1. For electrical test purposes, circuit board faults are defined as measurement results of the test system that deviate from those programmed to indicate a good board. These faults may or may not affect the functionality of the circuit board, although in most cases, they do.
2. Guidelines can be helpful in the absence of specific direction from the original specifier or designer. Gross shorts and opens in a circuit typically cause significant issues.
3. Can an electrical test system detect all faults? The definition of “all faults” is subjective.
4. The electrical test system will not identify faults related to aesthetics, annular rings, layer-to-layer registration, etc., unless they produce a measurable effect.
5. Furthermore, the effectiveness of electrical testing varies widely depending on factors such as the type of electrical measurement used, the test fixture, method of test program generation, and end-user requirements.
6. It is essential, for the purposes of this discussion, to clarify the distinction between a defect and a fault.
7. A fault is an item identified by the test system that does not meet the expected criteria.
8. A defect, on the other hand, specifically refers to issues with the board itself, including design, fabrication, appearance, etc.
9. Not all defects can be detected by the test system.
Image 1: Circuit board fault
Shorts, hard shorts, or short circuits are defined as erroneous low-resistance connections between two or more networks or isolated points. These connections typically exhibit a fairly low electrical resistance value. Shorts manifest as failures in the product’s isolation testing. They can occur due to various factors such as exposure problems, under-etching, contaminated photo tools, poor layer alignment, defective raw materials, and improper solder leveling.
Opens denote the absence of expected circuit continuity, resulting in a missing connection that divides a circuit network into multiple pieces. Opens are reported as failures in the product’s continuity testing. They can be caused by over-etching, under-plating, contaminated photo tools, impure raw materials, layer registration errors, and mechanical damage. During electrical testing, “false open” errors may occur, often due to localized contamination on the product or test probe hindering proper connection to the test system.
Of particular concern in substrates utilizing microvias are latent defects that may arise after testing during subsequent substrate assembly processes. Examples include improperly formed conductors at stress points where cracks may develop under thermal or mechanical stresses during assembly. Small HDI features are less tolerant of such defects, which may initially go undetected during testing. However, sensitive ohmic measurements may reveal limited conductor cross-sections where future open circuits could occur. Regular detection of these defects underscores the necessity for process adjustments to prevent field failures.
Image 2: electronic test pcba
Many circuits produced today are required to operate at very wide bandwidths. Examples include fast microprocessors, general digital circuits, and RF amplifiers in wireless devices. Just as we use specific types of cables to connect television antennas to TV receivers, maintaining specific RF characteristics in interconnections between components of fast electronic circuits on printed wiring boards is crucial. One commonly specified and measured parameter is the RF transmission line impedance of the signal traces. This impedance is significantly influenced by the materials used in board fabrication, trace thickness and width, and the proximity to ground planes and adjacent signals.
A widely adopted method for measuring RF impedance is Time Domain Reflectometry (TDR). TDR measurements provide a profile of RF impedance along the trace length. (Distance and time are directly related here, considering that electrical signals propagate through the board at speeds approaching that of light.) TDR testing is typically conducted on a test coupon attached during manufacture and later detached. Testing on actual product traces also occurs, but requires uninterrupted trace lengths of several inches, free from branches or other configurations. Typical RF impedance values on circuit boards range from tens of ohms to several hundred ohms.
Image 3: RF Hole
RF impedance should not be confused with ordinary DC resistance and cannot be measured using common ohmmeters, despite both using ohms as the unit of measure. RF characteristics of interconnections are typically evaluated in the frequency domain using instruments known as network analyzers, although this approach is not commonly employed in bare board testing. The need for RF impedance testing becomes more prevalent as signal frequencies surpass 100 MHz.
2. Guidelines can be helpful in the absence of specific direction from the original specifier or designer. Gross shorts and opens in a circuit typically cause significant issues.
3. Can an electrical test system detect all faults? The definition of “all faults” is subjective.
4. The electrical test system will not identify faults related to aesthetics, annular rings, layer-to-layer registration, etc., unless they produce a measurable effect.
5. Furthermore, the effectiveness of electrical testing varies widely depending on factors such as the type of electrical measurement used, the test fixture, method of test program generation, and end-user requirements.
6. It is essential, for the purposes of this discussion, to clarify the distinction between a defect and a fault.
7. A fault is an item identified by the test system that does not meet the expected criteria.
8. A defect, on the other hand, specifically refers to issues with the board itself, including design, fabrication, appearance, etc.
9. Not all defects can be detected by the test system.
Image 1: Circuit board fault
Shorts, hard shorts, or short circuits are defined as erroneous low-resistance connections between two or more networks or isolated points. These connections typically exhibit a fairly low electrical resistance value. Shorts manifest as failures in the product’s isolation testing. They can occur due to various factors such as exposure problems, under-etching, contaminated photo tools, poor layer alignment, defective raw materials, and improper solder leveling.
Opens denote the absence of expected circuit continuity, resulting in a missing connection that divides a circuit network into multiple pieces. Opens are reported as failures in the product’s continuity testing. They can be caused by over-etching, under-plating, contaminated photo tools, impure raw materials, layer registration errors, and mechanical damage. During electrical testing, “false open” errors may occur, often due to localized contamination on the product or test probe hindering proper connection to the test system.
Of particular concern in substrates utilizing microvias are latent defects that may arise after testing during subsequent substrate assembly processes. Examples include improperly formed conductors at stress points where cracks may develop under thermal or mechanical stresses during assembly. Small HDI features are less tolerant of such defects, which may initially go undetected during testing. However, sensitive ohmic measurements may reveal limited conductor cross-sections where future open circuits could occur. Regular detection of these defects underscores the necessity for process adjustments to prevent field failures.
Image 2: electronic test pcba
Many circuits produced today are required to operate at very wide bandwidths. Examples include fast microprocessors, general digital circuits, and RF amplifiers in wireless devices. Just as we use specific types of cables to connect television antennas to TV receivers, maintaining specific RF characteristics in interconnections between components of fast electronic circuits on printed wiring boards is crucial. One commonly specified and measured parameter is the RF transmission line impedance of the signal traces. This impedance is significantly influenced by the materials used in board fabrication, trace thickness and width, and the proximity to ground planes and adjacent signals.
A widely adopted method for measuring RF impedance is Time Domain Reflectometry (TDR). TDR measurements provide a profile of RF impedance along the trace length. (Distance and time are directly related here, considering that electrical signals propagate through the board at speeds approaching that of light.) TDR testing is typically conducted on a test coupon attached during manufacture and later detached. Testing on actual product traces also occurs, but requires uninterrupted trace lengths of several inches, free from branches or other configurations. Typical RF impedance values on circuit boards range from tens of ohms to several hundred ohms.
Image 3: RF Hole
RF impedance should not be confused with ordinary DC resistance and cannot be measured using common ohmmeters, despite both using ohms as the unit of measure. RF characteristics of interconnections are typically evaluated in the frequency domain using instruments known as network analyzers, although this approach is not commonly employed in bare board testing. The need for RF impedance testing becomes more prevalent as signal frequencies surpass 100 MHz.
