Design guides / DFM reference
A layout is manufacturable or it is not, and the difference is decided by a small set of dimensions that sit outside the process window. This page is the reference we review against: the numeric rules, the board areas where they usually break, the data package an assembler needs, and the self-check to run before the Gerbers are released.
Design rules
Each row gives the rule, the value this process holds to, why it matters at the machine, and what goes wrong when the layout breaks it. The values are the ones a DFM review is checked against, not a wish list.
| Rule | Value | Why it matters | What a violation costs |
|---|---|---|---|
| Trace width and spacing | 0.15 mm minimum on 1 oz copper; 0.10 mm only where the layout cannot avoid it | Etch tolerance is a fixed percentage of the copper thickness, so a narrower trace loses a larger share of its width to the same process variation | Open circuits at the etch step and a bare-board price 25 to 40% higher, with the yield loss carried on every panel of the order |
| Annular ring | 0.20 mm minimum on a plated through hole; 0.15 mm on a via with a 0.30 mm drill | The ring is what survives drill wander; below the minimum the hole breaks out of the pad on one side | Intermittent barrel-to-pad opens that pass AOI and fail thermal cycling in the field, which is the most expensive place to find them |
| Minimum drill and via | 0.20 mm finished via, 0.25 mm for a via in a thermal plane | Aspect ratio is what limits plating throw; a deep small hole plates thin copper in the barrel middle | Marginal barrel plating, barrel cracks after thermal cycling and a repair that cannot be reworked |
| Stencil aperture area ratio | 0.66 minimum, where area ratio is aperture area divided by aperture wall area | Below that figure the paste releases poorly from the stencil wall and the deposit is short and uneven | Insufficient paste on fine-pitch pads, which shows up as opens and weak joints after reflow rather than at print |
| Stencil thickness | 100, 120 or 150 um foil, chosen from the finest pitch on the board | Deposited paste volume is area multiplied by thickness, so thickness sets the volume for every joint on the board at once | Bridging where the paste is too thick for the pitch and starved joints where it is too thin for a heavy thermal pad |
| Component-to-component clearance | 0.30 mm between 0402 and smaller bodies, measured at the body and not the pad | Placement accuracy on a chip-shooter is plus or minus 0.05 mm, and the nozzle and feeder tolerances add to it | Tombstoning from unequal paste deposits, part-to-part contact after reflow, and design rules that stop the assembly being inspected |
| Courtyard and keep-out | IPC-7351 nominal courtyard on every part, with no via or test pad inside it | Placement, rework and inspection all need physical access to the part that a courtyard reserves | Irreparable assemblies where a part cannot be removed without damaging a neighbouring one |
| Thermal relief on planes | 0.25 mm spoke width with four spokes on through-hole and heavy pads | A pad connected to a plane by solid copper cannot reach reflow temperature with the rest of the joint | Cold joints and voiding on power devices, and hand rework on a joint that should have soldered in the oven |
| Via-in-pad | Filled and capped or plated over, never an open via inside a paste aperture | An open via below a pad drains the paste deposit into the barrel during reflow | Solder starvation and voids under the device, on the packages where rework is least possible |
| Through-hole land | 0.25 mm annular ring and 0.60 mm pad-to-pad spacing at the wave | Wave soldering needs a solder shadow-free path and enough land to form a fillet | Solder bridging between adjacent pins and insufficient fill on a connector row that has to carry current |
| Test point access | 100 test points maximum, 1.0 mm pads on a 2.54 mm grid | Probe count and pitch decide the fixture cost, and fixture cost is a fixed charge against the programme | An in-circuit fixture priced at the top of the 2,000 to 50,000 USD band, or a board that can only be tested by flying probe |
| Fiducials | 1.0 mm copper dot with a 2.0 mm clear area, three per panel and two per fine-pitch block | Global fiducials locate the panel; local fiducials correct the placement of a fine-pitch device that global alignment cannot hold | Placement offset across the panel and a first-article rejection that repeats on every panel until the fiducials are added |
| Panelisation | 5 mm breakaway rails with mouse-bites or V-score on a 300 x 250 mm carrier as a starting point | Assembly is priced per panel, so board count per panel sets the machine time each unit absorbs | Placement cost per unit 20 to 30% above a well-nested panel, and board edge damage where the rails are too thin |
| Silkscreen over pads | No legend inside a paste aperture, and no reference designator smaller than 0.8 mm | Ink on a land reduces the wettable area and can lift into the joint during reflow | Solder balls and poor wetting on the affected pads, plus inspection ambiguity when a designator is unreadable |
Board hotspot review
Rules are read one at a time, but they fail in clusters. These six areas are where a review of a released layout usually ends up, with the value that governs each and the consequence of missing it.
Area 01
Copper area, thermal via array and paste volume on the large pads. Governed by a 0.25 mm thermal spoke and a filled via array. Miss it and the joint runs cold while the rest of the board reflows correctly, which reads as a process fault rather than a design one.
Area 02
Anything at 0.5 mm pitch or finer, where the aperture area ratio rule bites first. Governed by an area ratio of 0.66 on a 0.12 mm stencil. Miss it and the symptom is insufficient paste on a percentage of pads, which appears as opens after reflow.
Area 03
Via diameter, drill size and the annular ring that survives drill wander. Governed by a 0.20 mm minimum annular ring. Miss it and the failure is intermittent rather than immediate, so it appears in the field instead of at final test.
Area 04
Through-hole land, wave soldering direction and edge clearance. Governed by a 0.60 mm pad-to-pad spacing at the wave. Miss it and adjacent pins bridge, or the breakaway rail tears a corner off a connector pad during depanelling.
Area 05
Controlled-impedance lines, a copper-free keep-out and a ground plane that has to stay continuous underneath the feed. Governed by a 50 ohm line within plus or minus 8%. Miss it and the board passes every visual check and fails radiated performance after assembly.
Area 06
Global and local fiducials, tooling holes and the panel frame. Governed by a 1.0 mm fiducial with a 2.0 mm clear area, three per panel. Miss it and the machine places to the wrong origin on every panel, and the first article is wasted discovering it.
Data package
An assembler works from these four. Anything that lives only in a conversation or a screenshot will be re-invented at the machine, usually in the wrong direction.
RS-274X or X2 with the NC drill file, and ODB++ or IPC-2581 where the CAD tool exports it. The output has to name the layer order and the stackup, because a mirrored inner layer is not visible on a routed board and only surfaces when the first assembly fails test. Include the impedance target for every controlled line.
CSV or XLSX with manufacturer part numbers, reference designators, quantities per board and the approved substitutes you will accept. A line that names only a value and a package forces the sourcing desk to guess, and a guessed part is a documented change request before the first build rather than a build.
Centroid file with reference designator, X and Y coordinates, rotation and the side of the board for every placement, plus the stencil aperture drawing or the paste layer. Rotation conventions differ between CAD tools, so the first article is where a 90-degree error is caught, and it is cheaper to catch it in the file.
Polarity, orientation, the workmanship class, any part that must not be substituted, and the coverage you need from AOI, X-Ray, in-circuit test, flying probe or functional test with the limits for each. Without the class on the drawing, Class 2 is assumed and a later move to Class 3 re-prices the assembly.
Pre-release self-check
Each row is a question with a numeric pass condition. Run it against the layout, not against the intent behind the layout.
| No. | Check | Pass condition |
|---|---|---|
| 01 | Trace width and spacing | Nothing below 0.15 mm on 1 oz copper anywhere in the design, checked on the inner layers as well as the outer |
| 02 | Annular ring on every drilled feature | 0.20 mm minimum on a plated hole, 0.15 mm on a via, measured against the finished hole size |
| 03 | Stencil aperture area ratio | 0.66 or better on every aperture for the foil thickness chosen, with window-pane splits on large thermal pads |
| 04 | Courtyard and keep-out | IPC-7351 courtyard present on every part and clear of every via, test pad and neighbouring courtyard |
| 05 | Thermal relief | Four-spoke relief at 0.25 mm on every pad connected to a plane, and no solid copper connection to a through-hole power pin |
| 06 | Via-in-pad treatment | Every via inside a paste aperture is filled and capped or plated over, and named as such in the fabrication note |
| 07 | Test access | 1.0 mm test pads on a 2.54 mm grid, underside access preferred, and no test point inside a courtyard |
| 08 | Fiducials and tooling | Three global fiducials per panel with a 2.0 mm clear area, two local fiducials per fine-pitch device, and the tooling holes dimensioned |
| 09 | Panelisation | Board count and utilisation stated, rails at 5 mm or more, and the depanelling method named |
| 10 | Assembly drawing | Polarity, orientation, workmanship class, no-substitute parts and the test coverage request all present on the released revision |
From drawing to tooling
The output of a review is not an opinion. It is a marked-up drawing with specific values changed, each one traced to a rule and to the cost of leaving it alone. These are the two changes that appear most often.
Where a controlled line is specified as a target with no stackup behind it, the fabricator picks the dielectric and the assembler inherits the result. The review writes the layer heights, the copper weights and the target impedance onto the fabrication drawing, so the coupon measured per lot tests the same construction the designer simulated. On a six-layer board this is usually a one-line change and it removes an entire class of late-stage rework.
A solid paste brick on a large thermal pad cannot release from a 0.12 mm foil, and a 0.5 mm pitch device drawn with full-area apertures will bridge. The review splits the thermal pad into a window-pane pattern, segments the fine-pitch apertures and states the foil thickness on the stencil drawing. Every one of those changes is free before the foil is cut and costs a new stencil after it is, which is why the review happens before tooling is raised.
Designer questions
No. Every dimension taken below the comfortable process window buys yield risk and cost: 0.10 mm trace and space instead of 0.15 mm typically costs 25 to 40% more on the bare board, and via-in-pad with planarisation adds a filled and capped via step. Design to the loosest rules the layout allows and keep the tight dimensions for the places that genuinely need them.
At layout freeze, before the panel is ordered. A DFM review on a released data package is not billed, and it is the cheapest point in a programme to change anything: a stencil aperture change is free before the foil is cut, a footprint change after tooling exists is a new stencil, and a stackup change after fabrication is a new bare board.
They change it more than most engineers expect, because assembly is priced per panel rather than per board. A layout that fits four boards on a 300 x 250 mm panel and one that fits six use the same machine time, so panel utilisation can move the placement cost per unit by 20 to 30% with no change to the design. We propose the panel layout and show the utilisation figure with the quotation.
IPC-7351 nominal density is a sound default for most industrial and consumer work, with least-density land patterns where hand rework or heavy thermal mass is expected. The footprint has to follow the paste aperture ratio rule as well as the pad geometry rule: a 0.5 mm pitch QFN with a 0.12 mm stencil needs a window-pane or segmented aperture, and a pad drawn for a solid paste brick will bridge in production even though it looks correct on screen.
Seven fields. A process engineer replies within one working day.
Seven fields. A process engineer replies within one working day.
Seven fields. A process engineer replies within one working day.