why leave empty (unpopulated) spaces on a pcb - Page 1
Forum section: electronics / projects

author: deth502
author_rank: Frequent Contributor
timestamp: January 07, 2014, 01:41:51 pm
content: ive noticed this a lot and i cant grasp the concept. just now, looking at a post in "what are you working on" this pic was posted that i will use as an example.https://www.eevblog.com/forum/chat/what-are-u-working-on-right-now-(electronics-related-and-with-picture-please)/msg361051/?topicseen#newthere are 4 empty, unpopulated spaces there. now im just curious as to why someone would design a pcb, and put those solder pads there if there was no plan on populating them?i understand, under certain circumstances, that they may be for an "upgraded" or added feature board, but again, i cant see how it can be feesable to sell, lets say, a $50 "loaded" board for $35 just because you left out $0.25 worth of passives to disable a feature?is there some other reason this is common practice that i dont know about?

author: amyk
author_rank: Super Contributor
timestamp: January 07, 2014, 02:03:29 pm
content: It can really be summed up in 3 words: just in case.

author: ampdoctor
author_rank: Frequent Contributor
timestamp: January 07, 2014, 02:09:58 pm
content: Any number of reasons, and yes marketing and making allowances for "feature creep" being one. Another is that sometimes you're not sure how the final product will differ from the prototype so just design the board fully loaded and only use what you need(just in case). Other times it's less expensive to just use an existing board and not go through the expense of a respin. here's a classic example of things going a bit sideways in production. A number of years back Peterson Tuners put out a product called the strobostomp. After it was initially released, they had a few problems with a single capacitor on the board. Don't know if it was a hot lot or what but it only appeared in the very first units that came out. They put out a service note to have that one part removed. When you pulled it the unit worked perfectly. Instead of doing a full respin on the board in a market where the margins are razor thin and time to market is critical, all they did was not load the capacitor, hence unpopulated footprint right out of the factory!

author: SArepairman
author_rank: Frequent Contributor
timestamp: January 07, 2014, 02:10:49 pm
content: Well if you don't know that you need something, you can prototype AND make the production board at the same time, if you are smart about it.(just an example)Lets say you have a chip that has an analog and digital supply at the same voltage. You figure that you can use one linear regulator for both in your circumstances, but you don't know, and you plan on hand assembling the boards. You design your circuit so you can either put down a linear regulator or you can put down a solder bridge to the other linear regulator. This way your time to market is shorter and Vinny won't break your wrists when you can't cough up the dough for the debt you went into to buy a spectrum analyzer.

author: mariush
author_rank: Super Contributor
timestamp: January 07, 2014, 02:16:10 pm
content: There's lots of reasons why someone would do that.For example, you may wish to use microcontroller with built in oscillator and eeprom memory but just in case the oscillator is not accurate enough you may want the pads to add separate oscillator along with its capacitors.You may also want to have room to add an eeprom in case you may save money in the future buying in volume version of the microcontroller without eeprom built in.You may have a switching regulator on the board which could work with a 22uF tantalum capacitor on the output but could also work with a 10uF ceramic + a 100uF electrolytic - if you want to reuse the pcb, for some applications (like automotive stuf maybe) you may not want to use tantalum.Sometimes small smd resistors or jumpers are used as sort of version control.. just solder a .01 cent resistor in a row of pads to signify the version of the pcb.Other times you add extra pads for resistors or capacitors in parallel/series with existing resistors or capacitors just in case you need to fine tune the design or you find out you need more filtering.etc etc etc

author: daveshah
author_rank: Unknown
timestamp: January 07, 2014, 02:48:19 pm
content: On that particular board, it looks like a footprint was left for a magnetometer, and associated passives, if one is ever needed. (Only a guess, based on the other components and footprint)

author: dannyf
author_rank: Super Contributor
timestamp: January 07, 2014, 03:20:08 pm
content: QuotePrice differentials are controlled by supply / demand, not cost differentials. Just ask owners of early BMW iDrive: those people paid a premium for that feature and had to settle for a discount when selling those vehicles (vs. comparable non-iDrive vehicles).In a market economy, cost has little bearing on prices.

author: DakLak
author_rank: Contributor
timestamp: January 07, 2014, 06:49:49 pm
content: If you have a product with options it is easy to add option components if a PCB has provision.Makes a single board more useful and reduces inventory of PCBs. Of course, the down side is that sharp people, such as in this chatroom, might be able to figure out what additional components are needed so reducing sales income.My employer uses modular construction and some PCBs can accept three different microprocessors (although not at once) which reduces our supply costs/delivery time. Other modules with unused space have a matrix of holes where custom applications can be accommodated.

author: Neilm
author_rank: Super Contributor
timestamp: January 07, 2014, 07:05:12 pm
content: I quite often put pads down in areas that could be affected by EMC. That way when the unit goes for testing I can populate them quickly at the test house if necessary.

author: Spikee
author_rank: Frequent Contributor
timestamp: January 07, 2014, 08:00:14 pm
content: This is a board is designed so i can explain a bit:The big package that is left out is a ST barometer , i left it out because in earlier tests it didn't like my lead free reflow profile (even a good one).The two 0805 that are next to each other in an horizontal way are unnecessarily pullup resistors for the I2C bus (i had one for each I2C device ...) .The two 0805 caps that are vertical are just for that barometer device. This is an earlier prototype and we have chose another barometer.It might also be that you are not sure if that cap/res/component is really needed and you have only time to design and build one prototype.Also one pcb design is way cheaper to produce than multiple ones. More advanced versions of "the project" just require to populate the empty pads.

author: tszaboo
author_rank: Super Contributor
timestamp: January 07, 2014, 09:02:51 pm
content: Quote from: deth502 on January 07, 2014, 01:41:51 pmThere could be a lot of other reasons for them. Lately I was designing with an onsemi chip, and it had 7 pages describing how your device will work if you connect your device this or that way. My head was full, it was easier to just add two more resistors, and try it in the final circuit, than figureing out which configuration fits my application.Also, on digital lines controlling stuff, a not yet programmed microcontroller can make funny stuff, so you can add an extra pull down resistor, just in case.Also, you simulated everything, but you cannot always be sure how noisy that analog line will be, so you add a small capacitor before the ADC just in case.You are not sure which I2C adress will be used.And re-spinning a board can be thousands of euros.My feeling is that a board with not populated components is usually better designed.

author: deth502
author_rank: Frequent Contributor
timestamp: January 07, 2014, 10:15:07 pm
content: i believe ive found the flaw in my thinking.most of the responses include something like "because we didnt know yet"i had just assumed, that if someone were to go through the trouble of getting a custom pcb manufactured in large quantities, that the design would already have been finalized and tested.logically, this is still a bit nonsensical to me, but i have never been involved in any mass production of anything. i guess i can understand that these things may be under such a strict time crunch that forcing them out as quickly as possible required things like this.

author: mariush
author_rank: Super Contributor
timestamp: January 07, 2014, 10:47:10 pm
content: QuoteThere are cases when a pcb has empty spaces because of various models of the same product... for example a company makes several motherboards using same pcb, the difference beeing one extra usb 3 controller, or extra controller for eSATA on the back and so on.Same with video cards, use same PCB for a several models but use only half the ram if you're doing a 64bit model, leave out some mosfets in the dc-dc converter if you're using the cut down gpu chip that only consumes 40 watts instead of 75 watts.Or maybe they designed the video card to use 8 gbit ddr5 chips but a few months later they can buy 16/32 gbit chips at a very good price so now they only have to use half the memory chip locations.I have here older video cards here that have missing stuff, basically the TV out option is not implemented.

author: Nerull
author_rank: Frequent Contributor
timestamp: January 07, 2014, 11:05:14 pm
content: Quote from: deth502 on January 07, 2014, 10:15:07 pmAnd how do you test the design without a board?Getting a design finalized often requires minor tweaks in board design and components. By designing the board to accept some level of component modifications you potentially reduce the number of board revisions you need to have made, which reduces cost.

author: tszaboo
author_rank: Super Contributor
timestamp: January 07, 2014, 11:24:02 pm
content: Quote from: Nerull on January 07, 2014, 11:05:14 pmNot to mention if they* see an LED blinking, they start ordering thousands of boards, without asking if it really works.*management

author: vk6zgo
author_rank: Super Contributor
timestamp: January 08, 2014, 02:13:14 am
content: This has been done as long as PCBs have existed--there is usually an upmarket version of the device with extra features,& sometimes the same PCB can be used for completely different purposes,using the same basic "guts".Even before PCBs,Tube radios often came with extra holes punched in the metal chassis for better versions.Sometimes a basic radio would have no RF stage,whereas one for use in remote areas would fill that space.Same with cars--the cheapie model doesn't have all the "bells & whistles" but it uses the same wiring loom.

author: aargee
author_rank: Frequent Contributor
timestamp: January 08, 2014, 02:26:05 am
content: I would hazard a guess that a lot of PCB geometries are controlled by the physical mounting of the boards in a case/housing etc.There are not a lot of boards that have their destination as bare bones on the workbench.

author: deth502
author_rank: Frequent Contributor
timestamp: January 08, 2014, 02:29:51 am
content: Quote from: Nerull on January 07, 2014, 11:05:14 pmwell, if it were me, i would be etching my own one-off boards to assure my design worked as intended before i would even think of ordering a batch of "professional" boards.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 08, 2014, 02:55:41 am
content: Quote from: NANDBlog on January 07, 2014, 11:24:02 pmAgreed. You must never forget the Management LED.Quote from: deth502 on January 08, 2014, 02:29:51 amI used to make my own boards. It's hardly worth it anymore, because even if I made my own, I had to make another.These days, with cheap access to many many board houses that can make prototypes for you, I spend my time thinking of what-if scenarios up front, so my first board is a prototype that has enough leeway for easy modifications that I can make those changes in testing, tweak the board, and possibly go to production with a tweaked board that is missing or has extra components. As long as I considered it up front .. For this reason I often use lots of 0-ohm resistors too, to logically break apart sections of the board for unit testing, the LDO feeds the power rails with a 0-ohm, I may cross feed an optional rail that may need its own LDO for better noise specs, I often put down extra pads for paralleling and serial resistors in an op-amp feedback loop I am not sure of the exact values needed. I often put extra pads for a bypass cap in a feedback loop, if it turns out to need that for an increased phase margin. I sometimes put pads for a low-pass RC filter between op-amp stages if I am not sure of that. In almost all cases, my connections between op-amp stages are through a 0-ohm so I can disconnect a stage and examine the frequency response of a single stage on the board.All of these can persist for a production board if the prototype board is flexible enough to permit it. And then you see a production board full of 0-ohm resistors with lots of odd looking placements on it and unpopulated pads, and you wonder why on earth the designer did what appears to be totally unnecessary and stupid

author: deth502
author_rank: Frequent Contributor
timestamp: January 08, 2014, 03:59:32 am
content: Quote from: codeboy2k on January 08, 2014, 02:55:41 amsee, i could never think that far ahead. id have to actually see a fuck up to address it. im not good at "what'ifs", and if i tried, im sure id miss everything that was actually important. i do like the 0 ohm resistor idea though.

author: Psi
author_rank: Super Contributor
timestamp: January 08, 2014, 04:14:16 am
content: Most of the time is either components left off to save money (once its confirmed the product works without the parts)And different versions of the product. You often see entire sections of the board unpopulated with products that have a budget version, normal, and pro versions all using the same PCB.

author: quanttrom
author_rank: Regular Contributor
timestamp: January 08, 2014, 06:00:23 am
content: Quote from: deth502 on January 08, 2014, 03:59:32 amMost of the time those "what-ifs" are things that come with more and more experience. Something related already bit you at some point before and you still remember the pain ....

author: Dago
author_rank: Frequent Contributor
timestamp: January 08, 2014, 06:08:44 am
content: Quote from: deth502 on January 08, 2014, 02:29:51 amNowadays ordering a custom board from china costs literally about as much as a (cheap) meal. You can get ten pieces of 50x50mm two-layer PCBs for ten dollars from itead/seeedstudio for example.

author: Unknown
author_rank: Unknown
timestamp: January 08, 2014, 06:24:09 am
content: Quote from: deth502 on January 08, 2014, 02:29:51 amIf you are running a company then taking into account the H&S requirements for all the chemicals involved and the space time and hassle of making your own boards it's cheaper to just order 10 from China than make a couple of your own now.

author: deth502
author_rank: Frequent Contributor
timestamp: January 08, 2014, 08:28:05 am
content: Quote from: Dago on January 08, 2014, 06:08:44 amfuck me, i cant buy blank, unetched pcb for that much! i think ive been missing out here.

author: westfw
author_rank: Super Contributor
timestamp: January 08, 2014, 11:08:43 am
content: Don't forget that there are plentyof reasons to not fully populate a board during testing, even if the full board will have everything. "on this unit we test the gyros, no need to mount the aero barometer chips and such.

author: g1pete
author_rank: Newbie
timestamp: January 12, 2014, 04:23:12 am
content: We have a 12V to 5V DC switcher and have the PCB made to support two sources. Since they do no have the same foot print, the board is designed to have two footprints. One of them is never populated in production boards.