qfn module#

The QFN Landpattern Generator#

The QFN landpattern has three basic entry points for generating a QFN landpattern, the QFN, QFNDecorated, and QFNBase classes. The QFN is the most off-the-shelf class which has the most defaults set, including a numbering scheme using LinearNumbering, with pads starting at .p[1] in the top-left, going counter-clockwise around the chip. The QFNDecorated class is the same landpattern, including silkscreen decorations but without a numbering scheme. It’s not usable as-is, and needs to be subclassed, mixing in a numbering scheme. Finally there’s the QFNBase class, which is the most basic class, which only declares the pads, with no silkscreen decorations.

A QFN is based on the QuadColumn class, which generates 4 “columns” of leads, and can be instantiated with either the number of leads (divisible by 4, distributed evenly on all sides) or the number of rows, which are laid out on each side. Optionally the number of rows can be a two-tuple to specify a different number of rows on left/right and top/bottom.

>>> class MyComponent(Component):
...     landpattern = QFN(num_leads=16)  # 4 leads on each side, 16 total

>>> class MyComponent(Component):
...     landpattern = QFN(num_rows=4)  # 4 leads on each side, 16 total

>>> class MyComponent(Component):
...     landpattern = QFN(num_rows=(4, 6))  # 4 on left/right, 6 on top/bottom

To further customize the landpattern, various customization methods are available, the main one being the lead_profile(), which allows specifying the lead profile for the landpattern using a LeadProfile object. If no other mechanism is used to declare size and placement, this call is required.

A full example of a QFN component could look something like this:

>>> from jitx.toleranced import Toleranced as T
>>> from jitxlib.landpatterns.generators.qfn import QFN, QFNLead
>>> from jitxlib.landpatterns.ipc import DensityLevel
>>> from jitxlib.landpatterns.leads import LeadProfile
>>> from jitxlib.landpatterns.package import RectanglePackage
>>> from jitxlib.landpatterns.pads import SMDPadConfig, WindowSubdivide
>>> from jitxlib.symbols.box import BoxSymbol
>>> class QFNComponent(Component):
...     A = Port(), Port(), Port(), Port()
...     B = Port(), Port(), Port(), Port()
...     C = Port(), Port(), Port(), Port()
...     D = Port(), Port(), Port(), Port()
...     GND = Port()
...     def __init__(self):
...         self.symbol = BoxSymbol()
...         width = T(5.0, 0.05)
...         height = T(0.8, 0.05)
...         pitch = 0.5
...         lead_length = T(0.4, 0.05)
...         lead_width = T(0.25, 0.05)
...         package_body = RectanglePackage(width=width, length=width, height=height)
...         self.landpattern = (
...             QFN(num_leads=16)
...             .package_body(package_body)
...             .density_level(DensityLevel.A)
...             .thermal_pad(
...                 shape=rectangle(3.7, 3.7),
...                 config=SMDPadConfig(paste=WindowSubdivide(padding=0.25)),
...             )
...             .lead_profile(LeadProfile(width, pitch, QFNLead(lead_length, lead_width)))
...         )

Note that we’re using the default pad-mapping here which will map the declared ports in order, ending with the thermal pad. If you want your ports organized differently you need to also declare a PadMapping where the pins are accessible through self.landpattern.p[1]` up to and including .p[16] with the additional thermal pad accessible as .thermal_pads[0].

It’s also worth noting that the density level is optional, and if not specified it will be read from the context which can be set for the entire design (or circuit) using the DensityLevelContext, which defaults to C if unset.

class QFNLead(length, width, lead_type=LeadProtrusion(name='QuadFlatNoLeads', fillets={<DensityLevel.A: 'A'>: LeadFillets(toe=0.4, heel=0.0, side=-0.04, courtyard_excess=0.5), <DensityLevel.B: 'B'>: LeadFillets(toe=0.3, heel=0.0, side=-0.04, courtyard_excess=0.25), <DensityLevel.C: 'C'>: LeadFillets(toe=0.2, heel=0.0, side=-0.04, courtyard_excess=0.1)}))[source]#

Bases: SMDLead

QFN Lead

This class specifies the default lead for QFN landpatterns.

Parameters:
lead_type: LeadProtrusion = LeadProtrusion(name='QuadFlatNoLeads', fillets={<DensityLevel.A: 'A'>: LeadFillets(toe=0.4, heel=0.0, side=-0.04, courtyard_excess=0.5), <DensityLevel.B: 'B'>: LeadFillets(toe=0.3, heel=0.0, side=-0.04, courtyard_excess=0.25), <DensityLevel.C: 'C'>: LeadFillets(toe=0.2, heel=0.0, side=-0.04, courtyard_excess=0.1)})#

Lead Protrusion Type

The default value for QFN leads is QuadFlatNoLeads.

class QFNBase(num_rows=None, *, num_leads=None)[source]#

Bases: CornerPadChamfer, ThermalPadGeneratorMixin, QuadColumn

QFN Landpattern Generator Base

Parameters:
class QFNDecorated(*args, **kwargs)[source]#

Bases: SilkscreenOutline, Pad1Marker, ReferenceDesignatorMixin, ExcessCourtyard, QFNBase

Decorated QFN Landpattern Generator, with no pad numbering scheme.

class QFN(*args, **kwargs)[source]#

Bases: A1, LinearNumbering, QFNDecorated

QFN Landpattern Generator