Posts Tagged ‘fiducials’

3 June, 2011
IPC introduced a new padstack naming convention in the IPC-7351B standard publication and it is used exclusively in the Mentor Graphics LP calculator. This article explains the breakdown of the new standard and its benefits.

The padstack consists of combinations of letters and numbers that represent shape, or dimensions of lands on different layers of printed boards or documentation. The name of the padstack needs to represent all the various combinations. These are used in combination with the land pattern conventions defined herein according to the rules established in the IPC-2220 Design standards.

The first part of the padstack convention consists of a land (pad) shape. There are six basic land shape identifiers. Note: All alphabetical characters are “lower case”. This helps discriminate numeric values.

Basic Land Shape Letters –

  • c = Circular
  • s = Square
  • r = Rectangle
  • b = Oblong
  • d = D-shape (Square on one end and full radius on the other end)
  • u = User defined contour (Irregular shape)

 The “b” was used for Oblong because the letter “o” can easily be confused with the character zero “0”.

 The next section of the naming convention addresses assumed defaults. This is to keep the default padstack name short and simple. Any deviations from these padstack defaults require the use of special modifiers.

  • Solder Mask is 1:1 scale of the land size
  • Paste Mask is 1:1 scale of the land size
  • The Assembly Layer land is 1:1 scale of the land size

  • Inner Layer Land is the same shape as the outer layer land

  • The Primary and Secondary lands are the same size

  • The inner layer land shapes are Circular

  • Vias are Circular

  • Thermal ID, OD and Spoke Width sizes follow the IPC Level A, B or C

  • Thermal Reliefs have 4 spokes

  • Plane Clearance Anti-pad size follows the IPC Level A, B or C

  • Mounting Holes are Circular

 Every board fabricator’s ability to register solder mask is different. The 1:1 scale solder mask default compensates for the variation, and so long as manufacturers are building to standard specifications such as the IPC-6012 that states you can’t have miss-registration of the solder mask. It’s important that when you are creating a CAD library that will be used for various trace/space combinations, that you leave the responsibility of the solder mask swell up to the fabrication CAM operator when they are panelizing your Gerber or ODB++ data. By having all of the solder mask sizes 1:1 scale of the land (pad) size, you are providing the manufacturer with a known starting point for them to work with.

I need to explain an exception to this rule for creating solder mask defined lands for BGA’s. IPC does not recommend solder mask defined BGA CAD library parts but some companies use this technique for very fine pitch parts that require a small diameter land size. In this case, the solder mask acts as an adhesive to secure the land to the PCB Prepreg to withstand drop testing for hand held electronic products. It has been proven in drop tests for hand held electronic devices that a fine pitch BGA solder joint is more secure than the land attachment to the Prepreg. i.e.: during drop testing, a fine pitch BGA pad will rip away from the PCB Prepreg material before the BGA solder joint fails. See the picture on the right side in Figure 1 as an example of a solder mask defined BGA land.   

Figure 1 - Solder Mask Defined BGA Land

Figure 1 - Solder Mask Defined BGA Land

 

 Solder mask defined lands are also used for Flexible circuit boards for the same reason, to hold the land (pad) to the PCB surface to prevent the land from ripping away from the PCB material. When you use solder mask defined lands you must indicate which parts deviate from the 1:1 scale solder mask rule in the fabrication drawing notes to notify the CAM operator not to swell these solder mask features.

 In the padstack naming convention there are illegal characters that should never be used. These include “ ” , ; : / \ [ ] ( ) . { } * & % # $ ! @ ^ =

 Examples utilizing the “Basic Land Shape Letters” (all padstack values are in metric units)

 Note: Every number goes two places to the right and as many places as needed to the left of the decimal

 Examples: 1150 = 11.50 mm or 11500 μm, 150 = 1.50 mm or 1500 μm, 15 = 0.15 mm or 150 μm

c150h90 - where “c” denotes a Circular land with a 1.50 diameter and H denotes a hole size of 0.90

v50h25 – where a “v” denotes a via with a 0.50 land (default Circular land) and H denotes a 0.25 hole

s150h90 – where “s” denotes a 1.50 Square land and H denotes a hole size of 0.90

s350 – where ‘s” denotes a square SMT land size of 3.50

r200_100 – where “r” denotes a Rectangular SMT land 2.00 land length X 1.00 land width

b300_150 – where “b” denotes a SMT Oblong land size of 3.00 X 1.50

b400_200h100 – where “b” denotes an Oblong land size of 4.00 length X 2.00 width and 1.00 hole

d300_150 – where “d” denotes land with one circular end and one square end (looks like a D) 3.00 X 1.50

v30h15l1-3 – where “v” denotes a 0.30 blind via with 0.15 Hole; 1 is the starting layer, 3 is the end layer

r200_100r5 – Rounded Rectangular 2mm X 1mm X 0.05mm radius corners

r200_100c10 – Chamfered Rectangular 2mm X 1mm X 0.1mm chamfered corners

v30h15l3-6 – where “v” denotes a 0.30 buried via with 0.15 Hole; 3 is the starting layer, 6 is the end layer

  Special modifiers are used when padstack features are different than the defaults. These are the “Variants” or “Modifiers” that go after the basic padstack naming convention.

 These are used when the User needs to change the padstack default values either by a different dimension or a different shape. In instances where shapes are different this becomes a two letter code with the modifier first followed by the land shape letter.

 

These are single letter modifiers –

n = Non-plated Hole

z = Inner Layer land dimension if different than the land on primary layer

x = Special modifier used alone or following other modifiers for lands on opposite side to primary layer land dimension

t­ = Thermal Relief; if different than IPC standard padstack – tid_od_sw for 4 spoke default      

m = Solder Mask if different than default 1:1 scale of land

p = Solder Paste if different than default 1:1 scale of land

a = Assembly surface land if different than default 1:1 scale of land

y = Plane Clearance (Anti-pad) if the value is different than the Thermal OD

o = Offset Land Origin

k = Keep-out

r = Radius for Rounded Rectangular Land Shape

c = Chamfer for Chamfered Rectangular Land Shape

 

These are double letter modifiers –  

ts = Thermal Square; if different than the top side land shape and dimensions

sw = Thermal spoke width

zs = Inner Layer Land Shape is Square (Note: The default is circular)

m0 = No Solder Mask

mxc = Solder Mask Opposite Side Circular

mx0 = Solder Mask Opposite Side No Solder Mask

xc = Opposite Side Circular

vs = Via with Square land

hn = Non-plated Hole

 

 Land shape change is the last letter in the string prior to the dimension.

 Examples of single letter modifiers with a Circular Plated Through-hole land –

c150h90 = Default padstack with a 1.50 circular land with a 0.90 hole (no modifiers used)

c150hn90 = Default padstack with a 1.50 circular land with a 0.90 non-plated hole (no modifiers used)

c150h90z140 = Inner layer land is smaller than external lands 1.40 or 0.10 smaller

c150h90z140x170 = Opposite side land is larger than top side land 1.70 or 0.20 larger

c150h90z140x170m165mx185 = Solder mask opening for top and bottom lands 0.15 larger for each

c150h90z140x170m165mX185a200 = Assembly drawing land in 0.50 larger than 1.50 primary land

c150h90z140x170m165mx185a200y300 = Plane clearance anti-pad diameter is 3.00

c150h90z140x170m165mx85 = Solder mask encroachment on opposite land by 0.65 smaller

c150h90m165 = adding a solder mask opening of 1.65 diameter or 0.15 larger than land

c150h90t150_180_40 = Thermal ID 1.50, OD 1.80, Spoke Width 0.40, Anti-pad 1.80

c150h90t150_180_40y200 = Anti-pad 2.00 (because the size is different than the Thermal OD)

c150h90t150_180_80_2 = Spoke Width 0.80 with 2 Spokes

c150h90m165t150_180_40 = Solder Mask 1.65

c150h90zc150 = where “c” is Circular 1.50 land with 0.90 Hole with 1.50 inner (Z) Layer Circular land

 

 Examples of single letter modifiers for an Oblong Surface Mount land –

b300_150 = Default padstack with a 3.00 length and 1.50 width land (no modifiers used)

b300_150m330_180 = Solder Mask is 0.30 larger than the land

b300_150m330_180p240_140 = Solder Paste is smaller by 0.10 width and 0.60 length

b300_150b-50 = Oblong Land 3.0mm X 1.5mm w/Offset Origin negative 0.5mm

r400_200po430_230 = Rectangle SMT land 4.00 X 2.00 with a Oblong Solder Paste size of 4.30 X 2.30

 

 Examples of a padstack with Oblong land with Slotted Hole –

Sample – b = Oblong Land Shape then “X” dimension (length) then Underscore _Y” dimension (width)

b400_200h300_100 = Oblong land 4mm length X 2mm width with slotted hole size 3mm X 1mm

b400_200hn300_100 = Oblong land 4mm X 2mm with non-plated slotted hole size 3mm X 1mm

 

 Chamfered & Rounded corner modifiers are used to indicate which corner(s) are modified.

 See figure 2 for the “order of precedence” that has been given to the first 4 modifiers.

Figure 2 - Chamfered Land Variations

Figure 2 - Chamfered Land Variations

 

 Modifiers:

  • bl = bottom left
  • br = bottom right
  • ul = upper left
  • ur = upper right
  • ulr = upper left & right
  • blr = bottom left & right
  • ubl = upper and bottom left
  • ubr = upper and bottom right

 Rounded and Chamfered lands in “one corner” Modifier Examples:

r100_200rbl50 = rectangular land 1.00 x 2.00 with 0.50 radius for rounded corner in bottom left corner

r100_200rbr50 = rectangular land 1.00 x 2.00 with 0.50 radius for rounded corner in bottom right corner

r100_200rul50 = rectangular land 1.00 x 2.00 with 0.50 radius for rounded corner in upper left corner

r100_200rur50 = rectangular land 1.00 x 2.00 with 0.50 radius for rounded corner in upper right corner

r100_200cbl50 = rectangular land 1.00 x 2.00 with 0.50 chamfer for chamfer corner in bottom left corner

r100_200cbr50 = rectangular land 1.00 x 2.00 with 0.50 chamfer for chamfer corner in bottom right corner

r100_200cul50 = rectangular land 1.00 x 2.00 with 0.50 chamfer for chamfer corner in upper left corner

r100_200cur50 = rectangular land 1.00 x 2.00 with 0.50 chamfer for chamfer corner in upper right corner

 

Chamfered and Rounded Rectangular with all four corners chamfered does not need a corner modifier.

Modifier Examples with Rounded Rectangle Land Shape: 

Rounded Rectangular Land Shape

Rounded Rectangular Land Shape

r200_100culr50 = rectangular land 2.00 x 1.00 with 0.50 chamfer for chamfered corners in 2 corners

r200_100c50 = rectangular land 2.00 x 1.00 with 0.50 chamfer for chamfered corners in all 4 corner

 

Modifier Examples with Chamfered Rectangle Land Shape: 

Chamfered Rectangular Land Shape

Chamfered Rectangular Land Shape

r100_200r50 = rectangular land 1.00 x 2.00 with 0.50 radius for rounded corners in all 4 corners

r200_100r50 = rectangular land 2.00 x 1.00 with 0.50 radius for rounded corners in all 4 corners

 

 Thermal pads can have a combination of chamfered and rounded corners however the typical application is 2 variations. The most prominent is a chamfered corner located near pin 1 and the second is a chamfered corner located near pin 1 with the other 3 corners rounded. These two variations are the default.

Square Configurations

Thermal Pad with 4 Square Corners

Thermal Pad with 4 Square Corners

s480p4s152 = 4.80mm Square Land with 4 Paste Mask Squares 1.52mm each

Thermal Pad with Chamfered Corner

Thermal Pad with Chamfered Corner

u480p4s152cul50 = 4.80mm Square Land with 4 Paste Mask Squares 1.52mm each with 0.50mm Chamfer in Upper Left corner

Thermal Pad With Chamfered Corner Rounded Corners

Thermal Pad With Chamfered Corner Rounded Corners

u480p4s152cul50r25 = 4.80mm Square Land with 4 Paste Mask Squares 1.52mm each with 0.50mm Chamfer in Upper Left corner with 0.25mm corner Radius

 

 Example of a Local Fiducial for Fine Pitch SMT Components:

c100m200k200 = Circular Land 1.00 with Solder Mask 2.00 with Keep-out 2.00

s100m200k200 = Square Land 1.00 with Solder Mask 2.00 with Keep-out 2.00

See Figure 3 for Local Fiducial application used for fine pitch components.  

Figure 3 - Local Fiducials

Figure 3 - Local Fiducials

 

 Did you know that you can download a free 10-day trial license for LP Wizard here – http://www.mentor.com/go/lpwizard

 After the 10-day trial license ends, the LP Wizard will run in “Demo Mode” as an IPC-7351B LP Calculator. The LP Calculator auto-generates padstack names using the convention mentioned in this article.  

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