Overview of DFM Technical Requirements for PCB Process

DFM (Design for manufacturability) is the design for manufacturing, which is the core technology of concurrent engineering. PCB design and manufacturing are the most important two links in the product life cycle. Concurrent engineering is to consider factors such as manufacturability and assemblability of products at the beginning of design. Therefore, DFM is the most important supporting tool in concurrent engineering. Its key is the process analysis of design information, evaluation of manufacturing rationality and suggestions for improving design. In this paper, we will briefly introduce the general technical requirements of DFM in PCB process.

1、 General requirements

1. This standard, as a general requirement for PCB design, standardizes PCB design and manufacturing and realizes effective communication between CAD and CAM.

2. Our company gives priority to the design drawings and documents as the production basis when processing documents.

2、 PCB material

1. Substrate

The base material of PCB is generally epoxy glass cloth copper clad plate, namely FR4. (including single panel)

2. Copper foil

a) 99.9% or more of electrolytic copper;

b) Thickness of copper foil on the surface of finished double-layer board ≥ 35? m（1OZ）； If there are special requirements, they shall be indicated in the drawings or documents.

3、 PCB structure, dimensions and tolerances

1. Structure

a) All relevant design elements constituting PCB shall be described in the design drawing. The appearance shall be uniformly represented by Mechanical 1 layer (priority) or Keep out layer. If it is used in the design document at the same time, generally keep out layer is used for shielding without opening, and mechanical 1 is used for forming.

b) In the design drawing, it indicates the opening of long SLOT holes or hollows, and the mechanical 1 layer can be used to draw the corresponding shape.

2. Plate thickness tolerance

3. Boundary dimension tolerance

The overall dimensions of PCB shall comply with the provisions of the design drawing. When the drawing is not specified, the tolerance of overall dimensions is ± 0.2mm. (except V-CUT products)

4. Flatness (Warp) Tolerance

The flatness of PCB shall comply with the provisions of the design drawing. If there is no specification in the drawing, the following shall be followed

4、 Printed conductors and pads

1. Layout

a) In principle, the layout, line width and line spacing of printed wires and bonding pads shall comply with the provisions of the design drawings. However, our company will have the following treatment: the line width and PAD ring width will be compensated appropriately according to the process requirements. For single panel, our company will generally increase PAD as much as possible to enhance the reliability of customer welding.

b) When the design line spacing fails to meet the process requirements (too dense may affect the performance and manufacturability), our company will make appropriate adjustments according to the pre manufacturing design specifications.

c) In principle, we suggest that when designing single and double sided boards, the customer should set the inner diameter of the through hole (VIA) above 0.3mm, the outer diameter above 0.7mm, the line spacing at 8mil, and the line width at 8mil. To minimize the production cycle and manufacturing difficulty.

d) The minimum drilling tool of our company is 0.3, and the finished hole is about 0.15 mm. The minimum line spacing is 6mil. The width of the thinnest wire is 6mil. (But the manufacturing cycle is long and the cost is high)

2. Conductor width tolerance

The internal control standard for width tolerance of printed wire is ± 15%

3. Mesh processing

a) In order to avoid blistering of copper surface during wave soldering and PCB bending due to thermal stress after heating, it is recommended to lay the large copper surface in grid form.

b) The grid spacing is ≥ 10mil (not less than 8mil), and the grid line width is ≥ 10mil (not less than 8mil).

4. Treatment of thermal pad

In large-area grounding (electricity), there are often legs of components and parts connected with them. The treatment of connecting legs takes into account the electrical performance and process needs and is made into cruciform bonding pads (heat insulation pads), which can greatly reduce the possibility of false solder joints due to excessive heat dissipation of the cross section during welding.

5、 Hole diameter (HOLE)

1. Definition of Metallization (PHT) and Nonmetallization (NPTH)

a) Our company defaults to the following methods for nonmetallic holes:

When the customer sets the nonmetallic property of the mounting hole in the Protel99se advanced property (remove the ticked item from the Advanced menu), our company defaults to the nonmetallic hole.

When the customer directly uses the keep out layer or mechanical 1 layer arc in the design file to indicate the hole punching (no separate hole setting), our company defaults to the non-metallic hole.

When the customer places the word NPTH near the hole, our company defaults to nonmetallic hole.

When the customer clearly requires the corresponding aperture non-metallization (NPTH) in the design notice, it shall be handled according to the customer's requirements.

b) Except for the above situations, element holes, installation holes, through holes, etc. shall be metallized.

2. Aperture size and tolerance

a) PCB component holes and mounting holes in the design drawing are the final pore size of the finished product by default. The aperture tolerance is generally ± 3mil (0.08mm);

b) Through hole (i.e. VIA hole) is generally controlled as: negative tolerance is not required, and positive tolerance is controlled within+3mil (0.08mm).

3. Thickness

The average thickness of the copper coating of the metallized hole is generally not less than 20? m. The thinnest part shall not be less than 18? m。

4. Hole wall roughness

The PTH hole wall roughness is generally controlled at ≤ 32um

5. PIN hole problem

a) The minimum locating pin of our CNC milling machine is 0.9mm, and the three PIN holes should be triangular.

b) When the customer has no special requirements and the aperture in the design document is less than 0.9mm, our company will add a PIN hole at the blank wireless path or the appropriate position on the large copper surface in the board.

6. Design of SLOT hole (slot hole)

a) It is recommended that SLOT holes be drawn with mechanical 1 layer (Keep out layer); It can also be expressed by connecting holes, but the connecting holes should be of the same size and the hole center is on the same horizontal line.

b) The minimum grooving knife of our company is 0.65mm.

c) When SLOT holes are opened for shielding to avoid creepage between high and low voltage, it is recommended that the diameter be more than 1.2mm to facilitate processing.

6、 Solder mask

1. Coating position and defect

a) The surface of PCB other than bonding pad, MARK point and test point shall be coated with solder mask.

b) If the customer uses FILL or TRACK to represent the disc, a figure of corresponding size must be drawn on the Solder mask layer to represent the tin coating. (Our company strongly suggests not to use non PAD presentation board before design)

c) If it is necessary to dissipate heat on the large copper sheet or spray tin on the wire strip, a figure of corresponding size must also be drawn with solder mask layer to indicate the tin coating.

2. Adhesion

Adhesion of solder mask shall comply with the requirements of Grade 2 of IPC-A-600F in the United States.

3. Thickness

The thickness of welding resistance layer shall conform to the following table:

7、 Characters and Etching Marks

1. Basic requirements

a) Characters of PCB shall be generally designed with a character height of 30mil, a character width of 5mil, and a character spacing of more than 4mil to avoid affecting the legibility of characters.

b) Etched (metal) characters shall not be bridged with conductors and adequate electrical clearance shall be ensured. Generally, the word height is 30mil and the word width is more than 7mil.

c) If the customer has no specific requirements for characters, our company will generally adjust the matching proportion of characters according to our process requirements.

d) When the customer has no clear regulations, our company will print our trademark, material number and cycle at the appropriate position of the silk printing layer in the plate according to our process requirements.

2. Processing of PADSMT in words

The PAD shall not be marked with silk screen layer to avoid false soldering. When the customer designs a PADSMT, our company will make appropriate movement, and the principle is not to affect the correspondence between its logo and the device.

8、 The concept of layer and the design of MARK point processing layer

1. For double-sided boards, our company takes the top layer as the front view by default, and the characters on the top overlay screen layer are positive.

2. For single panel, the Top layer is used to draw the Signal layer, which means that the line of this layer is the front view.

3. The single panel uses the top layer to draw the signal layer, which means that the line of this layer is a perspective surface.

MARK point design

4. When the customer has a SMT for the panel file and needs to use the Mark point for positioning, the MARK must be put in place, with a circular diameter of 1.0mm.

5. When the customer has no special requirements, our company uses the arc of Solder 1.5mm to represent the non blocking flux, so as to enhance the identifiability. Place one on the FMask layer

6. When the customer does not place MARK at the process edge of the surface patch for the panel file, our company generally adds a MARK point at the center of the diagonal of the process edge; When the customer has a surface patch without process edge for the panel file, it is generally necessary to communicate with the customer whether it is necessary to add MARK.

9、 About V-CUT

1. There is no gap between the V cut panel and the panel, but the distance between the conductor and the V cut centerline should be noted. Generally, the distance between conductors on both sides of V-CUT line shall be more than 0.5mm, that is to say, the distance between conductors in a single board and the board edge shall be more than 0.25mm.

2. The V-CUT line is generally represented by the keep out layer (Mech 1) layer, so the parts of the board that need to be V-cut only need to be drawn with the keep out layer (Mech 1) layer, and it is better to mark the V-CUT words at the board connections.

3. As shown in the figure below, generally the residual depth after V-cutting is 1/3 of the plate thickness, which can be adjusted appropriately according to the customer's residual thickness requirements.

4. The size of V-cut products will be slightly out of tolerance due to the phenomenon that the glass fiber is stretched after breaking, and some products will be larger than 0.5mm.

5. V-CUT knife can only go straight line, not curve and polyline; The thickness of the guyed plate is generally more than 0.8mm.

10、 Surface treatment process

When the customer has no special requirements, our company adopts the hot air leveling (HAL) method by default for surface treatment. (i.e. tin spraying: 63 tin/37 lead)

The above DFM general technical requirements (single and double sided boards) are the reference for our customers when designing PCB files, and we hope to reach some agreement on the above aspects, so as to better realize the communication between CAD and CAM, better realize the common goal of Design for Manufacturability (DFM), better shorten the product manufacturing cycle and reduce production costs.

11、 Conclusion

The above DFM general technical requirements (single and double sided boards) are only the reference provided by Century Core for customers when designing PCB files, and we hope to negotiate and reconcile the above aspects to better realize the communication between CAD and CAM, better realize the common goal of Design for Manufacturability (DFM), shorten the product manufacturing cycle, and reduce production costs.