How Many Fiducials Per Solder Paste Stencil?

We manufacture solder paste stencils for a variety of customers. We have seen as many as 180 fiducials per stencil with a step and repeat. Is this excessive? How many fiducials are recommended for a typical solder paste stencil?

R.C.

Expert Panel Responses

Typically 3 are sufficient.

Bill Coleman
Vice President Technology
Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

Any more than two marks on the stencil is a waste. Unlike a PCB, a stencil cannot be stretched or shrunk, you can only align it in "XY" and Theta. Two marks can accomplish this.

Scott Wischoffer
Marketing Manager
Fuji America Corporation

Scott Wischoffer has been in the SMT industry since 1986 with experience in service, training, sales & applications, and marketing.

Since fiducials are used for alignment, the amount can vary from 2-3 to as many as the customer wants. A couple things to consider/understand: For PCB alignment to the placement machine, there are 2-3 panel fids. Then 2-3 fids for each individual PCB in the panel.

Now add Fine Pitch, LGA, BGA or other alignment-critical parts, and you can have 1-3 fids for each of them conceivably. It doesn't take long to get to 180 fids per fab panel (plus add bad-board fids) just for machine and component alignment. Typically, all of these fids are in the gerber files for the PCB and stencils. Now, obviously this is excessive for stencil alignment, but could be critical for the pick and place alignment as well as component alignment. Typically, 2-3 fids spread out to the extents of the panel are good for the Solder Paste Printers.

You should check with your customer to ask them if they truly need that many fids. Most Solder Paste Printing machines that use fids for alignment will use 2-3 fids for alignment, but typically will not require/use more than that. More than likely, the additional fids are in the Gerber files and are not deselected or eliminated when the stencil file is created. I would ask the customer(s) if they truly require the additional fiducials for their process, more than likely they don't need them.

T.J. Hughes
Manufacturing Engineer
Esterline Interface Technologies

Mr. Hughes has been in the electronics manufacturing field for 20 years. Operating the processes and as a manufacturing engineer for the last 14 years. He is also a CIT as well as an SMTA Certified Process Engineer.

3 fiducials are the minimum number required, Assuming the 3 fiducials are referred to as A, B and C, fiducials AB are perpendicular to fiducials BC.

Fiducials AC is diagonally across each other, these fiducials are used to correct the theta offset(Z), Fiducials BC will correct the X offset, and Fiducials AB will correct the Y offset.

With newer SMT machines, Vision algorithms can correct the theta offset(Z), X offset, and Y offset using 2 diagonal fiducials provided that these fiducials are very close to the edges of the PCB's.

Typically manufactures that have multiple PCB's on a panel will make request for more the 4 fiducials per stencil. This is done, to increase their Printing Process window, as there may be step and repeat issues on the PCB panel (Typical caused by PCB manufacture of PCB warp-age), Having many fiducials enables the manufacture to adjust the offset best on the panel, hence, Every PCB offset with fiducials.

Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa

Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

For larger panels such as this, it is generally acceptable to utilize the board level fiducials on the four corner boards along with the panel fiducials that are on the rails. However, there are clients that will require all fiducials to be present.

Stephanie Nash
Director
Integrated Ideas & Technologies, Inc.

Stephanie Nash is the Director of Technical Services & Marketing for Integrated Ideas & Technologies, Inc., a premier manufacturer of SMT stencils. She has been instrumental in the stencil design and technical support.

Reader Comment

2 is sufficient but 3 is best. You are trying to accomplish a process called "Triangulation", which generally works "best" with 3 known points.

Kevin Hussey, The Morey Corporation

PLACEMENT fiducials are often repeated on the top and bottom PWB artwork to facilitate the Pick/Pace machine's ability to "re-zero" to a local fiducial as it places parts. Most often it was never the intent of these local PWB fiducials to be transferred to the Stencil artwork. The Experts are correct in stating that the paste printer only needs three GLOBAL stencils to accurately align the stencil to the PWB. LOCAL fiducials are seldom intended to be put in the stencil, but because they are part of the artwork, they often are inadvertently placed on the stencil design and overlooked by the checker.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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