Pcb Landpattern Design.pdf

Land Pattern Creation for Thomas Nau's and Harry Eaton's PCB

Stephen Meier

May 2 nd , 2003

Introduction:

PCB needs a land pattern for each device. The land pattern tells PCB how to draw the device pads or

pin holes, silk screen outline and device name. The land pattern can critically effect the

manufacturability of the board. If the pads are in the wrong place it can be impossible to attach the

device to its pads. If the solder mask doesn't cover traces near pads, the traces may become soldered to

the pads. Boards using Land patterns that have the pads in the correct spot but of the wrong size can

have a reduced manufacturing yield and possibly a reduced life.

See the standards document IPC-SM-782A Surface Mount Design and Land Pattern Standard for a

more complete discussion of the requirements and impacts of surface mount patterns.

The following is an example of a PCB land pattern for an 0603 chip resistor. It makes use of three

different macros to create the land pattern. These are Element, Pad and ElementLine. A 0603 chip

resistor requires two pads. Other devices may require hundreds of pads. The four lines are reproduced

only on the silkscreen layer.

Element(0x00 "Surface Mount Chip Resistor 0603" "R0" "" 0 0 -31 -82 2 100 0x00)

(

Pad(-2 0 2 0 39 30 50 "pad 1" "1" 0x00000100)

Pad(65 0 69 0 39 30 50 "pad 2" "2" 0x00000100)

ElementLine(-21 -35 87 -35 5)

ElementLine( 87 -35 87 35 5)

ElementLine( 87 35 -21 35 5)

ElementLine(-21 35 -21 -35 5)

)

Each PCB Pad impacts several layers. If the Pad is on the component surface it impacts the component

layer, the component mask (component side solder mask) and the component paste. If the component

layer has a polygon then the polygon is cleared away from the pad by an amount entered in the Pad

macro.

Each instance of a macro needs its parameters selected for the manufacturing techniques used to place

and solder the components to the board. The standards document (IPC-SM-782A ) will cover these in

detail. The scope of this paper will be how to use the standards document to generate suitable PCB

land patterns.

Element macro:

Element element_flags, description, pcb-name, value,

mark_x, mark_y, text_x, text_y,

text_direction, text_scale, text_flags

Element(0x00 "Surface Mount Chip Resistor 0603" "R0" "" 120 30 -31 -82 0 100 0x00)

element_flags - unsigned integer

description - string

pcb-name - string

value - string

mark_x integer One would believe that this would be the location of the

mark_y integer mark. However, the mark seems to be at point (0,0) no

mater what is entered here.

text_x integer - This does set the location for pcb-name string

text_y integer

text_direction - integer -

0 normal horizontal

1 vertical - counter clockwise 90 degrees

2 upside down horizontal - counter clockwise 180 degrees

3 vertical counter clockwise 270 degrees

text_scale - integer

text_flags unsigned integer

Within the Element macro body are the other components of the land pattern. Each pad or

pin for the device needs a pad or a pin hole. Generally a silkscreen outline is also

provided. The body is the code with in the parentheses. For the above example the body

is:

Pad(-2 0 2 0 39 30 50 "pad 1" "1" 0x00000100)

Pad(65 0 69 0 39 30 50 "pad 2" "2" 0x00000100)

ElementLine(-21 -35 87 -35 5)

ElementLine( 87 -35 87 35 5)

ElementLine( 87 35 -21 35 5)

ElementLine(-21 35 -21 -35 5)

Developing the Land pattern:

Determine the mark (center) of the pattern

Determine the rotation of the device around the pattern mark

Determine the Grid placement courtyard and its relationship to the center

Determine the soldering method to be used

Determine the pad locations and sizes

Determine the solder mask application method and its tolerances

Determine the solder mask size

Create the Element Macro

Add a PAD macro for each component PAD

Add enough ElementLine Macros to create the pattern outline. The pattern outline needn't

encircle the grid placement courtyard but doing so can be convenient for correct

placement.

Chip Resistor Land Patterns

Type

Grid

402

51.2

27.5

35.4

86.6

15.739.4x118.1

603

66.9

39.4

43.3

110.2

23.6157.5x118.1

805

74.8

59.1

51.2

126.0

23.6157.5x315.0

1206

110.2

70.9

63.0

173.2

47.2157.5x393.7

1210

110.2

106.3

63.0

173.2

47.2118.1x393.7

2010

173.2

106.3

70.9

244.1

102.4118.1x551.2

2512

220.5

126.0

70.9

291.3

149.6315.0x629.9

Dimensions C, X, Y, Z, G and Grid are all in mils.

Data is derived from the table on page 73 Of IPC-SM-782A Surface Mount Design and Land Pattern

Standard

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: