TOCREFGLOSSARYQUIT Section 5 Documentation and Dimensioning IPC Designer Certification Study Guide

TOC REF GLOSSARY QUIT

Section 5

Documentation and Dimensioning

IPC Designer Certification Study Guide


Section 5.1

Minimum Drawing Requirements



Minimum Drawing Requirements - 5.1

The documentation of a printed board assembly consists of several drawing types that are included or referenced in the documentation package.

All drawings are not always supplied to the fabricator or to the assembly company; however, the designer must have a good understanding of the minimum information necessary to convey the design intent.



The IPC-D-325 defines three types of documentation packages. These are:

Class A: Minimal Documentation This is usually used for internal use and consists primarily of a copy of the layout and artwork in hard copy or NC format. The information requires a great deal of coordination between the design and manufacturing disciplines. Notes on the layout convey much of the needed information. Class A documentation requires the use of a manufacturer that can produce a functional product from the information supplied.



Class B: Moderate Documentation This includes a complete board description without information as to the manufacturing allowances that are included in the design. The parts list and assembly drawing are also supplied to the assembler. CAD data on conductor routing describes the interconnectivity of the circuit and is used by the manufacturer to derive electrical test data. A schematic, logic diagram, or net list may also be supplied. Class B documentation requires working with a manufacturer and assembly company that has a strong CAD/CAM background and an understanding of what the designer expects.



Class C: Full Documentation This includes a complete procurement package that may be sent to multiple suppliers with each producing an identical part. The information is self sufficient and includes, as a minimum, the master drawing, the assembly drawing, bill of material (BOM), schematic or logic diagram, test specifications, artwork in hard copy and electronic form, and an electronic description of the design. In addition, a panel layout may also be included, especially if the assembly is to be built in panel format. The tooling features are defined and located and manufacturing allowances included in the design are identified.

D3251.2.4



The master drawing describes the unpopulated printed board and all the features that become a part of the board. Usually it is for a single board even though the board will be built as a panel by the board manufacturer. The assembly drawing defines the location of all the components that are associated with the assembly.



A reference designator is used to coordinate the part identification and location to the electronic diagrams (logic or schematic), and the parts list (BOM). The BOM may be supplied as a computer listing since the electronic forms are often used to tie in to the material issuing system of the assembler.

D3251.2.4



In the United States, drawing size formats are governed by ANSI Y14.1 and come in multiple sizes identified by the letters A, B, C, and D. The A size drawing format is 8 ½” x 11”. The next letter is B (i.e., 11” x 17”). Size C is 17”x22”, and D size is 22”x34”. There are also E and F sizes. But generally, multiple sheets of the four smaller sizes are preferred over going to the larger sizes.



Automated data is encouraged wherever possible. The preferred method is to provide data in GenCam, which describes the board assembly and electrical test in complete form. Other forms of electronic data are also permissible, i.e., Gerber plot data or Excellon NC data.

222111.2.4

D3251.4.2


Section 5.2

Minimum Requirements for Master Drawing



Minimum Requirements for Master Drawing - 5.2

The master drawing is used to describe the bare (or unpopulated) printed board. It shows the dimensional limits or grid locations that are applicable to any and all parts of a product to be fabricated. This includes the arrangement of conductors and nonconductive patterns or elements, the size, type, and location of all holes, and any other necessary information.



The master drawing identifies the locations of the datum reference planes and the coordinate dimensioning system for the board. This includes the relationships of the three datum planes (primary, secondary and tertiary) and the X, Y, and Z coordinate axes.



These conditions are usually established with the printed board oriented such that the side of the board on which the components are mounted is facing up. Since boards now have components on both sides the designer determines which side is facing up, and that side becomes the primary side. The backside of the board is therefore the reference for the primary datum plane.

D3254.2

Table4-1

Table4-2



Features of the board are used to locate the datum planes. The backside of the board is the feature that establishes the primary plane. Secondary and tertiary planes can be established using holes, symbols, or fiducials or board edges. It is preferred to use the features critical to mounting the board in its final package to establish datum planes.



When a feature, or pattern of features, is controlled by a geometric tolerance and the feature is also intended to serve as a datum feature, the feature control frame and the datum feature symbol are combined.

22215.4.3

Fig5-5Athru5-5E

Fig5-6

Fig5-7



There are classes of master drawing completeness just as there are documentation completeness requirements. A complete disclosure master drawing provides information on all the board details including, dielectric separation, overall thickness and tolerances, and allowable bow and twist of the final product.

Table4-1

Table4-2

4.2



Material descriptions include the base material, conductive and non-conductive properties as well as the plating, coating, and marking ink requirements. Conductors and hole requirements are documented. This includes shape, arrangement, and acceptance criteria.



Identification and traceability requirements are defined and all process allowances that were used in the design are detailed, such that the manufacturer doesn’t have to guess that they have the correct precision equipment needed to produce the part.



If a lesser master drawing is produced the liaison and understanding between the design and manufacturing disciplines increases dramatically. Nevertheless, a good working relationship can overcome many shortcomings in the documented requirements.

D3254.2

Table4-1

Table4-2


Section 5.3

Solder Resist Location Technique



Solder Resist LocationTechnique - 5.3

Many printed boards use solder resist (or solder mask) as an organic coating to protect the surface of the board from moisture or other forms of contamination that could impact the performance of the assembly. The function of the resist is to provide protection from the action of solder.

Extra1.4


However, the solder mask does much more after the assembly has been completed. One of the major recommendations to reduce the possibility of metal migration between two conductors is to add a solder mask over the conductive surfaces. The mask prevents surface moisture from providing a path for dendritic growth.


Extra5.3


There are many rules for the use of solder mask. Most of these are identified in IPC-SM-840. This document defines the performance requirements of the mask and the methods to determine the characteristics of the material.

However, the need for a solder mask is still determined by the design and the registration capability of the manufacturing process.


22214.5.1

SM7823.7.2


Since the main intent of the mask is to protect conductors from surface contamination, some designers find that by putting all conductors on internal layers of the board, the need for the mask is eliminated. The “surface lands only” approach is not new and has been used with many through-hole designs. With no conductors routed between lands on the outer layers, registration issues are all related only to the conductive pattern.

SM7823.7.1



Datum’s and/or datum features are used to register conductive layers to each other. This approach is also used for the registration of the solder mask to the surface conductive patterns. Since the application of the mask occurs after the board has been completed, the principles for registration are slightly different.



The goal is to have the solder mask cover all the conductive surfaces that are not to be soldered and leave open those areas where components must be attached, or where a test probe point needs to have access to the conductive surface. The precision necessary to accomplish this feat is often predicated on the density of the surface land and the pitch between lands.



Low density circuits can use a solder mask that is applied by silk screening methods; higher density circuits require photo-imageable solder mask. The registration for either mask type must still coincide with the concepts of registration for the conductive layers. The system uses similar datum features to define the location of the openings in the mask to the conductive patterns.



Solder Resist Location Technique - 5.3

The zero coordinate (panel target) is usually the same as for the rest of the board and patterns are intended to match each other. The design is based on a single board image that the manufacturer converts into multiple images. It is the manufacturers responsibility to establish the registration system used to have the mask align with the conductive lands to be left open.

SM7823.7.4


Solder Resist Location Technique - 5.3

The recommended practice is to provide a 1:1 artwork of the land pattern master and specify the clearance that the manufacturer must maintain between the solder mask opening and the land edge (i.e., 0.0 to 0.1mm [.000” to .004”] all around).

SM7823.7.3

Fig3-33

Fig3-34


Section 5.4

Dimensioning and Data Symbols



Dimensioning and Datum Symbols - 5.4

Dimensioning and tolerancing principles used for engineering drawings and related documents are defined in ASME Y14.5M. This publication establishes the uniform practices for stating and interpreting documentation related to dimensioning systems. The standard also establishes the symbols to be used for specifying geometric characteristics and other dimensional requirements. Specific requirements related to the construction, form, and proportion of individual symbols are also described.

26153.3.1

Table3-1

Table3-2



Datum features are the foundation of the dimensioning system and are selected on the basis of the geometric relationship to the toleranced feature and the requirements of the design. Datum features must be readily discernible on the part. To ensure proper part interface and assembly, corresponding features of mating parts are also selected as datum features, where practicable.



The symbol for a datum feature consists of a frame containing the datum identifying letter. Letters of the alphabet (except I, O, and Q) are used as datum identifying letters. For printed boards, usually three letters suffice to position the part in relation to a set of mutually perpendicular planes, which jointly are called a datum reference frame.

22215.4.3

Fig5-5Athru5-5E

Fig5-6

Fig5-7



The reference frame exists in theory only and is not on the part. Therefore, a simulated relationship is established between the board and the reference frame to restrict motion of the part as dimensions and measurements are made.

Measurements can be made for straightness, flatness, angularity, perpendicularity, concentricity, parallelism, and in fact any geometric condition.



The ASME Y14.5M defines the symbol to be used for each of these conditions.

As an example, the symbol for straightness is a single line ( - ), for parallelism the symbol is two slashes ( // ). Flatness is a parallelogram, and circularity is a circle. There are symbols for every geometric characteristic that may be used to describe the physical attributes of the board.

3.4.2

Fig3-5



The system also uses basic dimensions to control features of the board. A basic dimension is assigned a theoretical tolerance (i.e. not from the actual dimension itself). Printed boards use basic dimensions exclusively to define the location of features such as holes or land patterns. In these instances the location is thought of as being the true position or target location, regardless of the feature size. A basic dimension symbol may have a frame around the dimension much like the datum feature symbol.



Material modifying symbols are also defined in AMSE Y14.5M. The letter M or L in a circle identifies maximum and least material condition. If a basic dimension is given using either of these letters it indicates the tolerance is applied. The capital letter R is used to identify a radius. A circle with a slash through it is the symbol for diameter.



The symbol used for position is related to the true location of a feature. This symbol is a circle with a cross inside it. Symbols are combined in a frame to describe the feature and relate its characteristics to a datum or other feature on the same part.

26153.3.1

Table3-1

Table3-2


Dimensioning and DatumSymbols - 5.4

Some products that have a high degree of precision needed in a particular area have a second set of tolerances established. These are combined in a composite feature control frame. In this instance, one set of tolerances is more precise. This would be used when a set of features can move about within the board however, must remain in tight relationship to each other.


Dimensioning and DatumSymbols - 5.4

It is not unusual to have more than one set of datum planes for a panel. The second set is used for features that require a relationship to each other, but not necessarily to the rest of the panel.

26153.4.2.1

Fig3-6


Section 5.5

Master Drawing Hole and Conductor Description



Master Drawing Hole and Conductor Description - 5.5

The master drawing defines all the physical characteristics of the printed board. The description starts with the definition of the individual conductive layers that compose the board. Layer 1 is the first layer closest to the primary side and usually has many lands for component mounting. The lands may circumscribe a hole for through-hole components or be without a hole as in the case of surface mount lands.



Conductors may also be included on layer 1. The conductor definition, arrangement, and relationship to holes that are either part of the circuit or used for physical mounting must also be defined. Thus the shape and arrangement of both conductive and non-conductive patterns must be described for all the layers of the printed board. This may be accomplished using paper or film copies of the production master(s) or copies of the artwork if artwork is supplied with the documentation.

D3254.2

Table4-1

Table4-2



There are separate views of each layer and the conductor width and spacing is indicated. The conductor width and spacing is usually specified through a note, or for critical areas of the circuitry through a specific dimension and tolerance. In many instances finished conductor width and clearance is defined as a least material condition that is acceptable for design performance.



As an example a minimum conductor width might be defined as being 0.25mm [.010”]. Some define the conductor width using a range, i.e. 0.20-0.25mm [.008”-.010”]. In either case the manufacturer selects the target value that he thinks the designer wishes to achieve and tries to come as close to that condition as possible.

222110.1.1

Fig6-4

Table10-1

Table10-2



Processing allowances should also be defined on the master drawing. This is especially important when providing CAD data which indicates absolute location of the conductor arrangement. If two parallel conductors are routed on a 0.5mm [.020”] grid a good explanation of process allowance would be that there is 0.1mm [.004”] allowance between a minimum conductor 0.2mm [.008”] wide and a minimum spacing of 0.2mm [.008”]. The manufacturer can now adjust his process to meet the required conditions and compensate conductor size for his fabrication allowances.

222110.2.1



One problem exists in that the tolerances are assigned to conductors while the surface mount lands are often neglected. While the manufacturer is concentrating on the minimum conductor, the lands that should be at maximum material condition also come out smaller than required thus reducing the opportunity for a good solder joint.



Therefore, conductor definition should provide the target values for lands and conductors on each layer. This would usually be least material condition for the conductor (LMC) and maximum for the lands (MMC) to get good solder joints on the surface and no hole breakout on internal layers.

SM7823.2.3



Holes are also defined on the master drawing. This includes the location and whether the hole is unsupported or has plating in the hole. Although there should be an awareness of the drill size that should be used, it is usually best to only specify the range for the hole diameter in terms of LMC and MMC.



The master drawing also describes the minimum plating thickness that must be included in the plated-through hole to meet the reliability expectations of the design. The process allowance that was included in the hole to land calculation is also an important piece of information needed to determine the registration requirements so that holes do not break out of their circumscribing land.

22229.2.1.2

Table9-2


Section 5.6

Printed Board Profile Dimensioning and Tolerances



Printed Board Profile Dimensioning and Tolerances - 5.6

The master drawing defines the physical outline of the printed board as well as the conductive patterns and holes. This includes the printed board profile, construction in terms of dielectric spacing and thickness, and cutouts and notches included in the periphery of the finished board.



All mechanical dimensions are related to the datum reference of the board. Thus the circuit pattern and the board outline originate from the same location; their definition and tolerances describe the allowable variation.

D3254.2

Table4-1

Table4-2



Dimensions usually start from datum zero-zero. It is important to realize that when using coordinant tolerancing methods an accumulation of tolerances can result. When one feature location is dimensioned from another and a third feature is dimensioned from a second, the chain of dimensions all bring with them the allowable variation that can occur.



This incremental dimensioning is not a preferred method since the distance between the first location and the last location can vary by the total accumulation of all the members of the chain. Therefore most dimensioning techniques start at datum zero and provide information in either X or Y axis to the individual locations and where required make reference to other datum’s or datum planes.



One popular way to control the board outline is to use the Geometric dimensioning and tolerancing technique of profile dimensioning. The ASME Y14.5M symbol for defining the profile of a surface is a horizontal crescent (a straight line connecting to an arc above the line). This symbol is used in a feature control frame when describing features of the geometric tolerances.



The use of profile symbols can be further defined by a symbol that indicates that the profile tolerance applies to surfaces all around. This indicator is a circle that is located at the junction of the leader from the feature control frame. The board outline as a feature is controlled in relation to the primary, secondary and tertiary datum planes.

22225.4.2

Table5-3

26153.3.1

3.3.4

Table3-1

Table3-2

Table3-3



There are many ways in which the master drawing can control the position of features and their tolerance. Each technique is predicated on the need to control the dimension and to what degree. It is a good practice to be as liberal as possible in the range of allowable variation.



Board manufacturers will adjust tooling to accommodate the requirements. These will be checked as the first boards are produced. After initial first article approval, the tooling and tool maintenance control the end results.

22215.4.3

Fig5-5Athru5-5E

Fig5-6

Fig5-7


Section 5.7

Plated Through-Hole Dimensionsand Grid Location



Plated Through-Hole Dimensionsand Grid Location - 5.7

Plated-through holes become the vehicle that interconnects the various segments of the printed board. Defined as the “Interfacial Connection” it is the conductive path that connects conductive patterns of the external and internal layers of printed boards. They may be component holes in which component leads are attached, or vias which are plated-through holes that are used as an interlayer or interfacial connection where there is no intent to insert a component lead or other reinforcing material.

22219.1.2



• through viasvias that extend through the total board thickness

• blind viasvias that extend only to one surface of the printed board

• buried viasvias that extend only between conductive layers (not to any external board surface)

Vias come in three variations:

Extra1.1

9.2.7.1

9.2.7.2

9.2.7.3

Table9-4

Table9-5

22229.2.2.3

Table9-6



The three datum planes of a printed board (primary, secondary and tertiary) are all required to fully dimension the plated-through hole characteristics. It only takes two datum features to define the grid on which all the holes are positioned. One feature (hole or symbol) establishes the zero coordinate or secondary datum plane. The other feature fixes the tertiary reference plane and establishes the angular requirements as opposed to assuming that the plane is ninety degrees from the secondary reference plane.



What many documents for printed boards forget is to establish the relationship with the primary reference plane (or the backside of the board). The reason that this condition becomes important is to establish the perpendicular requirements for the plated-through holes.



This factor can become an important control especially for very thick printed boards that may have the drill move off-center as it removes material through the total board thickness. Drill “walk” or “splay” can cause hole breakout of the land furthest from where the drill first enters the board.

22215.4.3

Fig5-5Athru5-5E

Fig5-6

Fig5-7



The importance of properly dimensioning plated-through hole locations becomes apparent when determining the required land size. The designer must take all the factors into account that can cause the drilled hole to breakout of the conductive land formed with the original board copper foil. This includes the diameter of the drilled hole, the desired annular width to remain on either side of the hole after processing, and most important, the process allowance that needs to be included to ensure that all the conditions can be met.

22219.1.1

9.1.2

Fig9-3

22229.1.2



Process allowance factors include location of the feature (land) as a variation from the true position location. Hole location is also defined as a variation from true position and should include the drill splay perpendicular allowance. Etch factor and phototool tolerance is included to provide the variation that can occur between the tool datum and the physical datum of the individual boards of the panel.



If one were to do a worst case analysis it could probably be proven that printed boards could not be manufactured. Using statistical methods that say all tolerances will not be in the same direction, the RMS (root mean square) value can be used as the process allowance indicator. The industry has learned to accept a set of numbers for this value based on precision required (level) and the maximum board size.


Section 5.8

Tooling Hole Location Documentation



Tooling Hole Location Documentation - 5.8

Tooling holes are features of the printed board or the printed board panel. They are tooling features in the form of a hole, however, they may also be a slot as is used by many board manufacturers to optimize the tolerance conditions between pins on the tooling fixture and the holes or slots in the board.



Fabrication tooling holes are usually determined by the board manufacturer, but it is a good plan to interface with the assembler since they also use tooling features as part of the assembly panel.



The term tooling feature is more generically correct to define any physical feature that is used exclusively to position a printed board or panel during the fabrication, assembly, test, or inspection process.



With the increased use of vision systems in the manufacturing discipline, the whole issue of who owns the tooling concepts becomes a bit clouded. Before a design is committed to manufacturing a clear understanding and agreement is required to address all the goals of the various parties involved.


Tooling Hole LocationDocumentation - 5.8

The first step of the process is the design of the single image board. The tooling feature(s) are normally on the board, and may be holes, registration symbols, or fiducials. The reason they are on the board is so that when all processing is complete and the customer is holding the final product in his hand he can identify any location on the board in relation to the zero-zero origin that is established by one of the tooling features.


Tooling Hole LocationDocumentation - 5.8

Inspection, field maintenance, and handbook references find the tooling feature on the board most helpful to orient the operator in the function being performed. When boards are very dense with circuitry or very small there may be no room to have the tooling features located on the board. In these instances the zero-zero origin is off the board and a secondary location identified for visual orientation. Many times marking ink provides this function.

222111.1



Next, the board manufacturer repeats the single image on the production panel. The number of times that the board can be reproduced is a function of the board to panel size. Original tooling features that are part of the design are maintained, but are not used as a part of the manufacturer’s tooling system (which many times is optimized through the panel center to spread the material movement potential over the entire panel evenly instead of just coming from one edge).



The board manufacturer adds his own tooling features and only uses the original design tooling features to establish the relationship between and within board images. If the board manufacturer separates the boards to be delivered to the customer from the panel, they might use original tooling features to position the boards during electrical continuity testing of the bare board.

SM7823.6.4.5



The most significant change in the industry is the automation used to assemble products and the need of the assembler to have well thought out tooling schemes that match the capability of their equipment. Many conveyor systems have positive stops that require holes in the panel or that use the panel edges to obtain the original location of the product.



Vision cameras then look for symbols that help identify more precise locations to correctly mount parts. Additionally, once the assembly is completed tooling holes are usually used to position the assembly in the in-circuit test fixture. All these conditions should be taken into account and fully documented in order to achieve the best coordinated results.


Section 5.9

Non-Standard Parts Information



Non-Standard Parts Information - 5.9

To properly execute a design all pertinent information must be available to the designer. This information includes the electrical description in either schematic or logic form, the mechanical requirements of the board or mounting structure, and the information about the components to be interconnected. The component or materials list should contain part numbers and descriptions of the materials and components to be used in the assembly.

22213.4



Part standardization is a vital element of inventory control and cycle time reduction. The industry has established standard or registered component outlines in order to facilitate the acquisition process of having the right component available at the right time. Designers are encouraged to help in the effort by consulting standard parts lists and making a general effort to utilize standard or readily available parts.



The Electronic Industry Association (EIA) is responsible for standardization of passive and discrete components; the Joint Electron Device Engineering Council (JEDEC) is responsible for the packages that are registered or standardized for ICs.



There are many variations on some of the devices. A particular resistor, capacitor, or IC may come in various shapes. Over the years everyone has had their favorite variety of component. Component availability also plays a major role in determining exactly what type of component package is best for the application.



When engineering is not sure as to the availability of the part, many times designers start the design without really being sure as to the final part configuration. In these instances, the designer leaves room for the various types of packages that can be accommodated in the area left vacant.



If all the choices are of a group of standard parts leaving space can be a reasonable exercise. If the part is a nonstandard part the designer must receive mechanical information as to the physical size or the location of the output terminals before the layout begins.

8.14

22213.4

D3258.5



Once the design is complete, documentation should be produced that details the requirements of the nonstandard part. A Specification Control Drawing is the document of choice. This drawing shows the dimensional limits that are applicable to all portions of the part.



Also included are electrical performance requirements, plating characteristics, and, if appropriate, reliability data. A supplier number should also be referenced with the caveat that the part may be supplier XYZ or an equivalent supplier. This provides a method for procurement should the part be needed for another design or reorder of the original printed board assembly.

D3258.14


Section 5.10

Artwork Acceptance Criteria



Artwork Acceptance Criteria - 5.10

Artwork is the term applied to the accurately-scaled configuration of the printed board circuitry. The artwork master (usually a 1:1 pattern) is used to produce the production master. The reason for all the terminology is that early on the industry would interface with manufacturing at the phototool level and provide enlarged artwork. The manufacturer would then photographically reduce the enlarged single image and at the same time repeat the image for the production master.

222111.2.1



Much of the industry today sends electronic data since the boards are designed using Computer Aided Design (CAD) systems, but the term production master still applies. It is the 1-to-1 scale pattern that is used to produce rigid or flexible printed boards within the accuracy specified on the master drawing. The film or glass master used by the manufacturer may be a “single-image production master”, or a “multiple-image production master.”



In either case all the manufacturing allowances have been incorporated into the master that reflect the individual manufacturer’s process. When artwork was used as the method of data transfer the process allowances were incorporated photographically. With electronic data transfer the allowances are added by the Computer Aided Manufacturing (CAM) system.

222111.2.4



The CAM systems today are much more efficient and can add tooling features and, in many instances, can perform design rule checks on the data after the process allowances have been incorporated. However, the final production master is still film or glass that is used to transfer the image to the production panel. Photo-reproduction conditions are important and as the precision requirements increase so does the need for cleanliness and environmental (temperature and humidity) control in which the production master is prepared.



Usually there is one original production master made on dimensionally stable polyester film or on glass photographic plates. Copies are made from the original and used in the actual production process. Control and traceability of the copies to the original becomes important as does the revision control requirements.

222111.2.2



The masters used on the production floor have a limited life and are replaced frequently to ensure the integrity of the image transfer operation. Accuracy of one master to another ensures good registration capability. If one layer of a multilayer board is changed many companies reprint all the layers to avoid a difference in the conditions under which different layers of film were created.



Precision demands are becoming so acute that some manufacturers will not use copies of the original on the production floor and require that a full new film set is produced from the CAM data each time a floor production master is replaced.

These decisions are based on the complexity of the design and the production wear-out characteristics.

222111.2.2



Two other characteristics impact the quality of the production master. These are the occurrence of pin holes or opaque defects (specks) in the phototool. When either of these exist in the production master the imaged product becomes inferior; circuitry is left where it shouldn’t be, and conductor width is impaired by reductions caused by the pin hole or edge defect being passed along to the board resist that protects the copper during etching.


Section 5.11

Documenting Fastening Hardware



Documenting Fastening Hardware - 5.11

Hardware that becomes part of the final assembly needs to be properly identified and the requirements clearly prescribed. This is normally accomplished on the printed board assembly drawing. The assembly drawing is a document that depicts the physical relationship of two or more parts, a combination of parts and subordinate assemblies, or a group of assemblies required to form an assembly of a higher order.



The assembly drawing describes many characteristics related to the components including their location, reference designator, orientation, polarity, lead forming and solder joint requirements. Electrical test and conformal coating requirements are also described.

D3258.3

8.4


Documenting FasteningHardware - 5.11

With the concentration of requirements on electrical performance many times the mechanical hardware requirements are forgotten. Included in these requirements are, for example, the latches that may be needed to extract a board or the mounting hardware that stays with the assembly and is used to secure the product to the next level.


Documenting FasteningHardware - 5.11

The requirements on the assembly drawing should closely match the information provided on the parts list or BOM. The parts list may be separate or an integral part of the assembly drawing, however with the need to use computer inventory control most parts lists are separate and are computer generated.



When a separate parts list is developed it is recommended that the assembly drawing and the parts list have the same number. The parts list number is preceded by the letters PL, to distinguish it from the assembly drawing. It is also recommended that both documents maintain the same revision letter.

D3258.5

8.6

8.7



Every part appearing on the assembly drawing must be assigned an item number which must match the item number assigned on the parts list. In the case of like electrical parts they are assigned a single number but are located and distinguished on the assembly drawing by a unique reference designator that correlates to the schematic or logic diagram reference designator assignment.



Thus, for example, item number 15 might be a 50 ohm resistor and since there are three of them they would be identified as R11, R14, and R27 on the assembly drawing. The information between the assembly drawing, the parts list, and the electrical diagrams must be consistent.

D3258.8



Common mechanical hardware parts are shown and listed using the same item number. The quantity in the parts list shows the total number of the items that are required for that particular assembly. Other information that is necessary to properly document the requirements relates to the clearance that must be maintained between the hardware and any exposed circuitry.



Also, the torque required to properly secure the parts to the board should be documented. This may be shown in the notes on the drawing. The sequence of events may also be important. If the fastening hardware is to be riveted to the board it might be advantageous to do that operation prior to assembling the components so as not to stress the solder joints.



Moving parts might best be added after the conformal coating has been applied to the board. Assembly practices that are non-standard or unclear should be specifically detailed on the assembly drawing.



IPC-A-610 is the acceptability workmanship standard for printed board assemblies. This document covers clearance and attachment requirements for hardware as well as for soldering electronic components. Between the assembly drawing and the reference to the acceptability standard most requirements are well described.


Quiz 4

• Which part of the board should be used to define a datum plane?– the keying slot– the board edge– the connector tang– the corner tooling hole

Answer: the corner tooling hole


Quiz 4

• Which document describes the use requirements for fastening hardware on the printed board?– the master drawing– the assembly drawing– the source control drawing– the specification control drawing

Answer: the assembly drawing


Quiz 4

• From what side is the layout viewed and drawn in a fully documented printed board assembly?– the primary side– the primary datum– the tertiary plane– the secondary side

Answer: the primary side


Quiz 4

• Which two factors are the primary influence for the layer construction assignment of a printed board?– the bonding material used– the requirements for solder mask– the direction of conductor routing– the relationship of core to prepreg– the balance distribution of copper– the characteristics of the surface layer

Answer: the direction of conductor routing, the balance distribution of copper


Quiz 4

• Given a maximum through-hole component lead diameter of 0.5mm [0.020”], what is the minimum finished plated through hole size that should be used for a level “C” product?– 0.60mm [0.024”]– 0.65mm [0.026”]– 0.70mm [0.028”]– 0.75mm [0.030”]

Answer: 0.65mm [0.026”]


Quiz 4

• What is the main consideration in the standard fabrication allowance?– process variation– hole tolerance capability– aspect ratio characteristics– minimum annular ring requirements

Answer: process variation


Quiz 4

• How should the clearance be determined between a non-functional land and a conductive plane that surrounds the land?– By maintaining a minimum clearance of

0.2mm [0.008”]– by maintaining a minimum clearance of

0.25mm [0.010”]– by maintaining clearance equal to the conductor spacing

requirements– by maintaining clearance based on the copper thickness and

etch factor

Answer: by maintaining a minimum clearance of 0.25mm [0.010”]

Documents

TOCREFGLOSSARYQUIT Section 5 Documentation and Dimensioning IPC Designer Certification Study Guide