In the IPC's Technical Activities Executive Committee adopted Principles of. Standardization as a guiding principle of IPC's standardization efforts. IPC-AD. Acceptability of Electronic. Assemblies. Developed by the IPC Task Group (b) of the Product Assurance. Subcommittee (). IPC-A has criteria outside the scope of IPC J-STD defining handling, Contractual reference to IPC-A does not additionally impose the content of.
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IPC AF PDF - Download the latest revision of the most widely used electronics assembly standard in the world. IPC A ACCEPTABILITY OF ELECTRONIC ASSEMBLIES PDF download. IPC Standards and Publications are designed to serve the public When an IPC standard/guideline is updated and a new revision is pub-.
Current research indicates that voltages and spikes less than 0. However, an increasing number of extremely sensitive components require that soldering irons, solder extractors, test instruments and other equipment must never generate spikes greater than 0. As required by most ESD specifications, periodic testing may be warranted to preclude damage as equipment performance may degrade with use over time. Maintenance programs are also necessary for process equipment to ensure the continued ability to not cause EOS damage.
All ESD protection techniques and products address one or both of the two issues. ESD damage is the result of electrical energy that was generated from static sources either being applied or in close proximity to ESDS devices. Static sources are all around us. The degree of static generated is relative to the characteristics of the source.
To generate energy, relative motion is required. This could be contacting, separation, or rubbing of the material. Most of the serious offenders are insulators since they concentrate energy where it was generated or applied rather than allowing it to spread across the surface of the material. See Table Peeling adhesive tape from a roll can generate 20, volts. Even compressed air nozzles that move air over insulating surfaces generate charges.
Destructive static charges are often induced on nearby conductors, such as human skin, and discharged into conductors on the assembly. This can happen when a person having an electrostatic charge potential touches a printed board assembly. The electronic assembly can be damaged as the discharge passes through the conductive pattern to an ESDS component. Electrostatic discharges may be too low to be felt by humans less than static volts , and still damage ESDS components.
Examples of frequently encountered labels are shown in Figure Symbol 1 ESD susceptibility symbol is a triangle with a reaching hand and a slash across it. This is used to indicate that an electrical or electronic device or assembly is susceptible to damage from an ESD event. This is used to identify items that are specifically designed to provide ESD protection for ESD sensitive assemblies and devices. Symbols 1 and 2 identify devices or an assembly as containing devices that are ESD sensitive, and that they must be handled accordingly.
Note that the absence of a symbol does not necessarily mean that the assembly is not ESD sensitive. When doubt exists about the sensitivity of an assembly, it must be handled as a sensitive device until it is determined otherwise.
This protection could be conductive static-shielding boxes, protective caps, bags or wraps. ESDS items must be removed from their protective enclosures only at static safe workstations.
It is important to understand the difference between the three types of protective enclosure material: 1 static shielding or barrier packaging , 2 antistatic, and 3 static dissipative materials.
Static shielding packaging will prevent an electrostatic discharge from passing through the package and into the assembly causing damage. Antistatic low charging packaging materials are used to provide inexpensive cushioning and intermediate packaging for ESDS items.
Antistatic materials do not generate charges when motion is applied. Static dissipative materials have enough conductivity to allow applied charges to dissipate over the surface relieving hot spots of energy.
Do not be misled by the "color" of packaging materials. It is widely assumed that "black" packaging is static shielding or conductive and that "pink" packaging is antistatic in nature.
While that may be generally true, it can be misleading. In addition, there are many clear materials now on the market that may be antistatic and even static shielding. This is not necessarily the case now. Caution: Some static shielding and antistatic materials and some topical antistatic solutions may affect the solderability of assemblies, components, and materials in process.
Care should be taken to select only packaging and handling materials that will not contaminate the assembly and use them with regard for the vendor s instructions.
Solvent cleaning of static dissipative or antistatic surfaces can degrade their ESD performance. Follow the manufacturer's recommendations for cleaning. Safe workstations should include EOS damage prevention by avoiding spike generating repair, manufacturing or testing equipment. Soldering irons, solder extractors and testing instruments can generate energy of sufficient levels to destroy extremely sensitive components and seriously degrade others.
For ESD protection, a path-to-ground must be provided to neutralize static charges that might otherwise discharge to a device or assembly.
Provisions are also made for grounding the worker's skin, preferably via a wrist strap to eliminate charges generated on the skin or clothing. Provision must be made in the grounding system to protect the worker from live circuitry as the result of carelessness or equipment failure.
This is commonly accomplished through resistance in line with the ground path, which also slows the charge decay time to prevent sparks or surges of energy from ESD sources. Additionally, a survey must be performed of the available voltage sources that could be encountered at the workstation to provide adequate protection from personnel electrical hazards.
For maximum allowable resistance and discharge times for static safe operations, see Table Illustrations and text are copyright IPC and may be copied only for use in licensed translation of IPC-A Revision F. Table mat to ground megohms less than 1 sec. Wrist strap to ground megohms less than 0.
Note: The selection of resistance values is based on the available voltages at the station to ensure personnel safety as well as to provide adequate decay or discharge time for ESD potentials. Examples of acceptable workstations are shown in Figures and When necessary, air ionizers may be required for more sensitive applications. The selection, location, and use procedures for ionizers must be followed to ensure their effectiveness Figure Series Connected Wrist Strap E Fig Personal wrist strap 2.
EOS protective trays, shunts, etc. EOS protective table top 4. EOS protective floor or mat 5. Building floor 6. Common ground point 7. Ground Keep workstation s free of static generating materials such as Styrofoam, plastic solder removers, sheet protectors, plastic or paper notebook folders, and employees' personal items.
Tools and equipment must be periodically checked and maintained to ensure proper operation. Note: Because of the unique conditions of each facility, particular care must be given to "third wire" ground terminations.
Frequently, instead of being at workbench or earth potential, the third wire ground may have a "floating" potential of 80 to volts. Most ESD specifications also require these potentials to be electrically common. Whatever comes in contact with these surfaces must be clean. When boards are removed from their protective wrappings, handle them with great care.
Touch only the edges away from any edge connector tabs. These principles are especially critical when no-clean processes are employed. Care must be taken during assembly and acceptability inspections to ensure product integrity at all times. Table provides general guidance. Printed circuit boards and commonly used plastic components absorb and release moisture at different rates. During the soldering process heat causes expansion of the moisture which can damage the ability of the materials to perform as required for the product requirements.
This damage crack, internal delamination, popcorning may not be visible and can occur during original soldering as well as during rework operations. To prevent laminate issues, if the level of moisture is unknown, PCBs should be baked to reduce the internal moisture content. The baking temperature selection and duration should be controlled to prevent reduction of solderability through intermetallic growth, surface oxidation or other internal component damage.
Keep workstations clean and neat. There must not be any eating, drinking, or use of tobacco products in the work area.
Minimize the handling of electronic assemblies and components to prevent damage. When gloves are used, change as frequently as necessary to prevent contamination from dirty gloves. Do not handle solderable surfaces with bare hands or fingers.
Body oils and salts reduce solderability, promote corrosion and dendritic growth. They can also cause poor adhesion of subsequent coatings or encapsulates. Do not use hand creams or lotions containing silicone since they can cause solderability and conformal coating adhesion problems. Never stack electronic assemblies or physical damage may occur.
Special racks may be provided in assembly areas for temporary storage. Always assume the items are ESDS even if they are not marked. Personnel must be trained and follow appropriate ESD practices and procedures. Never transport ESDS devices unless proper packaging is applied Handling Considerations - Physical Damage Improper handling can readily damage components and assemblies e. Physical damage of this type can ruin the entire assembly or attached components Handling Considerations - Contamination Many times product is contaminated during the manufacturing process due to careless or poor handling practices causing soldering and coating problems; body salts and oils, and unauthorized hand creams are typical contaminants.
Body oils and acids can reduce solderability, promote corrosion and dendritic growth. They can also cause poor adhesion of subsequent coatings or encapsulants. Normal cleaning procedures may not remove all contaminants. Therefore it is important to minimize the opportunities for contamination.
The best solution is prevention. Frequently washing ones hands and handling boards only by the edges without touching the lands or pads will aid in reducing contamination. When required the use of pallets and carriers will also aid in reducing contamination during processing. When gloves or finger cots are used they should be discarded and replaced often.
Many sensitive assemblies will also be marked on the assembly itself, usually on an edge connector. To prevent ESD and EOS damage to sensitive components, all handling, unpacking, assembly and testing shall be performed at a static controlled workstation see Figures and Handling Considerations - After Soldering After soldering and cleaning operations, the handling of electronic assemblies still requires great care.
Fingerprints are extremely hard to remove and will often show up in conformally coated boards after humidity or environmental testing. Gloves or other protective handling devices may be used to prevent such contamination.
Use mechanical racking or baskets with full ESD protection when handling during cleaning operations Handling Considerations - Gloves and Finger Cots The use of gloves or finger cots may be required under contract to prevent contamination of parts and assemblies. This damage could be in the form of latent failures, or product degradation not detectable during initial test or catastrophic failures found at initial test. This section is primarily concerned with visual assessment of proper securing tightness , and also with damage to the devices, hardware, and the mounting surface that can result from hardware mounting.
Process documentation drawings, prints, parts list, build process will specify what to use; deviations need to have prior customer approval. Note: Criteria in this section do not apply to attachments with self-tapping screws.
Visual inspection is performed in order to verify the following conditions: a. Correct parts and hardware. Correct sequence of assembly.
Correct security and tightness of parts and hardware. No discernible damage. Correct orientation of parts and hardware. Conductive pattern 3. Specified minimum electrical clearance 4. Mounted component 5. Conductor Acceptable - Class 1, 2, 3 Spacing between noncommon conductors does not violate specified minimum electrical clearance 3. Figure Metallic hardware 2. Spacing less than electrical clearance requirements 4.
Conductor Defect - Class 1, 2, 3 Hardware reduces spacing to less than specified minimum electrical clearance Hardware Installation - Interference Figure D Acceptable Class 1, 2, 3 Mounting area clear of obstructions to assembly requirements.
Anything that interferes with mounting of required hardware Heatsinks Heatsinks - Insulators and Thermal Compounds This section illustrates various types of heatsink mounting.
Bonding with thermally conductive adhesives may be specified in place of hardware. Visual inspection includes hardware security, component damage, and correct sequence of assembly. The following additional issues shall be considered: The component has good contact with the heatsink.
The hardware secures the component to the heatsink. The component and heatsink are flat and parallel to each other. Figure Acceptable - Class 1, 2, 3 Not uniform but evidence of mica, plastic film or thermal compound showing around edges of component.
Figure Defect - Class 1, 2, 3 No evidence of insulating materials, or thermal compound if required. Thermal compound precludes formation of required solder connection Heatsink - Contact Figure Heat sink Target - Class 1, 2, 3 Component and heatsink are in full contact with the mounting surface.
Hardware meets specified attachment requirements. Heat sink Acceptable - Class 1, 2, 3 Component not flush. Hardware meets mounting torque requirements if specified. Figure Heat sink 2. This is when some IPC documents come in handy to set the level of acceptance criteria for each class of products. It is often true but Class 3 is not exclusive to aerospace or any other industry.
The criteria for the four IPC classes are based upon the application of the product. Therefore, Class 3 can also be the criteria for avionics, military, industrial, and medical applications. It makes sense that a lot of Class 3 boards are for aerospace. The products launched into space have to be highly reliable to prevent any failure that could be critical. And the additional inspection is just too pricey for the commercial and consumer market.
When you require a Class 3 circuit board, it implies that the product has to be built according to the complete IPC criteria. This means that the design and manufacture teams must take into account laminate selection, plating thickness, annular ring requirements, manufacturing processes, material qualifications, facilities arrangements, inspection criteria, etc. This is what we call a visual defect since it does not usually affect the electrical and mechanical performance.
It, therefore, does not matter for Class 2 circuit boards. However, Class 3 does not accept any imperfection and this type of assembly misstep will cause the circuit board to fail the inspection.
This way, you will have 7. Class 2 allows breakouts from the annular ring whereas Class 3 does not accept any lifted or fractured annular rings. Class 3 boards need to be highly reliable and when there is a breakout, it is too difficult to find out how much is really broken out and how much it really affects the connection with the pad.
For Class 2, 90 degrees breakout of the hole from land is allowed provided minimum lateral spacing is maintained. The conductor junction should never be less than 2 mils or the minimum line width, whichever is smaller.
For Class 3, the minimum internal annular ring cannot be less than 1 mil. The external annular ring cannot be less than 2 mils. This is due to shifting in materials during the circuit board manufacturing process. To meet the Class 3 requirements, Sierra uses Pluritec machines to discover the shift in material, software to re-scale the drill locations, and vision drilling to accurately place the drills.
Design rules for annular rings To achieve acceptance for Class 2 and Class 3, follow the tables below published by Altium.
PCB through-hole plating requirement Class 3 requirements are as well more astringent for voids in copper. These are just a few requirements that differ between Class 2 and Class 3.