PCB Tech - Summary of PCB Process DFM Technical Requirements

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

Noe, general requirements

1. As a general requirement for PCB design, this standard regulates PCB design and manufacturing and realizes effective communication between CAD and CAM.

2. Our company will give priority to design drawings and documents as production basis when processing documents.

Two, PCB material

1. Substrate

The substrate of PCB generally adopts epoxy glass cloth copper clad laminate, namely FR4. (Including single panel)

2. Copper foil

a) More than 99.9% electrolytic copper;

b) The thickness of copper foil on the surface of the finished double-layer board is ≥35?m (1OZ); when there are special requirements, specify in the drawings or documents.

Three, PCB structure, size and tolerance

1. Structure

a) All relevant design elements that make up the PCB should be described in the design drawings. The appearance should be uniformly represented by Mechanical 1 layer (priority) or Keep out layer. If it is used in the design file at the same time, generally keep out layer is used for shielding, without opening holes, and using mechanical 1 for forming.

b) In the design drawing, it means to open a long slot hole or hollow out, and use the Mechanical 1 layer to draw the corresponding shape.

2. Board thickness tolerance

3. Overall dimension tolerance

The external dimensions of the PCB should conform to the requirements of the design drawings. When the drawing is not specified, the tolerance of the external dimension is ±0.2mm. (Except for V-CUT products)

4. Flatness (warpage) tolerance

The flatness of the PCB should meet the requirements of the design drawings. When the drawing is not specified, follow the instructions below

Four, printed wires and pads

1. Layout

a) In principle, the layout, line width and line spacing of printed wires and pads shall be in accordance with the design drawings. However, our company will deal with the following: appropriately compensate the line width and PAD ring width according to the process requirements. In general, our company will try to increase the PAD as much as possible for the single panel to enhance the reliability of customer welding.

b) When the design line spacing does not meet the process requirements (too dense may affect performance and manufacturability), our company will make appropriate adjustments according to the pre-manufacturing design specifications.

c) In principle, our company recommends that when customers design single and double panels, the inner diameter of the via (VIA) should be set to 0.3mm or more, the outer diameter to be set to 0.7mm or more, the line spacing design is 8mil, and the line width design is 8mil or more. In order to reduce the production cycle to the greatest extent, reduce the manufacturing difficulty.

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

2. Wire width tolerance

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

3. Grid processing

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

b) The grid spacing is greater than or equal to 10mil (not less than 8mil), and the grid line width is greater than or equal to 10mil (not less than 8mil).

4. Treatment of thermal pad

In large-area grounding (electricity), the legs of components are often connected to it. The treatment of the connecting legs takes into account the electrical performance and process requirements, and is made into a cross-patterned pad (heat insulation plate). The possibility of excessively dispersing heat and producing virtual solder joints is greatly reduced.

Five, aperture (HOLE)

1. Definition of metallization (PHT) and non-metallization (NPTH)

a) Our company defaults the following methods as non-metallized holes:

When the customer sets the non-metallized properties of the mounting holes in the advanced properties of Protel99se (remove the plated item in the Advanced menu), our company defaults to non-metallized holes.

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

When the customer places NPTH near the hole, our company defaults that the hole is non-metallized.

When the customer clearly requires the corresponding aperture non-metallization (NPTH) in the design notice, it will be handled as required by the customer.

b) All component holes, mounting holes, via holes, etc. other than the above should be metallized.

2. Aperture size and tolerance

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

b) The via hole (ie VIA hole) is generally controlled by our company: negative tolerance is not required, and positive tolerance is controlled within + 3mil (0.08mm).

3. Thickness

The average thickness of the copper-plated layer of metallized holes is generally not less than 20 m, and the thinnest part is not less than 18 m.

4. Hole wall roughness

PTH hole wall roughness is generally controlled within ≤ 32um

5. PIN hole problem

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

b) When the customer has no special requirements, and the aperture in the design documents is less than 0.9mm, our company will add PIN holes at the appropriate positions on the blank wireless road or the large copper surface in the board.

6. SLOT hole (slot hole) design

a) It is recommended to use Mechanical 1 layer (Keep out layer) to draw the shape of the slot hole; it can also be represented by a connecting hole, but the connecting hole should be of the same size and the center of the hole should be on the same horizontal line.

b) Our smallest slot cutter is 0.65mm.

c) When opening the SLOT hole for shielding and avoiding creepage between high and low voltage, it is recommended that its diameter be above 1.2mm to facilitate processing.

Six, solder mask

1. Coating parts and defects

a) All PCB surfaces except for pads, MARK points, test points, etc. should be coated with solder mask.

b) If the customer uses the disk represented by FILL or TRACK, the corresponding size graphics must be drawn on the solder mask layer to indicate that the place is tinned. (Our company strongly recommends not to display disks in non-PAD format before designing)

c) If you need to dissipate heat on the large copper skin or spray tin on the lines, you must also draw a corresponding size figure with a solder mask layer to indicate the place where tin is applied.

2. Adhesion

The adhesion of the solder mask is in accordance with the Class 2 requirements of the US IPC-A-600F.

3. Thickness

The thickness of the solder mask conforms to the following table:

Seven, characters and etching marks

1. Basic requirements

a) PCB characters should generally be designed with a character height of 30 mils, a character width of 5 mils, and a character spacing of 4 mils or more, so as not to affect the legibility of the characters.

b) The etched (metal) characters should not bridge the wires and ensure sufficient electrical clearance. The general design is designed with a character height of 30mil and a character width of 7mil or more.

c) When the customer's characters have no clear requirements, our company will generally adjust the proportion of characters according to our company's process requirements.

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

2. PAD\SMT processing on text

There should be no silk screen layer logo on the disk (PAD) to avoid false welding. When the customer has designed PAD\SMT, our company will move it appropriately, and the principle is that the correspondence between the logo and the device will not be affected.

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

1. By default, our company takes the top layer (that is, the Top layer) as the front surface of the double panel, and the characters on the topoverlay silk screen layer are positive.

2. The signal layer is drawn on the top layer of the single panel, which means that the circuit of this layer is the front view.

3. The signal layer is drawn on the top layer of the single panel, which means that the circuit of this layer is a perspective surface.

MARK point design

4. When the customer has a surface mount (SMT) for the jigsaw file and needs to use the Mark point for positioning, the MARK must be placed, which is a circle with a diameter of 1.0mm.

5. When the customer has no special requirements, our company uses a 1.5mm arc of Solder to indicate that there is no solder resist to enhance the recognizability. Place one on the FMask layer

6. When the customer does not put a MARK for the jigsaw file with surface mounts and process edges, our company generally adds a MARK point to the middle of the diagonal corners of the process edges; when the customer has surface mounts and no process edges for the jigsaw file, Generally need to communicate with customers whether to add MARK.

Nine, about V-CUT (cutting V-shaped groove)

1. There is no gap between the V-cut jigsaw board and the board. But pay attention to the distance between the conductor and the V-cut center line. In general, the distance between the conductors on both sides of the V-CUT line should be more than 0.5mm, that is to say, the distance between the conductors in a single board should be more than 0.25mm from the side of the board.

2. The representation method of the V-CUT line is: the general shape is represented by the keep out layer (Mech 1) layer, then the place in the board that needs to be V cut only needs to be drawn with the keep out layer (Mech 1) layer, and it is best to draw it on the board The connection is marked with V-CUT.

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

4. After the V-cut product is broken, the size of the glass fiber yarn will be loosened, and the size will be slightly out of tolerance, and some products will be larger than 0.5mm.

5. The V-CUT knife can only go straight, not curved and broken lines; and the thickness of the drawable board is generally above 0.8mm.

Ten, surface treatment process

When the customer has no special requirements, the surface treatment of our company adopts hot air leveling (HAL) by default. (Ie spray tin: 63 tin/37 lead)

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

eleven, concluding remarks

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