Environmental Stress Screening (ESS) is a process to determine the reliability of an electronic component or assembly to a given standard or end-use requirement making it an integral design tool during the development phase. This process can be used to take the product to failure to determine how well the product performs under extreme conditions. For example, the reliability of solder joints can be tested using mechanical shock testing, one of many ESS processes. Mechanical shock is part of an ESS test plan that has great impact on solder joints, where failure can be critical. Mechanical shock testing simulates the sudden applied force or change of movement through handling, shipment, and field use. Some effects of mechanical shock are: 1) components detached from the printed circuit board; 2) cracks in the solder joints; and 3) multi-leaded component leads detached.

Following are some suggestions that may assist in successfully performing mechanical shock testing to PCB assemblies:

1. Mounting
Mounting to the shock machine will take a bit of planning and designing to accomplish the testing. In many cases, the board will require a fixture or mounting plate (Figures 4-1 and 4-2) to secure the board to the shock machine. When designing fixtures for the mounting of boards to the shock machine, the following should be considered:

Payload allowance
Payload allowance is the amount of weight of the test boards and its fixtures that the machine can handle without
compromising the machine performance.

Size limit
Size limit allowance is the dimensions of the test boards and its fixtures which the machine can handle without making the machine out of balance. The payload and size limit allowance will also affect the output of the desired spectrum profile from the test procedure.

Mounting screw pattern and dimensions
The mounting screw pattern and dimensions will center the boards and its fixtures.

Thickness of the fixture
The thickness of the fixtures is generally ¾ to 1 inch thick. This is to ensure that the fixture will not flex to the shock applied to the boards and the boards are the only items that are affected by the shock applied.

Material for the fixtures
The material generally used for fixtures is aluminum 6061, which can be easily machined.

Number of boards and positioning of multiple boards
The number of boards to be tested each time determines the positioning of the boards to the fixture and determines if stacking is necessary.

Support of the boards
Board support is critical to the design of the fixtures, as it will affect the result of the mechanical shock testing. In most cases, boards are designed to have mounting holes where screws, posts, and stand-offs support the boards in the housing. In other cases, the boards slide into guides, where they may be attached to an edge connector and locked with a lever. Board support during shock testing should simulate the board support in the board housing. For the purpose of testing solder joints, board support should be at least along all edges of the boards.

Follow the specified torque setting for the type of screw and bolt used. Refer to Machinery’s Handbook Guide or torque chart for screw, bolts and nuts. The depth of the tapped hole and length of the screw or bolt aids in the prevention of screws and bolts flying off during the testing.

2. Cable
If the procedure requires that the boards are to be monitored during the testing, then cabling will need to be addressed.

Some data collection systems generally are supplied with cables but may not comply with the rigorous conditions of mechanical shock testing. This compliance should also be a concern when purchasing cable assemblies. These cables should be thoroughly inspected for any flaws that may hinder the data collection.

If hand assembling the cables, the following should be considered:

• Connectors need to comply with the mechanical shock test conditions
• Wire preparation should meet J-STD-001C
• Soldering should meet acceptability of IPC-A-610 and/or IPC/WHMA-A-620
• Strain relief should meet acceptability of IPC/WHMA-A-620

Not all commercial off the shelf connectors will be suitable for mechanical shock testing. Choosing the connector that meets the acceptance criteria of IPC-A-610 and/or IPC-WHMA-A-620 should allow the cable assembly to conform to the test conditions.

Another consideration for data capture assurance is the mounting and securing of cables. The fixture should be designed not only for the boards but also for the cables used in testing. The positioning of the cables should be placed so that they do not lie on the boards or brush against the boards. Ideally, the cables should lie parallel to the edge of the board and have a space between the edge and the cable. Securing the cable should allow little or no movement at the connectors, unless the test also requires the evaluation of the connector and cable.

Securing the cable at the edge of the fixture should not add more weight or mass to the boards and fixtures. This will also limit the movement of the cable to the edge of the
fixture. The length of the cable, number of conductors, and insulation are all factors that add weight that can cause
damage to the cable at the edge of the fixture. Supporting the excess cable length should be planned for in the setup of the data collection equipment.

Conclusion
As the aforementioned Tech Tips clearly illustrate, planning is the key to successful ESS testing. A technically competent and well planned setup of the test will provide a higher probability of uncompromised results and a successful ESS process.