Guide

Interpreting Gerber Files: A Manufacturer’s Perspective

Interpreting Gerber Files

The purpose of the PCB is to mechanically support and electrically connect the components that are required to help make the PCB circuit function. PCB is the combination of layers of the conductors and insulating materials. Components are assembled on the top and bottom sides of the PCB.

PCB designers use different kinds of tools to design PCBs, such as Altium Designer, Cadence Allegro, Mentor Graphics Xpedition, Autodesk Eagle, KiCad., etc. However, PCB manufacturers can not use the design files for production. So PCB designers have to provide the CAD data in ODB+++ format or PCB Gerber files to manufacturers for circuit board fabrication.

PCB Layers in Gerber Files

What are Gerber Files from  A Manufacturer’s Perspective?

Gerber files are the most widely accepted file format for PCB manufacturers to produce circuit boards. Copper layers, solder mask layers, silkscreen layers and drill layers are all included in the Gerber files package, and the standard file extension is XXX.GBR or XXX.gbr. There are two major generations of Gerber format RS-274-D and RS-274X. In Gerber files, Gerber 274X must be accompanied by an independent net-list file (IPC-D-356). Designers have to always generate the Net-list file to check the electrical connectivity. Besides, stack up, read-me files, standard specifications, and special notes are also included in a standard Gerber files package.

PCB Gerber Files

What Information We Can Get From Gerber Files?

Some other factors to take care of while designing the PCB (Generally called the DFM process for PCB Manufacturers):

  1. General Information: For any PCB design, whether simple or complex, the designer will provide the board manufacturer with all the needed information. The data required are the board dimensions, board thickness, hole size, hole size location, copper thickness, solder mask, silkscreen data, each copper layer and mechanical size. All of this information is included in a PCB Gerber files package.
  2. Copper Traces: Copper traces are the critical part of Gerber files, CAM engineers from PCB manufacturers should check if they all are correct by tools, and check if the trace width and spacing are within their production capabilities.
  3. Drill Holes: All the drill positions and sizes exist in the PCB Gerber files. PCB engineers need to check and create their own drill data. For complex HDI PCBs with multiple blind and buried vias, CAM engineers may need to optimize the drill files.
  4. Edge Clearance: Designers have to provide clearance from the board edge otherwise this results in copper burrs on the board edge which affect the PCB quality.
  5. Copper Weight: Select copper weight according to minimum trace and minimum spacing. Important for the PCB manufacturer. Copper weight and layup may be written in Gerber files or readme files.
  6. PTH Annular Ring: Maintained minimum PTH annular ring for vias and plated holes. Especially when there is a Class 3 requirement. Engineers need to check the Gerber files carefully.
  7. Solder Mask: A solder mask is a non-conductive coating layer which protects the PCB areas from oxidation and unwanted soldering problems. PCB designers have to maintain sufficient spacing in the SMD array to give space for the solder mask web. Solder mask web is required between SMD pads to avoid shortage in the PCB assembly process. 3mil solder mask web between two SMD pads for provided spacing 7mil in copper. If the solder mask does not overlap on copper SMD pads solder mask clearance in SMD pads is very important. Different Solder mask colors have different solder mask web capabilities.
  8. Via-in-pads: Whenever the vias are placed on the SMD copper pads. This is called the via in pads, generally called VIPPO (Via in pads plated over). This kind of technology used in HDI boards or complex designs contains BGA or microcontroller pitch. It is best to fill these vias with non-conductive epoxy to avoid any problems in assembly soldering. High-tech or complex PCB designs use always micro vias (<6mil) in this technology and are filled with copper. To fill via-in- pads with copper is also an advance technology which is more effective.
  9. Copper Thieving:  To uniform electrolytic copper plating, the addition of the external copper thieving is recommended in low-density areas to create even copper distribution, designerS can add dots or square patterns on those isolated areas. The absence of copper thieving in Gerber files results in high or over plating of isolated features particularly impacting differential trace pairs.
  10. NPTH to Copper Clearance: Non-plated holes have sufficient clearance from copper. Generally low NPTH to trace or NPTH to SMD pads clearance creates the extra drilling process for PCB manufacturers. So, designers have to understand those things to create the design more efficiently.
  11. Hole-to-hole clearance: Hole-to-hole clearance is also important to avoid breakout issues between two holes. Minimum hole-to-hole (for vias) spacing should be 5mil for a standard PCB manufacturer.
  12. PCB Panel: Always confirm your requirement for PCB in single board (Individually routed) form or pallet/panel style in PCB Gerber files. This should be confirmed at the time of the quotation stage to avoid any problems in the process of the PCB production. The V-score (or V-cut)  is more efficient to hold the board but scoring requires more copper-to-edge clearance on the PCB board edge. So, PCB designers have to look into this to avoid the problem of copper burs on edge. Some of the assemble suggest using tab routed with mouse bites instead of a score when there is a hanging component. This is the best way you always suggest the array to the PCB manufacturer when there is a special array requirement.
  13. Stackup and Impedance: Every designer has to design the stack up according to the standard material selection. It is very important for designer to work closely with the PCB fabricator while designing on a controlled impedance PCB. The designer would need to consider advanced electrical parameters such as controlled Impedance. It is helpful to select the fabricator first and then work on stack up before completing the design.

PCB Gerber Files with Drills

Engineering Questions About PCB Gerber files

Many times, when the CAM engineer is working on the customer Gerber files data, needs to some changes or questions to match their manufacturing capabilities. Most of the PCB fabricators will create the DFM (Design For Manufacturing)  issues or EQs (Engineering Questions) form for the designer for approval of the recommended changes.

When EQs are sent to the designers, they have to approve their recommendation or provide the signed EQs (Engineering Questions) form to start the manufacturing process as soon as possible to avoid the delay or supply an updated Gerber files package if needed.

PCB manufacturers should check all the Gerber files carefully, and raise any questions if they have any doubts, to avoid mistakes which may cause huge losses to PCB manufacturers and end customers.

PCB Stackup in Gerber Files

The Parameters in Gerber Files That Affect the PCB Cost

Lots of parameters will affect the PCB cost, such as layer counts, dimensions, surface finish, copper thickness, dielectric thickness, drill size, copper trace width and spacing, and special technologies. All of the parameters are listed in the Gerber files.

Low-Cost Drivers Complex routing-scoring

Small process increase in NC programming

Medium-Cost Drivers Drilled hole quantity

Drilled hole size

Edge plate which adds an additional process prior to plating

Solder mask plugging

<8 mils holes

<4 mils copper trace width/spacing

Copper weight

Surface finishes

High-Cost Driver Layer count >20 layers

High speed, high frequency, Teflon, PTFE

Panel utilization

Blind buried vias

HDI

Plugged vias Conductive or Nonconductive

Selective plating

Nonstandard process

Conclusion

All PCB front-end operations today utilize advanced CAM Software to computerise manufacturing details such as work order, penalization, coupon generation, tracking and impedance control. As the design will generally use advanced technology, the price of the design will generally increase. To minimize the cost, proper design, material selection and choice of supplier is critical. But PCB Gerber files are the widely used PCB file format for PCB manufacturers.

The PCB designer always has to provide all the information in the proper format and without missing any specifications, and Gerber files are the most commonly used ones. So that the manufacturer works on it without any delay. Sometimes PCB designers will forget some information like drill files in data. This type of mistake will create a delay in the process of PCB manufacturing. The PCB designer always has to mention their special requirement in the fabrication drawing. Fabrication drawings should have clear information showing all the requirements needed in the PCB. The PCB designer always needs to touch with the PCB manufacturer about their requirement and standards.

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