We have seen as many as 180 fiducials per stencil with a step and repeat. How many fiducials are recommended for a solder paste stencil? The Assembly Brothers, Phil Zarrow and Jim Hall, address this question and share their own experiences. Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Transcript
Phil
Welcome to Board Talk with Phil Hall and Jim Zarrow of ITM Consulting, the Assembly Brothers. And today we are coming to you from the ITM elegante ballroom high atop Mount Rialto. If you can guess where that is.
But anyway we are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. And Jim, I believe today we have a stencil related question. Is that correct?
Jim
That's correct. It comes from R.C. R.C. manufactures solder paste stencils for a variety of customers. We have seen as many as 180 fiducials per stencil with a step and repeat, or what I would call a multi-up panel. Is this excessive? How many fiducials are recommended for a typical solder paste stencil?
Phil
Oh, good question. And from a stencil manufacturer. Very, very good. Well, I guess let's start off by saying that those multiple fiducials he is seeing, and then we will qualify it, are basically what we call localized fiducials.
You are seeing those because the propensity of high density fine pitch components on the board. And it is a good thing that the users are recognizing the use of localized fiducials for smaller pitch and high density components and things along those lines. That is a very best practice. However, with regard to the stencil.
Jim
There is no need to put more than three or four global fiducials on a stencil. Those fine pitch on the local fiducials that Phil was talking about on the individual panels are going to be used by later processes.
Pick and place primarily, but perhaps dispensing or inspection or some other process that needs more fine, accurate resolution of the patterns on the board. But these are totally unusable in the stencil printing operation. You cannot orient, or register a stencil to a specific pattern.
You have to register the whole, entire stencil to the board. So you really for stencil printing only need the global fiducials.
Phil
Right, so kudos to your customers for recognizing the use of localized fiducials for fine pitch. That is definitely best practice. However, it is totally unnecessary to be printing solder paste on.
Jim
Right, and kudos to R.C. for realizing that. Why does he need so many fiducials on the stencil. That is a very astute observation. It is good that their on the board, but you don't need to put them on the stencil.
Phil
That's right, don't need to solder paste them. Good, I guess we answered that question. And we thank you for the questions, keep them coming in. This is Phil and Jim, the Assembly Brothers.
Jim
Whatever you do, don't solder like my brother.
Phil
And please don't solder like my brother.
Comments
Generally 3, but sometimes up to 6, but I only intend to use 3. The extras are in case the location doesn't give the best alignment. That is usually if the boards are customer supplied, and I don't get to dictate fiducial location. I always specify half-etch fiducials for my stencils. Why would you want fully cut stencil apertures?
Joshua Turner, EMSG
Our suggestion is at least 2 fiducials (panel / global) and these should be as diagonal as possible, this to correct displacements in X and Y and rotation (theta), being recommended the use of at least 3 fiducials (triangular distribution) for panels / boards with a thickness <1.0mm and / or “large” formats (> 400 cm²), to also help correct the “torsion”. Currently, the new fiber laser technologies do not require the application of epoxy to protect and contract the fiducial. Eliminate this mechanical stopper/gap between pcb and stencil from your process, it can get in the way of deposition of solder paste on critical components. Your stencil supplier should help you!
Maurício Selmikaitis, Stentec
With multi up panels, I used the circuit fiducials at the panels 4 corners, allows a spare mark. The local fiducial had to be solder free to achieve the dispense and placement accuracies.
Larry Dzaugis, DZA
We'll use a minimum of 3 but leave a few in in case the epoxy or darkening material falls out. A 4 up panel with 2 per circuit, leave them all. Bigger panels, you can remove a few.
Kevin Mobley, Tektone Sound and Signal, Inc.
Usually You have use three global fiducials one of them as ¨Pokayoke¨ and you can include 3 more fiducials Local or Global Fiducial as Plan ¨B¨ if your 3 principal fiducial was damage. But in mayor of the case more than 9 fiducials are unnecessary and increase the cutting process time. In another hand, if have some fiducial near some fine pitch apertures, could be affect the coplanarity of printing process in that area.
Carlos A. Bobadilla, Photo Stencil Mexico
As a PCB designer and Stencil designer I use Fiducials with the solder paste layer removed for my local PCB unit and with the Solder paste layer left for the Panel/Array. These are half etched/lasered on the shim. At least two of my Stencil manufacturers will remove any more than the three global fiducials. I have had localised fiducials fully lasered on at least one stencil in the past, ones that I had forgotten to remove!! We use three globals to check for the board being inserted into the line 180deg out.
Steve Brine, Young Control Systems
We have found that sometimes the epoxy-filled stencil fids wear down with cleaning, and some before others. Although we use 2 or 3 fids for alignment, we specify 4-6 fids on the stencil so we have a back up plan.
Ben Thomas, OEM
As an engineer in PCB manufacturer, we usually need two fiducials on stencil. Two is sufficient for smooth manufacturing. However, when customers require more fiducials, we can accept more.
Dora Yang, PCBCart
As a PCB designer, I know the local fiducials aren't needed when I output the stencil files, but my design software doesn't have a convenient way to suppress them. Fortunately my manufacturing floor doesn't care if they get solder or not, and I suspect the majority of other manufacturers are of like mind. Sometimes my stencil maker spots them and asks if I'd like them deleted - sometimes I do, sometimes I don't, but I appreciate that they're paying that much attention to the design.
Joe Jorn, Grayhill Inc., USA
What are the chances the stencil manufacturer has mistaken test points for fiducials?
