PCBL - Footprint Expert [USER GUIDE] (2024)

The second thing to do after you install Footprint Expert isto open "Tools > Options" in the upper left.

You cannot edit or change any of the Default Options inFootprint Expert. To set up your own personal options and customize yourresulting PCB footprints and/or 3D STEP models, select "File > Save As... > Your Personal.opt"file (specify your own desired filename).

This turns on over 1,000 settings for you to custom edit toyour personal or corporate PCB library rules.

Select Pad Stack Rules

Pad Stack Rules will globally define the rules for everyfootprint created in the Surface Mount and Through-hole Calculators. Theserules are not used for custom FP Designer footprints because you can manuallydefine every pad stack in custom parts. Custom footprints always use thecomponent manufacturer recommended pattern and pad sizes because there is notany standard recommendations or guidelines for unique packages and theirrelated footprint patterns. The default values are best known practice, but youcan edit any value for your PCB library requirements.

1. Solder and Paste Masks

Solder Mask Note:
Solder mask is an epoxy coating that protects the circuit fromcorrosion and electrical shorts. It also provides electrical insulation thatallows higher voltage traces to be placed nearer to each other.

Most importantly, the solder mask keeps the solder on the pads,as opposed to flowing onto traces, planes, or empty board space. This reducesthe likelihood that solder will form bridges (unintended connections) from oneelement to another. Solder masks are critical for wave soldering, which is amass production technique. Solder masks also make hand soldering faster,easier, and more accurate.

a. Minimum Gang Solder Mask Web - 0.075 (3 mil)

• This rule is for PCB fabrication. Thesolder mask stencil must have a minimum 0.075 mm between pads. Anything lessthan that is not manufacturable. When the solder mask web violates the minimumvalue, Footprint Expert will automatically gang the solder mask into a solidblock. If you provide Gerber data with a solder mask web less than 0.075 mm tofabrication, they will automatically gang mask the row of pads for you, and nottell you. If you provide 1:1 scale solder mask to fabrication, they will swellthe mask to meet their fabrication tolerance for their solder mask application.Fabrication CAM tools will alert the operator of all solder mask webs less than0.075, and the operator will gang mask all solder mask areas that violate theminimum solder mask web.

b. Gang Solder Mask Shape - Contour or Block

• A Contour shape is used for Oblong, D-Shape, and Rounded Rectangle pad shapes. Thisis important if you are defining outer layer copper pour.
• This image illustrates Oblong pad shape with a Block Gang Mask using outer layercopper pour and the exposed copper problems that will have on your PCB. Allcopper must be covered with solder mask. Exposed copper, especially near pads,could cause solder bridging. Also, the physical appearance of exposed copper isnot desirable.

• This image illustrates Oblong pad shape with a Contour Gang Mask using outer layercopper pour and the exposed copper problems that will have on your PCB. Allcopper must be covered with solder mask. Exposed copper, especially near pads,could cause solder bridging. Also, the physical appearance of exposed copper isnot desirable.

• If you use a Rectangle pad shape, it does not make any difference if you select Contour orBlock because the solder mask will always conform to the shape of the pad.Image of Block Gang Mask on PCB.

c. Solder Mask Annular Ring Expansion - 0.00

• Some PCB designers use a solder mask expansion between 0.05 - 0.08 because they use thatas a visual for placing reference designators on a final part placement.Silkscreen Legend Ink should not be in an exposed solder mask area. If you placesilkscreen legend in a solder mask free area in your PCB layout, thefabrication process will automatically trim the silkscreen legend away from thefree solder mask area. But many PCB designers leave the solder mask 1:1 scaleof the pad and allow the PCB fabrication shop to automatically swell all soldermask features to meet their solder mask registration tolerance.

d. Paste Mask (% of Pad Size) - 100%

• Normally, paste mask on pads is 1:1 scale but this option allows you toincrease or reduce the amount of Paste Mask on a pad.

2. Corner Rounding

Corner Radius Note:
Some people say that rounded rectanglepad shape is best for lead free solder. This is a myth, as all pad shapes areOK with lead free solder. IPC has recommended Oblong pad shape since therelease of IPC-SM-782 in March 1987. The 782 Standard had both text and graphicillustrations. The text referred to Oblong (or Full Radius) pad shape but theimages portrayed Rectangle Pad Shape. This was confusing to the reader of theStandard. Most component manufacturer recommended patterns for surface mountparts illustrate Rectangular pad shape. The Rounded Rectangle Pad Shape is acompromise between the world standard Oblong and the component manufacturerRectangle. Also, it is important to note that paste mask stencil apertureopenings are laser cut with rounded corners. It makes sense to have the padshape match the paste mask stencil aperture shape.

When the PCB goes through a reflow oven,the paste mask tends to gravitate toward the terminal lead and away from thepad corners. It doesn't make sense to have rectangular pad shapes with nosolder in the pad corners. Here is a photo of both lead and lead-free solderwith no solder in the corners.

a. Corner Radius Size (% of Pad Width) - 25%

• The 25% value is applied to every corner, that is equivalent to 50% of the pad width.

b. Corner Radius Size Limit - 0.10

• The 0.10 default value sets the limit onthe corner radius. Some component manufacturers recommended solder patternsindicate a 0.05 mm corner radius. The corner radius size limit is totally up tothe PCB designer or CAD librarian. There is no recommended Standard for thisradius limit.

3. Manufacturing Tolerances

Manufacturing Tolerances Note:
Manufacturing tolerances have existed inthe IPC mathematical model for surface mount pad size calculation since 1987.The Fabrication Tolerance was used to compensate for the etching process. Butin the 1990's, the fabrication shops started to swell the outer layers tocompensate for their etch back tolerances so they could deliver pad sizesand trace widths the same size as the original Gerber data. But there is alsoan issue with the subtractive process where the etch acid gets slightly underthe photo resist and creates a trapezoidal effect in the trace and pad. IPCuses a 0.05 mm fabrication value in the pad size math to compensate for thissubtractive process. The average copper thickness on the outer layers is 1/2 OZ.or 0.017 mm. The acid under photo resist undercut value is only a smallpercentage of that. Basically, the Fabrication Tolerance added to the pad sizecalculation is no longer applicable.

The Assembly Tolerance used in the IPCmathematical model for pad size calculation is 0.025 and established in 1987. Thisvalue was to compensate for the assembly machine placement accuracy. Today'sadvanced assembly machines place components with a 0.01 accuracy or less. TheAssembly tolerance added to the pad size calculation is no longer applicabletoday.

When IPC developed the solder jointgoals for IPC-7351, side goals for fine pitch semiconductor packages had to beset at negative values to compensate for the robust manufacturing tolerances.If you decide to turn off the manufacturing tolerances, you should also updateevery negative value in the Terminal solder joints to 0.00. By turning off themanufacturing tolerances and changing all negative solder joint goals to 0.00,the pad length & width size is only affected by +/-0.05 (2 mil).

a. Fabrication Tolerance (+/-) = 0.00

b. Placement Tolerance (+/-) = 0.00

4. Minimum SMD Trim and Spacing

a. Minimum Pad to Pad - 0.15

• The minimum pad to pad rule is intendedfor surface mounted parts. When the pad spacing violates this rule, FootprintExpert will automatically trim the pad to enforce this rule and an errormessage will be created that looks like this:

• The value of 0.15 is arbitrary and not ahard-coded standard. The minimum pad to pad spacing rule should never violateyour minimum PCB clearance rule. If your PCB layout is using a minimumtrace/space rule of 0.20 mm then you should set the minimum pad to pad rule to 0.20.
• Also, if you want to ensure a 0.08minimum solder mask web, and your solder mask swell is 0.05, then change thevalue to 0.18. If the minimum pad to pad is 0.15 and the pads violate that rule,the Gang Mask feature will kick in and mask the entire row.

b. Minimum Pad to Thermal - 0.20

• The minimum pad to thermal rule isprimarily established by the majority of component manufacturers' recommendedpatterns, and not any industry standard. If the thermal pad is too big and violatesthis rule, the pads will be trimmed, and a warning message will be created thatlooks like this:
  

c. Minimum Trim Standoff Height - 0.03

• The minimum stand-off is the A1 package body bottom to PCB space dimension in gull wing packages.

• The default "minimum" value of 0.03 mmwill only trim pad Heel under the component package if the A1 value is lessthan 0.03. The main purpose for pad trimming under low profile packages is toeliminate paste mask from touching the plastic body during reflow. The pad Heelis an important part of the gull wing solder joint, but if the Heel pad isunder a low profile package, the solder on the pad Heel is pushed out by theweight of the package creating excess solder.
• IPC-J-STD-001 mentions in table 7-7 Dimensional Criteria - Flat Gull Wing Leads.Note 4: Solder should not extend under the body of surface mount components whoseleads are made of Alloy 42 or similar metals.

5. Thermal Tabs

a. Thermal Tab Solder Mask Expansion (+/-) - 0.00

• Typically, the thermal tab solder maskis 1:1 scale of the pad size. It's OK for the fabrication solder mask toleranceto be +/- 0.05, which could result in the solder mask creeping over the thermalpad. The minimum paste mask aperture is 0.10 mm from the edge of the thermalpad. Most component manufacturers recommend a 1:1 scale solder mask on thermal pads.

b. Thermal Tab Pattern Defined Solder Mask - on/off check box

• The default setting for this feature isturned off because you must accept the responsibility for turning it on.The solder mask defined thermal pad places solder mask in-between the pastemask apertures, basically damming in the paste mask from flowing. The IPC-7093Adescribes all the benefits of this feature.
• Ability to add vias in the solder mask area, so the paste maskwill not flow down the via hole. This could potentially save on fabricationcosts to avoid via plugging which stops the flow of paste mask into the via hole.

• This image provides some basic values for defining a solder mask defined thermal pad.

• This image illustrates a 0.10 mm paste mask stencil thickness for the image above.

• Because the paste mask is dammed in by solder mask, this reducespaste mask voiding because the solder has no where to flow. The amount ofvoiding post reflow should not exceed 25% per IPC-A-610. Voiding occurs whenthe thermal pad is not solder mask defined.

c. Thermal Tab Paste Percentage - 50%

• The default setting is 50% paste maskcoverage. IPC-7093A figure 3-10 recommends that the stencil design provide 50 -60% solder paste coverage on the thermal pad area.

d. Thermal Tab Minimum Pattern Space - 0.20

• The default setting for this feature is0.20 mm. It is important to have a minimum stencil aperture spacing to increasethe stencil web strength. This will make the stencil last longer and work better.

• If you use the solder mask defined thermal pad feature and intendto place vias in-between the paste mask, you need to figure out the via holesize and add 0.20 for the minimum pattern space. Example: if the via hole sizeis 0.20 + 0.20 clearance for the solder mask web, then the thermal tab minimumpattern space should be set to 0.40. Or you can move the calculator part to FPDesigner and edit the pad stack paste mask to set the aperture size and spacing.
• Note 1: The Solder Mask and Paste Mask aperture opening are the same size and location.
• Note 2: 9 vias provide the maximum thermal relief for most packages with thermal tabs.

6. SMD and TH Miscellaneous

a. Cathode/Anode Pin Names - Alphanumeric or Numeric

• The default setting is Alphanumeric, andproduces pin names A & C. This setting is primarily used for schematicsymbols that cannot be Alphanumeric. The preferred pin label assignments forDiodes and LEDs are C and A for Cathode and Anode. This avoids making amistake between the schematic symbol and the CAD library. But if you usenumeric pin names on these polarized devices, you must make sure that theschematic symbol pins 1 & 2 match the PCB footprint pins 1 & 2.