Once upon a time, before any PCB design took place, circuit designers used to take an entire tour of a PCB shop. They communicated with fabricators and all face to face, over their expectations in input and output. It helped them to understand and prevent errors. But today’s era of outsourcing has changed this scenario. People now outsource most of their boards. Therefore, Sierra Circuits is really transparent about its manufacturing process. Knowing the process in and out may not eliminate but can surely trace down the possibilities of errors to a great extent.
Before we dive into manufacturing steps let’s be clear about our core material.
Design and Output
Manufacturing of a PCB commence with a design. PCB manufacture always start with a specific scheme: the designer laying out a blueprint for the PCB that abides by all the outlined requirements. The customer provides the manufacturers with a basic design to the designers in Gerber format.
Once a design layout for the PCB is fed into the Gerber Extended software, all the different aspects of the design are looked over to ensure no errors.
The completed PCB design is taken to a PCB fabrication house. On arrival, the design undergoes a second check by the fabricator, known as a Design for Manufacture (DFM) check. A proper DFM check ensures that the PCB design is manufacturable.
From File to Film
Manufacturers use a special printer called a plotter, which makes photo films of the PCBs, to print circuit boards. Manufacturers will use the films for imaging the PCBs. Although it’s a laser printer, it isn’t a standard laser jet printer.
But this filming process due to miniaturization and technological advancements is not adequate anymore. It is, we can say becoming obsolete. Now we mostly use LDIs.
The traditional imaging method needs a photo-tool and UV-light to transfer images. However, LDI solely uses a computer-controlled, irradiation to directly outline the circuit pattern onto the board. The panel is coated with a layer of photoresist. The CAM files are preloaded into the laser. The circuit pattern gets digitally printed onto the board.
For perfect alignment of all films, registration holes are done.
Image Transfer: Inner Layers
In PCB manufacturing, cleanliness is a real matter of concern. During this stage, it’s crucial that no dust particles settle on the laminate. Next, the panel is given a layer of photo-sensitive film called photo-resist. The photo-resist comprises a layer of photo reactive chemicals that polymerize with the exposure to ultra-violet light. This panel is now positioned under a computer-controlled laser. Computer scans the board surface into a digital image. Matching the digital image to a pre-loaded CAD/CAM design file that contains the specifications for the desired image intended for the board, the laser is used to directly generate the image on the board. Then a negative image is developed on the inner layers of the board.
Removal of Unwanted Copper
The film and board gets a flash of UV light. The light passes through the required parts of the film, hardening the board traces on the photo-resist.
The unhardened photoresist is removed and the hardened resist protect the desired copper, the board proceeds to unwanted copper removal. We use acidic etchant to wash off the excess copper. Meanwhile, the copper we wish to keep remains fully covered beneath the layer of photo-resist.
Etching process is a very significant step in the PCB manufacturing, therefore, requiring some special attention.
In PCB fabrication, etching is a process of removal of unwanted copper (Cu) from the circuit board. The unwanted copper is nothing but the non-circuit copper that is removed from the board. As a result, the desired circuit pattern is achieved. During this process, the base copper or the start copper is removed from the board.
Before the etching process, the designer’s desired image of the circuit is transferred on to a PCB by a process called Photolithography. This forms blueprint with that decides which part of the copper must be removed.
The PCB manufacturers usually employ a wet etching process. In wet etching, the unwanted material gets dissolved when immersed in a chemical solution.
There are two methods of wet etching:
1. Acidic etching (Ferric chloride and Cupric chloride).
2. Alkaline etching (Ammoniacal)
The acidic method is used to etch off the inner layers in a PCB. This method involves chemical solvents like Ferric chloride (FeCl3) OR Cupric Chloride (CuCl2).
The alkaline method is used to etch off the outer layers in a PCB. Here, the chemicals utilized are chloride copper (CuCl2 Castle, 2H2O) + hydrochloride (HCl) + hydrogen peroxide (H2O2) + water (H2O) composition. The alkaline method is a fast process and is a bit expensive.
The important parameters to be considered during the etching process are the rate of the panel movement, spray of the chemicals, and the amount of copper to be etched off. The whole process is implemented in a conveyorized, high-pressure spray chamber.
Once the unwanted copper is removed, the board is processed for stripping where the tin or tin/lean or the photoresist is removed from the board.
Inspection and Layer Alignment
With all the layers clean and ready, the manufacturers ensure alignment punches for line up. The registration holes align the inner layers to the outer ones. The technician places the layers into a machine called the optical punch for a precise inner and outer layer alignment.
Once the layers are aligned, it’s impossible to correct any errors occurring on the inner layers. Therefore, a machine performs an automatic optical inspection of the panels to confirm an absolute defect free board.
The aligned layers now await to get united. In this stage with the layers ready and confirmed they just require to be fused. The PCB takes its shape in here.
It all begins by placing a prepreg layer over alignment top. Next, sheets of prepreg are stacked on the top of the copper layer. Now it’s ready for pressing.
The bonding press computer orchestrates the process of heating up the stack, applying pressure, and allowing the stack to cool at a controlled rate.
All components slotted for later on use, such as copper-linking via holes and leaded aspects, rely on the precision of drill holes. To find the location of the drill spots, an x-ray locator is used. Then, proper registration holes are punched to secure the stack for the series of upcoming holes. Before drilling, the manufacturers place a buffer board above and beneath the drill target to ensure a clean and burr-free hole is enacted.
Plating and Electroless Copper Deposition
The first step in the plating process is the chemical deposition of a very thin layer of copper on the hole walls. After a thorough cleaning, the panel goes through consecutive chemical baths. A one-micron thick copper layer gets deposited on the surface of the panel. The copper goes into the recently drilled holes.
Outer Layer Imaging
In inner-layer imaging, we applied photoresist to the panel. We shall do that again except this time, we image the outer layers of the panel with PCB design. We begin with the layers in a sterile room to prevent any contaminants from sticking to the layer surface, then apply a layer of photoresist to the panel. Then use LDIs to print the image. The image developed here is the positive image.
Here we return at the plating. The uncovered areas of the panel from the outer layer photoresist stage get the copper electro-plating. Following the underlying copper plating, the panel for the most part gets tin plating, which allows the removal of all the copper left on the board slated for expulsion. The tin protects the segment of the panel intended to stay secured with copper amid the following etching stage. Etching scraps, the undesirable copper foil from the board. In the process we deposit minimum of 1 mil (25 micron) of copper plating.
Once the panel has been plated the photo-resist becomes undesirable and needs to be stripped from the panel. This is done in a horizontal process containing a pure alkaline solution which efficiently removes the photo-resist leaving the base copper of the panel exposed for removal in the following etching process.
The tin, guards the ideal copper amid this stage. The undesirable exposed copper and copper underneath the rest of the resist layer experience removal. In this etching we use ammoniacal etchant to etch off the undesirable copper. In the meantime, the tin secures the required copper during this stage.
The conducting regions and connections get legitimately settled at this stage.
Post etching process, the copper present on the PCB is covered by the etch resist, i.e., the tin, which is no more required. Therefore, we strip it off before proceeding further. You can use concentrated Nitric acid to remove the tin. Nitric acid is very effective in removing tin, and does not damage the copper circuit tracks below the tin metal. Thus, now you have a clear distinct outline of copper on the PCB.
Solder Mask Application
Solder mask is applied to both sides of the board, but before that the panels are covered with an epoxy solder mask ink. The boards receive a flash of UV light, which passes through a solder mask. The covered portions remain unhardened and will undergo removal.
Finally, the board is put into an oven to cure the solder mask.
Green was chosen as the standard solder mask color because it doesn’t strain on the eyes. Before machines could inspect PCBs during the manufacturing and assembling process, it was all manual inspections. The top light used for technicians to check the boards doesn’t reflect on a green solder mask and is best for their eyes.
PCBs receive hot air-levelled pads after solder masking for better solderability. The hot air levelling results in uniform pads. To add extra solder-ability to the PCB, we sometimes chemically plate them with gold or silver. Sierra Circuits can process multiple types of surface finish according to customers’ specific demands.
HASL, HASL-free, ENIG, ENEPIG
It is the most artistic process of PCB manufacturing. The almost completed board receives printing of human readable letters, normally used to identify components, test points, PCB and PCBA part numbers, warning symbols, company logos, date codes and manufacturer marks. The PCB finally passes onto the last coating and curing stage.
As a final detection, PCB manufacturers test the board for functionality. They use an automated procedure to ensure the boards conformity to its design. Bed of nails, flying probe etc. are such type of tests.
Flying Probe Testing
The flying probe test originally worked only for bare board testing. But now we know FICT or FPT are efficient in PCBs that pose accessibility issues. Also, in prototypes and low to mid-volume production. That is because the Flying Probe extends in-circuit test benefits to applications where this bed of nail is impossible to use (no access), too long to build (prototype) or too expensive to justify (high mix/low volume). It uses software instruction to move the flying probes. electro- mechanical controllers in flying probes access PCB. Equipped with four headers flying probe tester along X-Y axis and it moves at a high speed. We equip a camera on the flying probe tester to inspect component polarity. The flying probe tester can therefore get down on the number of test fixtures required. It is also becoming much easier to introduce changes, especially to features such as a pad or component positions. It takes just a software change.
Again, flying probes can access component pins directly or through automated test point probing. They don’t require test points. This improves test coverage over in-circuit tests that use beds-of-nails. It uses the vias on the board as embedded test points and uses them as test points. Therefore, the test points are no longer in action.
Profiling and V-Scoring
The final manufacturing stage is to profile the PCBs and cut them out of the production panel. The method employed either use a router or a v-groove. A router leaves small tabs along the board edges while the v-groove cuts diagonal channels along both sides of the board. Both ways permit the boards to easily pop out from the panel.