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DFT Basics
Why Perform Manufacturing Testing?
Functional testing verifies that your circuit performs as it was intended to perform. For example, assume you have designed an adder circuit. Functional testing verifies that this circuit performs the addition function and computes the correct results over the range of values tested. However, exhaustive testing of all possible input combinations grows exponentially as the number of inputs increases. To maintain a reasonable test time, you must focus functional test patterns on the general function and corner cases. Manufacturing testing verifies that your circuit does not have manufacturing defects by focusing on circuit structure rather than functional behavior. Manufacturing defects include problems such as the following:
What Are Fault Models?
A manufacturing defect has a logical effect on the circuit behavior. An open connection can appear to float either high or low, depending on the technology. A signal shorted to power appears to be permanently high. A signal shorted to ground appears to be permanently low. Many of these manufacturing defects can be represented using the industry-standard stuck-at fault model. Other faults can be modeled using the IDDQ, or quiescent current fault model.
Functional testing verifies that your circuit performs as it was intended to perform. For example, assume you have designed an adder circuit. Functional testing verifies that this circuit performs the addition function and computes the correct results over the range of values tested. However, exhaustive testing of all possible input combinations grows exponentially as the number of inputs increases. To maintain a reasonable test time, you must focus functional test patterns on the general function and corner cases. Manufacturing testing verifies that your circuit does not have manufacturing defects by focusing on circuit structure rather than functional behavior. Manufacturing defects include problems such as the following:
• Power or ground shortsManufacturing defects might remain undetected by functional testing yet cause undesirable behavior during circuit operation. To provide the highest-quality products, development teams must prevent devices with manufacturing defects from reaching the customers. Manufacturing testing enables development teams to screen devices for manufacturing defects. A development team usually performs both functional and manufacturing testing of devices.
• Open interconnect on the die caused by dust particles
• Short-circuited source or drain on the transistor caused by metal spike-through
What Are Fault Models?
A manufacturing defect has a logical effect on the circuit behavior. An open connection can appear to float either high or low, depending on the technology. A signal shorted to power appears to be permanently high. A signal shorted to ground appears to be permanently low. Many of these manufacturing defects can be represented using the industry-standard stuck-at fault model. Other faults can be modeled using the IDDQ, or quiescent current fault model.
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