Layers, stacks & processes

A design is drawn as components and made as a stack of bonded slabs, each formed by a process.

The stack

A stack is an ordered list of layers — slabs bonded face to face along one axis. Layer 0 is the bottom of the stack and the numbers count upward, so a design anchors at its lowest layer and builds upward.

Only the layers you pattern belong to the stack. The glass slide a PDMS chip is bonded to carries no features of yours, so it is not a layer — a typical soft-lithography chip is a one-layer stack, with the glass understood to sit against the face the channels open onto. Add it as a layer only if you need to cut something into it.

Each layer carries:

Thickness.
Material, with an optional laminate backing — a second material filling the space below the deepest feature.
Default face, depth and walls — what the features cut into the layer inherit.
An optional process of its own, which overrides the stack’s.

A bond sits between adjacent layers and carries the alignment tolerance of that joint. The stack’s registration scheme says how those tolerances accumulate. Common datum locates every layer against one reference, while Sequential · RSS and Sequential · worst case locate each layer against the one below it and differ in how they add the error up.

Faces, depth and the roof

A feature is cut into a slab from one side, not floating inside it. Which side it is cut from is the face, Top or Bottom, and how far in it reaches is the depth.

The material left between a feature and the opposite face is the roof, thickness − depth. A thin roof sags shut in a soft material, so the design rules measure the roof directly. Where depth reaches the thickness the feature goes all the way through and the layer is open on both faces there.

A channel is an open trough until a face closes it into a conduit — the neighbouring layer’s, or that of a cover the stack does not model.

A slab in cross-section: one feature cut from the Top face with its depth and roof measured against the slab thickness, one cut from the Bottom face, and one cut through; below, the same trough closed into a conduit by the layer bonded above it

Draw on the Top face

Both faces work, and each feature says which one it is cut from. Draw on Top wherever you can: it is what a new layer defaults to, and the viewport is opaque, so a feature cut from Bottom lies under its own roof and the layer looks unbroken from above until you explode the stack. Much of the app is tuned for Top and more of it will be. How many faces one layer may use at all is the process’s to say — a design rule reports a layer that opens from more faces than the process can form.

The convention puts the cover above your layers, so the exposed outside face is the bottom of the stack. A PDMS chip is one layer whose channels open upward, with the glass understood to sit on top of it; a Quake device is two layers — control at layer 0, flow at layer 1 — with the glass above both. A finished PDMS chip is normally held glass-down, so the viewport shows it the other way up; your design and the files you export are the same either way.

Several stacks, one active

A design carries as many named stacks as you like and fabricates against one active stack. Switching the active stack switches thickness, material, layer count and forming process together, so a prototype target and a production target live in one document.

The base layer is the layer of that stack the open component anchors at. It is a design edit and not a view setting, and the viewport always shows the design at that anchor.

Beside it, the active layer is view state: it is where a newly placed child lands. Changing it edits nothing.

Processes

A process is how one layer is formed. Processes live in a shared pool: each stack names one as the default its layers inherit, and any layer may override it, so a molded base under a laminated film is two processes in one design.

A process is data you author, and it holds:

Deliverables — the artifacts this process hands a shop, which export writes. See Deliverables.
Compensation — the measured shrink of this process, and the scale export derives from it so the finished part lands on size. It is applied at export and nowhere else, so the viewport, the design rules and every number you read are the nominal part. Left unmeasured, the process says so. See Fab processes window for the shrink you author and the scale it derives.
Corner radii it rounds by default — one for inner corners, at a corner of the void, and one for outer corners, at a corner of the material. 0 means sharp, which is an ordinary formed state rather than a missing value.
Wall kinds it can form, each with a forming default: Vertical, Fillet (a radius) and Draft (an angle). A kind the process does not list, it cannot form.
A rule deck — the subject of Design rules.

Presets seed a process you then tune: Soft lithography, Injection molding, Laser cutting, Micromilling, Hot embossing and Xurography. Loading one replaces everything the process holds: walls, corner radii, compensation, deliverables and rule deck. Its name and the stacks pointing at it stay put.

A change of process

Switching a layer’s process rewrites nothing you have drawn. The outlines, centerlines, widths and depths stay as you authored them, and a process your design violates is not refused: the model goes on building, and the Problems section says what that process cannot form.

What does follow the process is what you left inherited. Corner radii come from the process, as does a wall parameter you have not set, so the same drawing is formed at one fab’s radii and then at the next one’s. Override a value and it stays where you put it.

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