Chapter 6: How to Build an SMT Leaded Package

A variety of part types and package styles use leads that extends out from the side or bottom of the package body which are soldered to the PCB. These leads offer some compliance to the solder joint, meaning that in general leaded parts have improved thermal cycling solder fatigue performance.

ID Tab

  • Part Type: Surface mount leaded packages are a common style for components with a silicon die. Part Types include Diode, IC, Transistor, Transducer, and so on.

    Tip:  Lead geometries vary within this category. Expect to see gullwing, C-bend, L-lead, stub, and so on. See below, SMT Leaded- Lead Geometry.

  • Failure Class: This can be left blank. The property is not used in any Sherlock stand-alone analysis. It is only used when Sherlock is part of a multi-tool analysis workflow.

SMT Leaded Part Properties:

 

6.1. SMT Leaded Package Properties

Package Tab

  • Package Name: Refer to the part data sheet. Supported values are defined in the package data files provided by Sherlock or user-defined package data files. See Package Management in the Sherlock User's Guide.

    Tip:  You can enter the Package Name manually, or you can use Sherlock's Package Chooser which helps you select a prebuilt part that approximates the one you are building. Sherlock then auto-populates all the relevant properties. These package properties are based on industry standards, so you may have to edit the properties to match the part you are building. To open the Chooser, click the … icon (A, below). See The Package Chooser in the Sherlock User's Guide for more information.

  • Package Type: Refer to the part data sheet.

  • Package Mount: SMT

  • Package Length: Refer to the part data sheet drawing.

  • Package Width: Refer to the part data sheet drawing.

  • Package Thickness: Refer to the part data sheet drawing.

  • Model Part: ENABLED

  • Corner Shape: Typically, SQUARE but refer to the part data sheet drawing.

    A = Square Corner • B = Miter Corner • C = Round Corner • Arrows show corner radius.

  • Corner Radius: Does not apply to SQUARE. See image above for MITER and ROUND corners.

  • Corner Face: TOP_BOTTOM, FRONT_BACK, or LEFT_RIGHT

    D = TOP_BOTTOM • E = FRONT_BACK • F = LEFT_RIGHT

  • Material: OVERMOLD-LEADED. See notes below.

    • Most SMT leaded parts consist of a copper lead frame, connected internally to a silicon die, and overmolded with an epoxy resin.

    • Manufacturers usually do not provide details on the overmold materials. As a general rule, you can use the OVERMOLD-LEADED material from the Sherlock library. It approximates the effective properties of the copper/epoxy content of the package.

    • Military or aerospace rated SMT leaded parts are often constructed with a ceramic package body.

Example only. Refer to manufacturer's part data sheet.
 

6.2. SMT Leaded- Lead Geometry

Tip:  The content below assumes you are using Sherlock's default lead modeling. If you have enabled Advanced Lead Meshing, refer to Advanced Lead Meshing in the Sherlock User's Guide for additional information on how lead properties are defined.

Most lead geometry inputs are clearly stated on the data sheet. The sections below explain the various dimensions Sherlock requires for each lead type. You can also find this information in the following places:

Lead Tab

Tip:  You will find Lead Frame Percentage among the Lead properties for SMTs. This is the percentage of lead material in the layer of the package beneath the die. It is only used when exporting a project to Ansys Electronics Desktop for thermal conductivity calculations.

  • Lead Count: Refer to the part data sheet.

  • Lead Geometry: Refer to the diagram below. Options are C-LEAD, J-LEAD, GULLWING, STUB, L-LEAD.

    A = C-Lead • B = J-Lead • C = Gullwing • D = Stub • E = L-Lead

  • Lead Material: Typically COPPER unless otherwise specified on the part data sheet.

  • For the various lead dimensions (Lead Height, Lead Width, and so on), refer to the relevant Lead Geometry in the subsections below.

Lead geometry subsections:

 

6.2.1. Gullwing Lead Geometry

  • Lead Height: If not provided by the data sheet, use this approximation: half the package thickness plus the standoff.

 

6.2.2. C-Lead/J-Lead Geometry

Sherlock models C and J leads using the same basic elements (Shoulder, Leg, and Foot for example) in the same relative arrangement, with the Foot tucked back under the part. In practice, C leads usually have equal Shoulder and Foot lengths (C, below), whereas the Foot is usually longer than the Shoulder in J leads (J, below). In the real world, J leads also have a toe that curls toward the bottom of the part, but for simplicity and ease of modeling, Sherlock ignores the toe when modeling the J lead. The following diagram depicts how each of the lead properties are used by Sherlock to construct a C_Lead or J_Lead model.

When incorporated into the FEA model, each C or J lead is bonded to the part using the side Shoulder face and bonded to the PCB using the bottom Foot face.

Lead Height is measured from the Foot of the Lead to the center of the Lead Thickness at the shoulder.

  • Lead Height: If the mechanical drawing does not provide this, use the following formula:

    Lead Height = Lead Standoff + 0.5(Package Thickness)

  • Lead Shoulder and Lead Standoff: Leads are often molded into a package body or closely folded around it. In those cases, you can set the Lead Shoulder and Lead Standoff to a small nominal value of 0.1.

  • Lead Standoff = Lead Thickness

 

6.2.3. L-Lead Lead Geometry

Tip:  Lead height is often not given for L-Leads, particularly for V-Chip electrolytic capacitors. In these cases, 1 mm is a good estimate.

 

6.2.4. Stub Lead Geometry

  • Lead Standoff: Refer to the data sheet drawing, but often this information is not provided. If the stub lead protrudes from the bottom of the package, a dimension between 0.05 and 0.1 mm is a good estimate.

  • Lead Shoulder: Typically equals Lead Foot Length.

The following list provides guidance for the different stub lead types:

Stub Foot Only

  • Lead Shoulder = 0

  • Lead Standoff = Lead Thickness

Stub Extended Foot

  • Lead Shoulder < Lead Foot Length

  • Lead Standoff = Lead Thickness

Stub Shoulder Only

  • Lead Shoulder = Foot Length

  • Lead Standoff = 0

Stub Foot & Shoulder

  • Lead Shoulder > 0

  • Lead Shoulder < Foot Length

  • Lead Standoff > 0

  • Lead Standoff < Thickness

 

6.3. SMT Lead- Solder Properties

Solder Tab

  • Solder Material: Typically, DEFAULT

    When set to DEFAULT, Sherlock uses the Default Solder Type specified in the Circuit Card Properties. To view or edit these properties, right-click the circuit card in the Project Tree and select Circuit Card Properties in the context menu.

    If a solder other than the default is used for this part, you may enter the name of the solder in the Solder Material field. To add a custom solder to the Solder Library, see Solder Management in the Sherlock User's Guide.

  • Solder Model: LEADED MODEL

  • Solder Thickness: Sherlock calculates this value. It is based on the stencil thickness as recorded in the Circuit Card Properties and is assumed to be half that value.

    To view or edit Circuit Card Properties, right-click the circuit card in the Project Tree and select Circuit Card Properties in the context menu.

 

6.4. SMT Leaded- Die Properties

Refer to the section Die Properties Guide.