The capability extract shows in thirty seconds what runs as standard and what carries a surcharge.
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To the data check →The green layer does three jobs: it stops solder going where it should not, it protects the copper underneath, and it insulates adjacent tracks. The width of resist left standing between two pads — the dam — is what limits how fine your assembly can go.
Modern solder mask is a liquid photoimageable resist: coated, exposed through a film or by direct imaging, developed and thermally cured. Direct imaging is what has pushed registration down to ± 50 µm and made 100 µm dams routine — with film, both figures were roughly double.
The design decision that matters most is how pads are defined. A non-solder-mask-defined pad has resist pulled back from the copper, so the copper edge defines the joint; this is the default and gives the most reliable solder fillet. A mask-defined pad has resist overlapping the copper, so the mask opening defines the joint — used mostly for BGA balls where a consistent opening diameter matters more than fillet geometry.
The minimum dam between adjacent pads sets the finest pitch that can be assembled reliably. It is a mask constraint, not a copper one.
Laser direct imaging holds ± 50 µm registration without a phototool, which is what makes fine-pitch mask geometry practical.
White for LED reflectivity, black for optical assemblies and contrast, green for inspection legibility. It is not purely cosmetic.
| Parameter | Standard | Advanced | Note |
|---|---|---|---|
| Min. dam width | 70 – 150 µm | — | By colour: green 70, red 100, blue / black 135, white 150 µm; green on flex 100 µm |
| Registration | ± 65 µm | ± 50 µm | Laser direct imaging |
| Mask clearance | 60 µm | 50 µm | Opening larger than pad, per side |
| Thickness on copper | 15 – 25 µm | 10 – 15 µm | Over track edges — thinnest point |
| Thickness on laminate | 25 – 40 µm | 20 – 30 µm | Between tracks |
| Min. opening | 150 µm | 125 µm | Smallest reliably developed feature |
| Colours | green | black, white, blue, red | Non-green may add lead time |
| Standard | IPC-SM-840 Class T | Class H | Class H for higher reliability |
Guide values for pre-selection. Binding values are confirmed per project against your data.
The points below are what most often comes back as a query after data review. Settling them before layout freeze saves a loop.
Both are aggressive baths that attack the resist edge. A dam that survives HASL may undercut in an ENIG line — allow 20 – 30 µm more where the design permits.
Pulling the mask back from the copper gives a proper fillet and a more reliable joint. Reserve mask-defined pads for BGA balls where opening consistency matters more.
Keep at least 0.3 mm clearance between resist and any wear surface. Mask on a contact interferes with the socket and can flake into it.
A 50 µm clearance with ± 50 µm registration means the mask can just touch the pad edge. On critical fine-pitch areas, allow more.
Yes. White reflects, which matters for LEDs. Black absorbs, which matters for optical sensing and looks better on consumer products but makes inspection harder. Non-green colours are sometimes slightly thicker and can carry a small lead time penalty. Colour also sets the minimum dam: 70 µm in green, 100 µm in red, 135 µm in blue and black, 150 µm in white.
With a 100 µm dam and ± 50 µm registration, 0.4 mm pitch is comfortable and 0.35 mm is achievable. Below that you are into mask-defined pads and tighter process control.
Yes, and if you tent a via it must be both sides — one-sided tenting traps chemistry that outgasses at reflow.
Class T covers general industrial and consumer work. Class H tightens the requirements on adhesion, moisture and insulation resistance and is normal in automotive and high-reliability applications.
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