Avoiding Common PCB Failure Modes
Considering that almost all modern electronic equipment make use of Printed Circuit Boards (PCBs), seemingly, PCB failures are hard to avoid. Although most PCB failures are unique in some way or the other, and analyzing the failures can point to different root causes, eminent Printed Circuit Boards manufacturers and assemblers such as Rush PCB generally classify PCB failures into two broad categories—failures that occur during manufacturing, and failures that occur after delivery to the end user.
Failures Occurring During Manufacturing
The complex task of PCB manufacturing involving numerous processes leads to several opportunities of failure. Unless the fabricator exercises proper care and adheres to the exact tolerances the designer has set, the result can be catastrophic. For instance, failing to maintain adequate insulation between two metal traces, one carrying high-voltage and the other a lower voltage, can lead to an arc charring not only the board, but also destroying the entire equipment.
Populating the board with components requires following precise standards, while selecting the proper solder or reflow temperature can define whether the board has a successful life cycle or suffers a premature demise. Defects due to improper solder and reflow temperature settings vary widely. While initial testing some uncover some soldering defects, others may crop up only through rigors in the field.
Eminent manufacturers such as Rush PCB often choose to follow Destructive Physical Analysis (DPA), which involves sacrificing a small portion of one of their PCBs to ensure manufacturing failures do not crop up, that they have met all specifications, and there exist no indicators of potential process problems.
Failures Occurring in the Field
Failures occurring in the field mostly due to conditions unfavorable to the PCB. Although there may be cases of marginally good PCBs failing after a wearing down through a moderate amount of use. Thermal or mechanical stresses the PCB suffers in the field beyond its normal expectations can lead to acceleration of wear on the PCB. For instance, presence of large amounts of ionic contamination, the result of an unclean environment, is likely to enhance corrosion leading to an early life failure.
The primary task of failure analysis is to determine whether the failed PCB belongs to the first category or the second. A good failure analysis report, therefore, is an invaluable tool for the manufacturer providing them with crucial data for creating better PCBs.
Common PCB Failures During Manufacturing
Depending on the processes the fabricator uses during manufacturing, some common failures can be:
- Mismatched or Misaligned Layers
- Broken Component Packages
- Incomplete or Over-etching
- Improper Copper Weight
- Solder Flux Corrosion
- Broken Solder Joints
- Electro Static Discharge
With modern electronic products requiring sophisticated multi-layered PCBs, the fabricator must precisely align each layer to allow them to work in tandem, thereby avoiding short circuits, incomplete or open circuits, and crossing of signal lines.
The protective packaging of an electronic component keeps it sealed off hermetically from the environment. Mishandling can break this protective covering, thereby exposing the inner structure of the component to humidity and oxygen, causing it to age quicker. Breakages can occur due to process faults, manufacturing flaws, chemicals, exposure to UV rays, or due to excessive heat.
PCB fabricators immerse the laminate substrate in corrosive chemicals to remove the unwanted copper. Timing is of extreme importance in this process, as removing the laminate from the etching solution too soon will cause incomplete removal of copper, while allowing it to remain immersed for long will over-etch the traces. Post-etching, it is necessary to wash the substrate thoroughly with cleaning solution to remove all traces of the etching solution, as small amounts may remain behind causing slow deterioration of copper traces.
While maintaining thick traces of copper may not hamper the performance of a PCB, its cost will go up. On the other hand, thin traces may cause heating as they present more resistance to current flow. Therefore, the manufacturer must adhere to the copper weight specified by the designer.
Assemblers use flux to ensure removal of oxide layer from the metal surface, allowing better wetting by solder and resulting in a proper solder joint. It is necessary they remove the excess flux from the PCB surface as the remaining flux can corrode metals with which it comes into contact.
Stress from mechanical or thermal cycles, use of improper type of solder, growth of tin-whiskers, and improper temperature settings on reflow machines for lead-free solder can cause improper, broken, or dry solder joints, leading to failure between a component lead and the PCB pad.
Electric charge stored on human skin and synthetic cloth worn by operators can discharge as they encounter a grounded or uncharged object such as a PCB mounted with components. The resulting lightning discharge dissipates current through the components to ground, and may pass through the PN junctions of semiconductors and ICs, causing them to fail immediately or weaken them to cause latent failures. Assemblers must guard against ESD by wearing grounded conductive clothing, using static dissipative mats, and following other steps to prevent ESD.
Avoiding PCB Failures
It is very important for manufacturers to set up their internal processes to get the maximum yield of good PCBs. Although they follow several traditional techniques for analyzing failure in PCB and correct their process parameters, Rush PCB uses a combination of external techniques, such as visual inspection, electrical testing, X-ray inspection, and micro-sectioning of the relevant area. Other tests include solderability, and surface contamination.
One of the most common and simplest test methods is the visual inspection. Preferred for detecting faults, defects, and problems associated with soldering and assembly. Manual inspection may involve optical microscopy and visible light to help with inspection of denser boards, while high volume PCB inspection may require automated optical inspection (AOI).
AOI uses video camera and oblique lighting to capture images of the PCB surface. It then uses a computer to compare this image to a good image stored in its memory. The computer has special algorithms to identify areas that deviate from the base image.
This helps in detecting problem areas very fast, and the fabricator can take necessary measures to change the process parameters to counter them.
While visual inspection works efficiently for the top surfaces of the PCB, it cannot test the inner layers. Electrical testing is one of the simplest methods of testing these inner layers. This test mainly checks for opens and shorts where they should not be any. A good PCB establishes this and a fixture with probes tests the PCB and compares the readings. However, this is not a conclusive test, as only traces that appear on the outer layers are accessible for testing. Electrical tests cannot probe buried traces.
X-ray testing is one of the established methods of testing PCB areas that are not accessible and not visible. X-rays can penetrate multiple substrates and show up inner buried features. A video camera records the results for later viewing. This method can bring out soldering defects present under fine pitch components such as BGAs and gull-wing components.
With X-ray testing, it is possible to examine board integrity, as this is a major concern to manufacturers. This test can pin-point improper construction, such as that of buried vias, and expose other flaws at certain levels. For instance, X-ray inspection can easily reveal defects such as:
- Inadequate, poor, or excessive presence of solder
- Trace integrity on a specific layer
- Voids between substrate layers
- Die attach quality
- Internal wire dressing (within the IC)
- Presence of Internal particles.
The major advantage of X-ray testing is its non-destructive nature, as all components on the PCB remain undamaged after the test.
This is a detailed destructive examination of a section of the PCB that manufacturers such as Rush PCB use to investigate:
- Raw material
- Processing failures from solder reflow
- Opens and shorts
- Component defects
- Failures related to thermal and/or mechanical stresses
The method uses a thin slice of the PCB sample that uncovers the troubling feature within the PCB. The slice undergoes potting, solidifying, and curing in an epoxy resin. Receding the resin by abrasion and polishing it brings the defect to the surface ready to inspection and testing.
Although micro-sectioning is a destructive testing method, it a precise technique for isolating the relevant defective part of the board. The analyst can highlight the damage and identify the nature of the defect. Rush PCB follows quality standards such as PC-MS-810 and ASTM E3 for testing using micro-sections.
A PCB is useless unless it can allow soldering of components on the copper on its surface. Manufacturers therefore, are very concerned about the ease with which molten solder wets the copper surface on the PCB under minimum realistic conditions. Issues related to misapplication of the solder mask and oxidation can lead to PCB assembly problems during the manufacturing process.
PCB manufacturers minimize the probability of failure by testing the solderability of component leads and PCB pads. This ensures the robustness of the surface to increase the probability of forming a reliable solder joint.
Reproducing the contact between the material and its covering solder, it is possible to evaluate the quality and strength of the solder wetting the joint. Rush PCB employs the wetting balance method to measure the wetting force by evaluating the time it takes from contact to generating the necessary wetting force.
Rush PCB also employs special solderability testing techniques to determine the effects of storage on solder joints in PCBs. By providing an accurate measure of why a fault may have occurred, solderability testing essentially works for multiple applications such as:
- Quality control
- Flux evaluation
- Solder evaluation
- PCB surface coating evaluation
However, the failure analyst must understand the acceptance requirement criteria for the testing technique, while being able to differentiate the various surface conditions. Rush PCB complies with the IPC-J-STD-002 and 003 standards governing all solderability tests.
PCB Surface Contamination Testing
Contamination of the PCB surface can cause a variety of failures including degradation, corrosion, metallization, and rapid deterioration of interconnects. Although manufacturers process and assemble PCBs in clean environments, infection is possible from human byproducts and handling, flux residue, and reaction products.
PCB surface contamination may also occur from the aggressive chemistry processes that manufacturers often use during PCB fabrication, such as from:
- Water soluble soldering
- Electrolytic solutions
- Hot air leveling fluxes
- Copper etching liquid
Using the above chemicals typically requires cleaning via a special process. Rush PCB typically uses ionic contamination testing for measuring the cleaning efficiency and the stability of their cleaning process.
Precision and accuracy are the main advantages of the surface contamination analysis. Rush PCB conforms to the standards IPC-TM-650, method 2-3-25 for ionic cleanliness of their PCB surfaces.
By continuously testing the PCB during manufacture, Rush PCB tunes all processes to their maximum efficiency, allowing them to effectively produce the highest yield, by avoiding common PCB failures as outlined above.