Solving Problems of Overly Constrained Boards

Posted by Chris Montgomery on Aug 19, 2016 9:30:00 AM

Overly-Constrained-Boards.jpgToday’s technologies have placed increasing performance demands on boards, and designers are left to contend with the challenges of smaller spaces, higher temperatures and fitting more components on boards.

Overly constrained, bending boards represent more than a conundrum. They can cause failures.

Coefficient of Thermal Expansion (CTE)

When the effective CTE of a board is elevated, it can induce highly localized bending and ruin the ideal flat board. CTE mismatch can occur in three critical areas:

  • Between the board and the components: The board CTE value is no longer valid if it is being affected by an external source.
  • Between the board and the aluminum: Temperature is the culprit here – if it’s too high, the aluminum expands more than the board; too low, and the aluminum shrinks more than the board.
  • On solder joints: Board bend places a shear load on solder joints, creating an additional tensile load. 

Improper Mounting Conditions

Improper mounting conditions have a significant impact on board reliability and, therefore, have been extensively tested to understand their impact. Using Sherlock Automated Design Analysis™ software, the effects of improper mounting conditions in the following situations were predicted and proven:

  • Board bonded to plate: A layer was added in the stackup that represents the epoxy and the aluminum plate. Maximum strain of 700 microstrain was seen at 100°C and 200 microstrain at -40° Board bending was not significant as compared to the stretching witnessed. Effective CTE may be suitable in this instance.
  • Heat sink attached to board: The heat sink was attached to the board using standoffs and a connector supported the whole assembly. In this case, maximum board deflection was 9mm at the tip, with one side constrained by the connector. Thermal-induced bending was seen at 900 microstrain. Effective CTE will not work since the strain is concentrated close to the mounting points; in fact, there is no good model for predicting solder fatigue from board bending.
  • Board attached to housing: In this test, two scenarios were run. In both instances, the board was attached to the plate with standoffs and the plate was constrained only at the bottom:  
    • Scenario 1: Mount points were at the four corners of the board, resulting in a bending motion with .3mm deflection over the span, and 600 microstrain on the bottom of the board. The board was stretched more at the bottom than at the top.
    • Scenario 2: Mount points were at the four corners of the board, as in Scenario 1, but more mount points were added in an attempt to prevent bending. At 100°C, 500 microstrain was seen at the top of the board and 700 microstrain was seen at the bottom of the board – all close to the additional mounting points. At lower temperatures, the board experienced out-of-plane deflection and board buckling due to higher shrinkage in the housing than in the board.

Proving Board Bending

Sherlock can identify the effects of CTE through:

  • Simulation: Isothermal load results simulation reveals the impact of mounting conditions and tracks the solder strain energy density for life prediction.
  • Testing: Boards with strain gauges attached or subjected to digital image correlation are tested using temperature cycling and monitoring, and the results on various mounting conditions are compared. 

Resolving Board Bending

Proper mounting conditions can resolve board bending and, by extension, improve board reliability:

  • Shoulder bolts are designed with non-threaded sections to create a small gap, or shoulder, between the fastener and the board. This prevents board squeezing and allows for slight board adjustments in response to conditions instead of systematically tightening everything to the board with no give.
  • Slotted holes allow the housing to expand more freely without causing friction between the bolt and board or prohibiting Z direction constraint. Slotted holes are slightly wider than the shoulder bolt and at least .1mm larger than the bolt diameter. 

Advancements in technologies are making overly constrained boards an increasingly common occurrence. Sherlock Automated Design Analysis™ software addresses – and solves – board bending issues and improves board reliability. Find out what Sherlock can do for you! Request your free trial now by clicking the button below.

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Topics: Sherlock, Design for Reliability

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