kortuk - What does that mean ? \$\endgroup\$ – Rocketmagnet Aug 11 '12 at 15:37
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    \$\begingroup\$ @Rocketmagnet At the time I posted it both stevenvh and RussellMcMahon had posted answers within a minute and were both editing and improving each time the other did. Russell won though, pictures always win. Just me joking around. I get to make chatty comments occasionally too, right? Just hope one of those mods does not spot it. \$\endgroup\$ – Kortuk Aug 11 '12 at 15:41
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    \$\begingroup\$ It may be less a "battle" and more experience than is obvious :-). I've learned that I can prepare a longish anser that takes some while and when I post it a similar long answer has been posted by someone else. Or a short but good enough (or better) answer. So I tend now to post a basic square answer in a paragraph or so, with a longer answer largely formed in my head, and see what else is ahppening, and repeat N times as needed. Hence my "More soon ..." appelation. I do note that other answers do sometimes tend to assymptote tpwards mine when they have originally been somewhat different ... \$\endgroup\$ – Russell McMahon Aug 11 '12 at 21:28
  • \$\begingroup\$ . No doubt that sometimes happens with mine as well :-). \$\endgroup\$ – Russell McMahon Aug 11 '12 at 21:28
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BSC means "Basic Spacing Between Centers". This is often used if the reference lines don't refer to a physical point or edge, like in the case of the pins: the reference is in the middle of the pin, instead of one of the edges.

BSC SQ = BSC square, i.e. in both X and Y dimensions.

The abbreviation BSC is misused in the case of the package outline, and overall dimensions, as they are specified by physical edges which can be measured.

Usually a mechanical drawing of a package will show minimum and maximum values. BSC SQ must be regarded as nominal, so may deviate, but it doesn't say by how much.

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  • \$\begingroup\$ @downvoter - it might be helpful if you would tell us what you think is wrong here. Until you do, I only can assume that this is just a random downvote; I think the answer is correct. \$\endgroup\$ – stevenvh Oct 5 '12 at 6:03
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    \$\begingroup\$ St even - I just received a down vote also (+6/-1). I'm aware of a person who occasionally downvotes me for the Hallibut (a fishy thing to do). You may have fallen a-fowl of him as well. \$\endgroup\$ – Russell McMahon Oct 5 '12 at 10:48
  • \$\begingroup\$ @Russell - yes, I remember several comments from you expressing your irritation with this. But when you commented on it here I went to have a look and at that moment it was a month ago since the last downvote on an answer of yours, so I assumed that person stopped his actions, and that this downvote was from someone else. But now you got three again this week. Like you said: very sad. Consolation: it can always be worse: in this tread my posts got a total of 7 downvotes in 2 days :-) \$\endgroup\$ – stevenvh Oct 5 '12 at 11:08
  • \$\begingroup\$ Steven - I actually don't mind (too much) downvotes when they make sense - and sometimes I agree that they do. But the specific shall-be-nameless person who occasionally decides to downvote me plus possibly some random occasional others often demonstrate more their lack of knowledge of engineering or their general antipathy. My complaint is that others use the downvote as a guide as to the answer quality and if that is not what it reflects then in such cases it misleads the seekers. \$\endgroup\$ – Russell McMahon Oct 5 '12 at 12:04
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In its most generally used form "BSC" means "distance between two nominated points.

"BSC SQ" when referring to an IC package means that the side is BSC units long and that the package is "square" so that the side at right angles to this one is the same length.

The words may literally stand for "Basic Spacing between Centers" but the term is used even where there are no specific centers involved (as in eg the package side measurement in the example given).

Where holes are involved the distances are measured between the centers of the holes.

Where eg pins are used the distances are between the centers of the pins.

A side or other dimension that has nothing to have a formal center means that the dimension is measured between the mean edge locations.


Some explanations say it stands for "Basic Spacing between Centers" but that seems to be more an attempt to put meaningful words to the term than a genuine meaning".

Regardless - you can expect that either all the left hand edges OR all the right hand edges or all the centers of pins along an edge are spaced at a spacing of BSC within the specified accuracy tolerances.

enter image description here

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  • \$\begingroup\$ The distance between the lines that say "9.00 BSC SQ" is 11.5 times distance between pin centers. I'm confused. \$\endgroup\$ – endolith Sep 25 '15 at 21:43
  • \$\begingroup\$ @endolith You are confused :-) as 11.5 x 0.8mm = 9.2mm <> 9.00 mm.|| However: Pin to pin centre = 0.8mm x 7 spaces = 5.6mm. pin widths are 0.45mm so distance between outer edges of 8 pins = 5.6 + 0.45/2*2 = 6.05mm approx. As package is 7mm square this means space from outside outer pins to package edge = (7mm- 6.05mm)/2 or about 0.5mm. This is nowehere spefified but seems reasonable. After that, the pin lengths must be (pmm_BSC - 7mm_PSC)/2 = 1mm long. So overall it looks reasonable so I don'y see what ypu re finding confusing. I may have missed something? \$\endgroup\$ – Russell McMahon Oct 1 '15 at 10:09
  • \$\begingroup\$ There is a horizontal dimension line across the top that says "9.00 BSC SQ". So if "BSC" means "distance between pin centers", then this distance would logically mean "9 times the distance between pin centers". But it's actually 9 mm, which is 11.25 times the distance between pin centers. Likewise for the dimension of "7.00 BSC SQ". \$\endgroup\$ – endolith Oct 11 '16 at 20:56
  • \$\begingroup\$ @endolith I think if you read through my answer and look at my diagram then it makes sense. If, as it seems, they use the term BSC differently for pin to pin and for package size you "just need to accept" their usage. As I note above" - ' "BSC SQ" when referring to an IC package means that the side is BSC units long and that the package is "square" so that the side at right angles to this one is the same length.' - That's for the package - no CENTRES - it's a edge delimited length. That's what they do and you have to accept it (assuming I'm correct, co course :-) ). THEN when they come ... \$\endgroup\$ – Russell McMahon Oct 12 '16 at 1:45
  • \$\begingroup\$ @endolithn... to the pins they DO mean centre lines. | And as I noted, when you combine the results from thse two methods they do seem consistent dimensionally, if not terminologically. \$\endgroup\$ – Russell McMahon Oct 12 '16 at 1:46
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"BSC SQ" is an abbreviation for "Basic square". "Basic" has a specific meaning defined in the dimensioning and tolerancing standard (ASME Y14.5M for the curious), but it roughly means "nominal". "Square" simply means that the feature is square, so there is no need to clutter the drawing with an extra dimension for the perpendicular direction. Neither abbreviation has anything to do with spacing between centers.

To get technical, "basic" really means that we aren't going to specify the tolerance of the feature through the dimension, nor through an inherited tolerance coming from the title block. Instead, there would be a feature control frame defining the region that the part's surface could lie within. For example, something like this pointing at the side of the package:

enter image description here

would mean, "The surface of the package must lie within a volume 0.2 mm wide centered on the ideal location given by the basic dimensions."

This kind of detail isn't shown on the data sheet, but it would be on the company's internal and proprietary mechanical drawings of the package and tooling for manufacturing it. The referenced JEDEC standard likely defines details of the package shape somewhere between that of the data sheet and a full mechanical drawing. For most purposes the detail on the data sheet is sufficient.

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I think one of the actual questions being asked by the poster, is what are the units of terms like "7.00 BSC". Since the datasheet linked has a mechanical drawing that specifies that all dimensions are given in millimeters, the units are millimeters. The "BSC" part doesn't change the units.

Thus 7.00 BSC is equivalent to 7mm, with the spacing measured between "centers" (not really the centers of anything in the specific example of one side being 7mm between "centers").

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I've always interpreted it as meaning a 'basic dimension'... A basic dimension is an ideal, exact dimension, and as such has no tolerance. Important features of the device are often shown like this. Pitch is pretty commonly basic.

Of course the real life device won't be exact. What is ideal is what is relevant here though...

Apparently Microchip agrees with me: datasheet excerpt

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