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I am going to put a part on a board and the position of this component needs to be perfect. I am talking about 1/10th of a mm accuracy. What is the best practice for this type of alignment issues, what precautions I can take to ensure this accuracy during manufacturing.

UPDATE Ok. I get it, I will loose the tolerance, let me ask, what type of alignment can be achieved with ease?

This is for a complex lens system focusing on a Photodiode, the size of the active area is 2x2mm, so more error placing = less signal hence the concern.

W5VO
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Frank
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    For interest. Why do you need the degree of accuracy that you do? – Russell McMahon Jan 13 '12 at 11:24
  • @RussellMcMahon Good question. I would bet it is interacting with a case or another board or something like that. – Kellenjb Jan 13 '12 at 14:10
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    Even if it's fit with another board or a case, that's still very tight tolerance. Is the hole in the case really that accurate? The mounting holes so tight that they have less than 4 mil slop? The different thermal coeficients similar enough for less than 4 mil difference over temperature, etc? This sounds like someone not knowing what they are doing. – Olin Lathrop Jan 13 '12 at 15:09
  • More error leads to less signal only if the spot size created by the optics is about the same size as the photodiode active area. If you design your optics to give a 1 mm diameter spot (for example), you have +/-0.5 mm of play in each direction instead of the 0.1 mm you were asking for. If your optics have an output spot 4 mm in diameter you have some loss, but the loss is pretty consistent regardless of alignment to within ~ 2 mm tolerance so it should be "straightforward" to account for when designing amplifiers for the received signal (requiring more gain, but not more dynamic range). – The Photon Jan 16 '12 at 06:00
  • @thephoton, well said. currently I designed it such that the optics are covering a larger area (about 10% larger). Based on my calculation 10% loss is eminent but it will be consistent, which is what I was shooting for. I actually asked the question to make sure my thinking wasn't flawed and I shouldn't shoot for perfect alignment. It turns out, I was right but I missed out on the tolerance level. I should have shoot for probably 0.4mm or so, currently I shoot for less than 0.2mm only. – Frank Jan 16 '12 at 07:56
  • How many of these do you need to make ? – Rocketmagnet Aug 04 '12 at 11:11

4 Answers4

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First you need to pop up two levels in the overall design and ask why you really need 4 mil position accuracy. Even if you do achieve that most of the time, it will complicate inspection and lead to reliability problems. This smells like a a bad higher level design choice was made. Find out what the real problem is and then address it in a more reasonable way.

To get the best placement accuracy, you need fiducials as others have noted. Even when you don't put explicit fiducials on a board, someone at the board house will find two pads near the corners and set their system to work with that, right after grumbling something about "rookie" and "moron". Sometimes they will tell you they want fiducials, but usually they'll just cover up your mistake with maybe a little more position error and the aforementioned grumbling.

The best answer however is to talk to your assembly house. At that level of accuracy, you can't just slap a couple of fiducials on the board and figure everything will be fine. You need to have a dialog with them because different machines and processes have different capabilities.

However, even if the assembly house says they can do 4 mil accuracy and you do what they tell you to, how are you going to test this board? A manufacturing requirement is no requirement at all if it isn't tested to. Again, this gets back to my first point. The only right answer is to not be here in the first place. Not everything is possible or reasonable just because you can write a spec for it.

Added:

I meant to say this before, but got distracted and forgot before hitting send.

Even if the assembly house can guarantee original placement to 4 mils, that does not mean the part will end up there. Molten solder has a pretty strong surface tension, especially with respect to small parts (look up "tombstoning" for a extreme but real example). Once the solder melts, the original placement is largely irrelevant. The surface tension of the solder pulling on each of the pins will pull the part into the sortof "center" alignment. Usually this is a good thing since it allows for some initial placement error and still have the part end up in the right place.

In your case, this means the real placement is not so much a function of where the machine plunked down the part (this is the part of the process that fiducials help with), but where the pull of the solder surface tension on all the pads averaged out to. With a symmetrical chip and the pads only a little larger than the pin footprints, this is going to be pretty predictable. With a assymetric part, like a SOT23 transistor for example, this is not so obvious. The surface tension pull is also in part a function of how much solder got placed on each pad. So now the eveness of the solder paste thickness matters.

Again, what you are asking will be difficult at best, possibly unreasonable.

Olin Lathrop
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Agree with Majenko answer, but two fiducials per board sometimes is not enough.
Following is an excerpt from Sibex Electronics PCB Design Guidelines:

4 Global Fiducials - 1.20mm diameter shall be placed on all 4 corners of rails. CENTER of Fiducials shall be 10mm X 5mm from the corners as show below: enter image description here

Fiducials:

These should be included in 3 main categories:

A. Local Fiducials – For each fine pitch device (25mil lead pitch or less), fiducial marks should be located near the component on all 4 corners of the IC or diagonal at minimum.

B. Global PCB Fiducials – For each single PCB, fiducial marks should be included on all 4 corners or diagonal at minimum and spaced to encompass the entire placement area on the PCB.

C. Global Panel Fiducials – If the PCBs are panelized in an array form, fiducial marks shall be included on the panel rails on all 4 corners (both sides if SMT parts are located on top and bottom) and spaced to encompass the entire placement area on the panel. The fiducial mark should be round, and 1.20mm in diameter, and should have clearance of at least 50mil around from any traces, silk, stamps, mask or anything that can obscure the fiducial image.

Andrejs Cainikovs
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Put Fiducial Markers near 2 opposite corners of the component.

enter image description here

The camera in the P&P machine (if it's a good one) should identify them and use them to calibrate the positioning of the components around it.

Majenko
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Currently we're working on a Led pcb with a 50 micron accuracy requirement. You need two fiducials per module or pcb and what we are using to reduce reflow oven variation is glue underneath the component which cures at a lower temperature compared to the solder paste securing the component's position.

Jpineda
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