PCB Manufacturing

What are Gerber files for Printed Circuit Boards, and how are they used by PCB manufacturer?

What is Gerber files?

Gerber files are a type of file format used to describe PCB designs. They contain the necessary data for a PCB manufacturer to produce a given design, including the copper layers, solder mask, component placement and silkscreen.

Gerber files are named after Gerber Scientific Instrument Company, which first developed the Gerber format. Gerber files are the standard for PCB design and can be imported by most CAD/CAM tools to allow for more complex editing and manufacturing of the board.

Gerber files are the most commonly used PCB files for PCB manufacturer. Engineers in PCB company read Gerber files to get the information needed such as copper trace pattern, drill size, PCB material, dimension, thickness, stack-up,surface finish etc, and then convert to production files which used in the production.

Three normally used formats for Gerber files

Gerber files usually has three formats as below:
RS-274-X (Gerber and aperture are combined together)
RS-274-D
RS-274 (Gerber and aperture separated )

Typical layers in Gerber files

Typical layers in Gerber files

Introduction for layers in PCB files

In Gerbers, each layer are separated as a files. We can use CAM350 or any other software to import Gerbers. Usually, Gerber files has a standard .suffix which is helpful to recolonize the layers, for example a 6 layer PCB should contain the following files in the package:

 

    • ↝Top Paste .GTP –top paste layer (for top layer solder stencil)
    • ↝ Top Overlay .GTO –top silkscreen layer
    • ↝Top Solder .GTS –top solder mask layer
    • ↝Top Layer .GTL –top copper layer
    • ↝MidLayer1 .G1 –inner layer 1
    • ↝Internal Plane1 .GP1 –inner power plane layer 1 (negative)
    • ↝MidLayer2 .G2 –inner layer 2
    • ↝Internal Plane2 .GP2 –inner power plane layer 2 (negative)
    • ↝Bottom Layer .GBL –bottom copper layer
    • ↝Bottom Solder .GBS –bottom solder mask layer
    • ↝Bottom Overlay.GBO –bottom silkscreen layer
    • ↝Bottom Paste .GBP –bottom paste layer(for bottom layer solder stencil)
    • ↝Mechanical1 .GM1–mechincal layer 1 (for inner slots)
    • ↝Mechanical2 .GM2–mechincal layer 2 (for inner slots)
    • ↝Keep Out Layer .GKO –outline of the PCB
    • ↝Aperture Data.APR –aperture file
    • ↝Drill Data .DRL –drill file
    ↝Drill Report.LDP– drill report
Layers in PCB files

Layers in PCB files

How are Gerber files used by PCB manufacturer

Gerber files are used by PCB manufacturer to produce PCBs. And engineers in PCB manufacturing company will read the Gerber files and fully understand the printed circuit boards before converting to production files.

Generally, the data in Gerber files are divided into two parts as picture below. One part is layout file (generally in the form of GERBER and NC Drill, and some simple flexible boards can also be in the form of CAD format file), the other part is the specification of PCB fabrication, which generally describes the material properties, board thickness, copper thickness (some multi-layer boards define lamination), solder mask&silkscreen color, surface treatment, and special process.

PCB Gerber files

PCB Gerber files

If the customer has no specification documents, and there are no fabrication instructions in the Gerber file, we should confirm the fabrication technique with the customer.

In the inspection of specific data, we need to grasp the main information first, so as to determine the difficulty of the board, and then determine the basic price and delivery of the board.

Information we get from Gerber files

We can get all enough information to produce a printed circuit board from a complete Gerber files package, such as base material type, stakup, copper trace and solder mask pattern, drill size and position, surface finish, panel and all other information needed.

1) Base material,include copper thick&board thick,, rigid PCB or flexible PCB, and then confirm its layers, Usually, these are also the two most important parts that affect PCB price, PCB material information are normally described just like photo below.

In this photo,we can see that this board must choose IS410 FR4 raw material or equivalent TG180 material,1oz on inner layer and 1.5oz on outer layer,and 0.063inch board thickness.

PCB base material in Gerber files

PCB base material in Gerber files

The normal raw material for PCB have FR4,PI,high Frequency,aluminum substrate and copper substrate.,etc.

In addition to raw material type, it must be mentioned that the TG value is another key parameter for FR4 material.

TG value determines the heat resistance, moisture resistance, chemical resistance, stability and other characteristics of printed boards.

The higher the TG value is, the better the temperature resistance function of the plate is. Especially in the lead-free tin spraying process, high TG is used more often.

TG values are generally divided into three types: low TG 135, medium TG 150, and high TG 170

If the customer does not specify it, it is necessary to find the specification of the PCB type for comparison.

Some customers do not know the TG value and attributes of PCB. Our factory can give reasonable suggestions according to the use scenario of the board.

However, the higher the TG value is, the higher the cost of PCB is, so don’t blindly ask for higher TG, just find the appropriate TG value of the product itself.

About high Frequency substrate, usually choose Panasonic and DuPont brand, Customers who design high-frequency boards are professional and know which type of materials to choose, our more consideration is the inventory and procurement cycle of material.

For Aluminum PCB, Aluminum substrate is composed of circuit layer (copper foil), insulation layer and metal base layer. Compared with traditional FR-4, aluminum substrate can minimize thermal resistance and make aluminum substrate have excellent thermal conductivity. Therefore, it is common in LED lighting products. The thermal conductivity of aluminum substrate pcb is 1.0, 1.5, 2.0, 3.0, 5.0, and the most used are 1.0 and 2.0 W/m.k, meanwhile,Different thermal conductivity parameters correspond to different prices of aluminum substrate materials.

2) Stack-up , Standard stack up should including layers order,dielectric thick,copper thick, the finished board thickness, if HDI board, blind & buried holes demonstration will be also shown,the professional stack-up even includes Core and Prepreg like below,
HDI PCB stack-up

HDI PCB stack-up

If flex-rigid board, need to point out rigid and flex part separately, to make the lamination look more clear, normally, we use the following photo to show them.
Rigid-flex PCB stack-up

Rigid-flex PCB stack-up

3) Solder mask and silkscreen, usually, involving its part number and solder mask color, such as LPI TAIYO PSR-4000 series, the usual color for solder mask must be green, meanwhile, the blue color and black color is also normal, and sometimes, there are description of “dummy color”,it is one of special color in solder mask ink, this case we have to pay attention and clearly show on manufacturing instruction,normally,white solder mask are used for LED board,black S/M used for camera product, the green S/M used in general product. Just like below, need to use matte black instead of normal black.
Solder mask layer in Gerber files

Solder mask layer in Gerber files

4) Surface finish , normally, including HASL – Hot Air Solder Leveling, Lead Free HASL, ENIG- Electroless Nickle/Immersion Gold, ENEPIG-Electroless Nickel Electroless Palladium Immersion Gold,Immersion Silver , Immersion Tin, OSP – Organic Solderability Preservatives.

the thickness of tin thick in the HASL process and the thickness of gold are also to be paid attention to. If the thickness is not defined, it is normally implemented according to the IPC-A-class 2 standard. IPC-A-class 3 is a high standard for processing, and needs to be clearly explained by the customer. We can only judge whether the class 3 standard is based on individual parameter requirements.

Among them, special attention should be paid to the requirements of gold fingers, electroplating of thick gold, and related beveled edges and requirements, normal 10-30u” gold plating for gold finger area. (notes,some pad arranged like fingers, which is used to insert into the card slot and is in contact with the metal spring plate in the card slot. Because this kind of design has high requirements on the wear resistance and electrical conductivity of the pad surface, a layer of nickel and a layer of gold will be plated on the pad surface, so it is generally referred to as a gold finger),A row of pads and slots with finger shape like the following is a typical gold finger board.

Gold connectors in Gerber files

Gold connectors in Gerber files

5) Other special tolerance ,technique and standard
5.1 one of main parameter affecting the difficulty of PCB fabrication should be tolerance, including board thickness tolerance, holes tolerance,War-page tolerance and impedance tolerance.,etc. If these tolerances are within the range of various standards, such as IPC-A-600 class2 and class3, we can only note the manufacturing standards. If some tolerances are beyond the standard tolerance range, we should pay special attention to them. If they can be controlled separately, we should find ways to achieve them. But if they are beyond the factory’s manufacturing capacity, we should feed back to the customer, issue an EQ confirmation, further communicate with the customer, and strive to relax the tolerance requirements, So as to meet the process capacity of the factory. The following is the impedance tolerance.+/- 5% is actually difficult to control, and the normal tolerance is+/- 10%.
PCB controlled impedance in Gerber files

PCB controlled impedance in Gerber files

5.2 The other difficulty is special technique,
HDI PCB
The priority is to pay attention to the type of HDI, which belongs to several levels of HDI, and then check the thickness and requirements of the dielectric layer in the laminate. If there is a specified thickness requirement of the dielectric layer, it is usually necessary to communicate with the customer to meet the actual blind hole processing requirements, and also pay attention to whether the blind hole is on the SMT pad. If there is, it is usually necessary to do VIPPO, which has met the customer’s SMT requirements.
The following photo is the demonstration of a typical 2+N+2 HDI board, L2-3/L10-11 for the first laser drilling, L1-2/L11-12 for the second laser drilling.
2+10+2 HDI PCB

2+10+2 HDI PCB

Flex-rigid PCB
The difficulty of rigid-flexible boards is processing and manufacturing, especially the control of yield. The following picture is the classic rigid-flexible board data. The relatively narrow area is the flexible part, and the other is the rigid area. Of course, the details should be based on the data.
Rigid flex PCB manufacturing

Rigid flex PCB manufacturing

Thick copper PCB
he requirement for copper is relatively thick. Generally, 3OZ-6OZ is thick copper plate, which tests the etching control ability of the factory.
Other Techniques
Peelable solder mask or Kapton tape:
It is usually used instead of DIP fixture to cover the position of some holes to prevent tin from entering some holes during wave soldering. The previous blue glue process is gradually replaced by yellow tape named Kapton tape.
Edge plating:
As shown in the following picture, when the copper is laid outside the board and the solder mask is opened outside the board, most of them need to be electroplated at the board edge, but most of them will be explained by the customer. If not, our engineers will also communicate with the customer to confirm whether the electroplated at the board edge is necessary
PCB edge plating

PCB edge plating

Vias in pad VIPPO Via in Pad Plated over technique,VIPPO is also used to ensure the welding ability of SMT pads, especially BGA pads.

Carbon Mask:

The carbon oil board is mostly used for key boards, making full use of the conductivity of carbon oil. After heating and curing, the ink film is not easy to oxidize, and the performance is stable.,etc. The following picture is the classic key board with carbon oil technique for reference.

Carbon mask on PCBs

Carbon mask on PCBs

There are also some unusual technique related to holes ,such as Countersink holes, Press fit hole

6) layout checking Layout checking is the last part of all data inspection, and it is also related to the process capability of PCB processing and cam engineering production. For the circuit partcking , usually check the size and spacing of BGA, the minimum line width and line spacing, and whether there is edge plating, etc.
BGA layout checking

BGA layout checking

In the solder resist part Layout checking is the last partwe will generally check the solder mask bridge problem, check whether the solder mask opening is matched with the corresponding pad, and pay special attention to the problems such as missing solder mask opening or exposure of the line and base material, if our engineer can’t make sure how to do, normally make EQ confirmation with the customer.

In the silkscreen part, it is mainly to check whether silkscreen characters are on pads ,whether the part number and version number on silkscreen layer are consistent with the product model, and often encounter that the original silkscreen frame is beyond the outline of board,in this case, we should consider the problem of Panel and SMT assembly.

The drilling part, mainly checks the size of holes, the spacing between holes, the spacing between holes and pads, and whether the Vias are on SMT pads, etc

For the panel part, In addition to the reasonable utilization rate of plates and the convenience of PCB processing, the convenience of SMT mounting and processing should also be considered. if there are component beyond the board, it is usually necessary to add frame or adjust the panel spacing and the panel direction to facilitate SMT processing.

PCB panel design

PCB panel design

The following 4UP is the panel we adjusted according to this situation, that is, we need to reverse single piece, and increase the piece spacing and breakaway tabs to meet the actual SMT manufacturing requirements.

PCB breakaway tabs

PCB breakaway tabs

The following is the picture of a single finished product after SMT.

Finished PCB after SMT

Finished PCB after SMT

At last, Other matters not mentioned and relatively tiny hidden problems need to be judged and dealt with by engineers’ experience. Those that need to communicate with customers should be communicated, and those that do not need to communicate should be dealt with as usual.

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