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Mechanical Torque Stress Assessment for Surface-Soldered Leaded Modules

IP.com Disclosure Number: IPCOM000038337D
Original Publication Date: 1987-Jan-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 3 page(s) / 61K

Publishing Venue

IBM

Related People

Engel, PA: AUTHOR [+3]

Abstract

The surface soldering of components for assembly to a printed circuit card, or the like, leaves these surface-soldered components vulnerable to interconnection fracture during handling. A method of determining the risk of fracture is provided by an analytical model using an equation to assess design alternatives including various component carrier sizes, component sizes, material changes and application stress levels. This stress application works as follows. As seen in Fig. 1, a circuit card 1 is held in a fixture 2 that secures the bottom of card 1 against translation while the top of card 1 is displaced linearly, perpendicular to the x, y plane of card 1. The approximate shape of a card subjected to torque follows from the boundary (i.e.

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Mechanical Torque Stress Assessment for Surface-Soldered Leaded Modules

The surface soldering of components for assembly to a printed circuit card, or the like, leaves these surface-soldered components vulnerable to interconnection fracture during handling. A method of determining the risk of fracture is provided by an analytical model using an equation to assess design alternatives including various component carrier sizes, component sizes, material changes and application stress levels. This stress application works as follows. As seen in Fig. 1, a circuit card 1 is held in a fixture 2 that secures the bottom of card 1 against translation while the top of card 1 is displaced linearly, perpendicular to the x, y plane of card 1. The approximate shape of a card subjected to torque follows from the boundary (i.e., support) conditions; it is an antisymmetrical deformation:

(Image Omitted)

w (x,y) = wo . x.y/ bL

(1) where: w = card under deflection

wo = maximum card deflection

b = half base width of card

L = height of card The torque on a card, by a development based on thin plate theory is related to the maximum excursion at the top corners: 3 f L T

wo = Ec h3c (2) where: f = shape factor for card, equal to
1.31 for rectangular

shapes.

T = torque (in .lb)

hc = card thickness

Ec = card modulus Forces in the leads, and thus the solder joints, arise because of the tendency of the card to force bending of the module attached to it. The leads are located evenly spaced along the periphery of the module, and have finite stiffness. When torque is exerted on a card, solder joint failures occur predominantly on the corner leads, without preference for any particular corner. For a plastic leaded chip carrier lead as shown in Fig. 2, a crack opens first on the outside of the joint, and then follows the outline of the lead. A maximum tensile force arises in the leads at the corners of the module, tending to cause mechanical failure. Therefore, the strength of the module attachment depends on the tensile load carrying capacity Pu of the solder joints. To experimentally determine Pu, modules can be cut away leaving individual leads soldered to a card surface; then a tensile load can be exerted to pull the lead out of the joint. A finite element NASA STRUCTURAL ANALYSIS (NASTRAN) program has been us...