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SMT Design Guidelines For PCB Drawings

Drawing Definition

A PCB design document is a controlled document that contains all necessary information about schematic, electronics components, physical properties and any other vital details which are required to produce a PCB.

Purpose & Application

The purpose of this document is to establish a guide in order to aid Designers to fully complete drawings.  All Drawings shall be fully detailed in order to allow potential users of the information to produce or supply components for the PCB without error or confusion.

Drawing Details

Methode PCB Assembly drawings will consist of multiple sheets organised as follows:-

– Manufacturing notes and full revision history

– Schematic and last revision

– PCB assembly view, component BOMs, Manufacturing notes and last revision.

Assembly drawing numbers will begin 16xxx, where xxx will be defined by the part number issuing authority.

Bare Board drawings will be prepared separately from the assembly drawings. Their numbers will begin 44xxx, where xxx will be defined by the part number issuing authority

Amendment Block

Zone

Zone shall be filled in to identify the area where an amendment has taken place. The Y axis is lettered and the X axis is numbered.  The X-axis shall be typed in front of the Y axis, e.g. 4G (First the number followed by a letter.) All drawings shall be prepared on the standard template.

Index

All drawings shall be issued with a “00A” index and marked “DRAWING ISSUED”.  Index level shall start from top to bottom of the amendment block.  When the amendment description is longer than one line, the index number shall be typed on the first line of the amendment and the date and name shall be typed in line with the end of the amendment description.  

Amendments

Amendments shall be recorded as concisely as possible, but shall contain a clear record of what has been changed. All modifications shall be cross referenced to Change Notice (CN) and Change Reference (CR) as applicable.  This number shall be recorded in the next column – Supporting Document field (SPRT DOC).  Any modifications in the drawing shall be marked with an encircled amendment index letter placed on the location where the amendment has taken place, as shown below.  The supported document should refer to the Index No.  

  

                                                                   

In those sheets where no changes have been effected, the entry in the Amendments field shall read “REFER TO SHEET x”, where x denotes the sheet number where changes are present. In case of multiple sheets containing changes, these should be all listed in the same entry reading “REFER TO SHEETS x, y, z”. 

CR & CN Usage

A  CR is to be used to amend typing errors and to bring drawings in line with the mechanical PCB (due to mechanical tolerances and so on).

A CN is to be used for anything else like change in resistor value, change in component and so on. 

Date/Name

Date/Name shall be entered when Drawing is issued or amended.  Name shall be written in the following manner: first name initials only and surname in full, both in block form.  

DWG NO.  (Drawing Number)

All drawings shall be initially issued with “00A” index.  The drawing shall get the next higher number and letter index for amendments effected by an Engineering Change Notice e.g. from 00A to 01B.

The Drawing shall get the next revision letter only for every amendment effected by a Change Reference e.g. from 00A to 00B.

Title Block 

DWN BY (Drawn By)

‘Drawn by’ shall be typed in by the PCB Designer.

CHK BY (Checked By)

‘Checked by’ shall be typed in by someone within the Product Development Department. This shall commonly be the product designer nominated as Lead Engineer for the project.

APVD BY (Approved By)

‘Approved By’ shall be typed in by the PCB Designer’s Principal, or in his absence the Product Development Manager.  

CAD Filename

The full Drawing file name and extension shall be identified in the field.

Title

The title of the drawing shall identify the PCB name and Product Function. e.g. “PCB Assy. – Main Board – SAS” etc. It is not acceptable to simply write “PCB Assembly”.

IV. Drawing Types

The above guidelines shall apply to all PCB drawings. 

The PCB Assembly Drawing – 16xxx

All sheets of all PCB Assembly drawings will carry the standard border, including the Title Block and Amendment Block (see Figure 1). Special notes related to standards, finishes and any other notable features which affect the PCB assembly shall be included in the first sheet. All notes should be collected in a single numbered list, except where individual notes may relate specifically to a particular view. In that case it is acceptable to place the unnumbered note close to the corresponding view.

The number of sheets in the drawing shall be kept to the minimum amount possible, by using larger page sizes where necessary. However, it is important that the drawing maintains clarity and avoids clutter; hence when necessary it is acceptable to increase the number of pages if this enhances readability. All sheets should have the same page size, where possible.

Where multiple variants of a PCB exist, each variant shall have as unique assembly number. All assemblies will refer to the same bare board number. The assemblies will traceable through their use of a common bare board.

 

Figure 1: The Standard PCB Assembly Border. Note that that Amendments block should be extended to the maximum amount of lines possible on the sheet, to allow room for detailed descriptions of modifications.

The Schematic View  

The circuit Schematic shall be included in the PCB assembly drawing. This shall include the following:-

The circuit diagram.

The connector pin numbering and functions.

Where a mechanical movement or contact system influences inputs or outputs of the circuit, such mechanical features should be represented in the schematic to aid understanding.

Passive component values shall be shown on the schematic.

Semiconductor part numbers.

All components will be identified by a designator that shall be referred to in the bill of materials and PCB assembly drawings. Refer to the table below for the prefixes to be used for each component type. 

 

Components shown on the schematic but not fitted must be identified by drawing a broken line box around the component. There should be text adjacent to the box “NOT FITTED”. Alternatively, the component may be crossed out on the schematic, and a note reading “CROSSED OUT COMPONENTS IN THE SCHEMATIC VIEW ARE NOT TO BE FITTED” shall be included. The best way to denote this shall be chosen according to the complexity of the schematic and the grouping of the components in question, in such a way as maintain clarity and avoid clutter in the schematic.

Signals and terminal names required to improve understanding in other documents and software should also be shown in the schematic.

For simplicity the schematic may be drawn in blocks with off page connectors.

The schematic may be spread over several sheets if necessary for clarity.

Bill Of Materials

The Bill Of Materials (BOM) Table must identify all electronic components and their description in an unambiguous way.  The component reference designators used in the BOM must be consistent with those shown in the Schematic View.  All components shown in the schematic view have to be listed in the BOM. Those which are not to be mounted should be placed in separate lines and have an entry of ‘0’ in the ‘Quantity’ column (see Table 1).

The table shall have the following format: –

 

Table 1: Bill Of Materials Table

If PCB is to be assembled in-house at MEM, a new column with the MEM part number s MUST be added.  

The preferred and alternative suppliers for the components, as well as the part number of the preferred supplier have to be listed in the BOM.

Microcontrollers should be identified and the software part number should be shown in the BOM as a component. Software numbers and revisions should be assigned according to the Software Development Procedure as follows: –

 

For example: 16234-U16-S02 refers to the software which should be in microcontrollers with reference designator U16 of PCB assembly 16234 and has revision 02.

Connector Pin-Out  

The connector numbering shall be shown on the schematic view as described previously. The signals shall be described in a separate table that relates to the same connector pin numbering. 

The table shall have the following format: –

 

Table 2: Connector Pin-Out Table

In-Circuit Testing

An In-Circuit Testing table shall be included in the PCB assembly drawing. This table should indicate which electrical tests are to be performed on the PCBs after the components have been assembled. The table shall have the following format: –

 

Table 3: In-Circuit Testing Table

In the case that any programming and/or calibration procedure is required, this has to be specified in a separate process instructions document. The PCB assembly drawing shall contain a note (either in the general notes list or in this ICT table) referring to this process instructions document, including the file name and version.

LED illumination intensity bins

In most cases, different resistors may be fitted on the PCB, depending on which of the specified LED illumination intensity bins are being assembled. In this case a table showing which resistor values are required for each LED intensity bin shall be included for each LED. It should be noted that two identical LEDs on the same PCB may require different resistor values for same bins, because the illumination may have different optical paths in the product.

When such resistor/LED bin tables are included, an additional table showing similar fields to those in the BOM table will be required for these resistors.

New Boards and or new Plastic will require LED tweaking across all acceptable BINs, resistors values for non-tweaked BINs shall be entered as TBE. This value will then be updated via a CN once the PCB assembler provides PCBs mounted with the available BINs.

A note in the 16xxx drawing is to require the PCB assembler to allow two weeks from the receipt of the new BIN PCBAs for a tweak and drawing update to occur.

PCB assembly Views

The PCB assembly views should show the view of the PCB with the components placed on the board.  Dimensioning is not necessary unless to define the location of a component with respect to some PCB datum.  However it should be noted that SMD component locations are determined by the pad location and pad design and the tolerances will be wide, so if special tolerances are required, they should be indicated accordingly. Other dimensions shall be shown as REF DIMS.

The drawing shall show top and bottom view, whether or not components are assembled on both sides. 

Each component shall be shown in outline.

Polarity or orientation indicator and component reference designator shall be shown.

The component reference shall be traceable to the schematic view and the BOM table. 

Test points shall be identified with the same detail as components.

A 3D view of the PCB may be shown if this will improve understanding of the assembly, but it is not strictly necessary.

Dimensions, notes and information which are related to the bare board shall NOT be included in the PCB assembly drawing, but in the Bare Board drawing.

The PCB assembly views normally occupy the last sheet of the drawing.

Connector Detail View

Where applicable, views showing connector dimensions, materials, plating and coining details shall be included in the PCB assembly drawing. 

The Bare Board Drawing – 44xxx

The PCB bare board drawing is a separate drawing from the assembly drawings. The PCB 44XXX drawing will have a full title block and revisions control as shown in Figure 2. Since the PCB assembly refer to the single board, any changes will affect both drawings. 

 

Figure 2: The standard bare board drawing border

This drawing shall detail all the mechanical dimensions and tolerances that are important to the design. It shall also clearly identify and label the Top and Bottom layers of the PCB. When applicable, intermediate inner layers shall also be identified and labelled. These shall follow the naming convention of Inner 1, Inner 2, etc…, with the first inner layer being the one closest to the top layer.  Dimensioning of the holes shall only be included if critical to the assembly. The drawing shall also detail the following as applicable: –

Board material.

Copper and all other plating thickness specification and tolerances.

Areas of the PCB requiring additional plating processes.

Solder resist finishes.

Silk screen and identification where applicable.

Reference to any manufacturing standards that must be observed.

Carbon finishes tolerances, potentiometer resistance and linearity.

Edges or edge sections which shall be routed.

Edges or edge sections which should be scored for snap-off.  

Indicate if the circuit tracks should be tested for continuity.

Indicate conformal coating if required and coating type and locations.

Indicate where the supplier may place their traceability identification data.

Indicate an unused area for the panelisation reference co-ordinates.

Indicate ‘keep out’ zones

The panelisation details shall only be defined when Methode are to purchase PCBs in panelised form. Otherwise a note reading “Panelisation to be defined by PCB assembly supplier” shall be included with the general notes list.

 

The bare board drawing does not need to the show the artwork pattern unless it aids the understanding of the design. 

Flexible PCBs shall be drawn with the same details.

Artwork data

The bare board drawing shall include a table containing the file names and description of the manufacturing data of the PCB. The table shall have the following format: –

 

Table 4: Artwork Data Reference Table

A ZIP file with the number of the PCB and revision shall be created for each PCB and each revision. The file will contain all Gerber files, Excellon drill files and any other output files needed for the manufacture of the PCB, as listed in Table 4. The Excellon drill files shall be called ‘ContourNonPlated.ncd’, ‘ThruHoleNonPlated.ncd’ and ‘ThruHolePlated.ncd’, and shall be placed in a folder within the zip file called ‘NCDrill’. The file name of the zipped file shall be 44xxx-nn.zip, where 44xxx is the bare board number and nn is its revision number, which shall correspond to the drawing revision number. 

Tolerances

Tolerances on dimensions of PCBs, placement of components, hole diameters, etc… will vary between PCB suppliers, depending on their equipment. Thus, when tolerances are critical on particular dimensions it is recommended that these are specifically highlighted during a design review with the supplier, in order to verify that they can be maintained.

The table below gives an overview of typical dimensions that can be achieved by most PCB suppliers and can be used as a guideline.

 

General Recommendations

Block letters shall be used for all text, except for units.

All tables shall be numbered.

Ensure the standard Notes are included in the panelisation and single PCB drawings.

All drawings must be readable when printed on A4. Use appropriate scaling to ensure readable prints.

The schematic should be laid out in blocks if necessary with named block to block connectors to aid legibility. Avoid a “rats nest appearance”.

Make notes on the schematic that will aide suppliers in the understanding of the circuit and provide them with the information they need to test the PCBs.

Name signals where this aids understanding.

LED tables should be included on the BOM sheet where resistors are defined for each LED bin. Ensure LEDs are defined fully by colour range and intensity bin range.

Drawing may only be release on the standard PCB border or the reduced standard PCB border.

Standard Drawing Notes.

The notes below will be shown on all drawings. Those maked “A” are assembly drawing notes and those marked “B” are bare board drawing notes.