PCB Case Studies & Practical Application 2

Question 1

A high-speed DDR4 memory interface is experiencing intermittent data errors. The trace lengths between the controller and DIMMs vary by 800 mils. What is the most likely cause?

Accepted Answer

Length mismatch causing timing skew violations

Answer

DDR4 requires tight length matching (typically within 25 mils for data groups) to prevent setup/hold timing violations from propagation delay differences.

Question 2

A power supply PCB fails thermal inspection. The 5A trace connecting the inductor to output capacitors is 10 mils wide on 1oz copper. What corrective action is most appropriate?

Accepted Answer

Widen the trace to at least 80 mils or add copper pours

Answer

A 10-mil, 1oz copper trace can only safely carry about 0.5A; a 5A trace requires roughly 80 mils width on 1oz copper using the IPC-2221 standard.

Question 3

During EMC pre-compliance testing, a PCB radiates excessively at 120 MHz. The clock frequency is 60 MHz. Which layout change is most likely to reduce this emission?

Accepted Answer

Reduce the clock trace loop area by routing return path directly beneath the signal trace

Answer

The 120 MHz emission is the second harmonic of the 60 MHz clock; minimizing the loop area of the clock signal and its return path reduces the antenna effect causing radiation.

Question 4

A USB 2.0 full-speed device fails eye diagram testing. The differential pair has 3 vias and a 2-inch stub. What is the primary issue?

Accepted Answer

The via stubs create resonances that distort the signal at USB frequencies

Answer

Via stubs act as transmission line stubs that cause resonant notches in the frequency response, degrading signal integrity even at USB 2.0's relatively modest 480 Mbps.

Question 5

A medical device PCB must meet IPC Class 3 standards. The manufacturer reports 4-mil annular rings on 10-mil drill holes. Does this meet Class 3?

Accepted Answer

No, Class 3 requires minimum 5-mil annular ring for supported holes

Answer

IPC-6012 Class 3 requires a minimum 5-mil (0.127mm) annular ring for supported holes, so 4-mil rings would be a defect in a Class 3 medical device.

PCB - Design Practice Test

PCB - Design Practice Test

PCB Case Studies & Practical Application 2