Manufacturing Processes for a Multi-layer PCB
Initial Process Flow Chart:
1. DFM→2. Cut Material→3. Buried Via Drilling→4. Platting Copper→5. Inner Layer Dry Film→6. Inner Etching→7. Black Oxide →8. Laminating→9. PTH Drilling →10. Plated-through hole→11. Outer Dry Film→12. Pattern Plating→13. Outer Etching→14. Solders→15. Silk Screen→16. Surface Treatment→17. Routing→18. E-test→19. Final Visual Inspection(FQA、FQC)→20. Packing and Output
A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.
PCB can be single-sided (one copper layer), double-sided (two copper layers on both sides of one substrate layer), or multi-layer (outer and inner layers of copper, alternating with layers of substrate). Multi-layer PCB allow for much higher component density, because circuit traces on the inner layers would otherwise take up surface space between components. The rise in popularity of multi-layer PCB with more than two, and especially with more than four, copper planes was concurrent with the adoption of surface mount technology.
What Are The Structure of PCB?
There are four main parts to a PCB:
Substrate: The first, and most important, is the substrate, usually made of fiberglass. Fiberglass is used because it provides a core strength to the PCB and helps resist breakage. Think of the substrate as the PCB’s “skeleton”.
Copper Layer: Depending on the board type, this layer can either be copper foil or a full-on copper coating. Regardless of which approach is used, the point of the copper is still the same — to carry electrical signals to and from the PCB, much like your nervous system carries signals between your brain and your muscles.
Solder Mask: The third piece of the PCB is the solder mask, which is a layer of polymer that helps protect the copper so that it doesn’t short-circuit from coming into contact with the environment. In this way, the solder mask acts as the PCB’s “skin”.
Silkscreen: The final part of the circuit board is the silkscreen. The silkscreen is usually on the component side of the board used to show part numbers, logos, symbols switch settings, component reference and test points. The silkscreen can also be known as legend or nomenclature.
We’ve gone over the basics of PCB and PCB anatomy now, we’ll walk through the whole process of how to make a PCB.
How Is A PCB Made LHD tech?
When LDH tech get a PCB Orders from customer, our MI&CAM engineers will make DFM for the PCB design(274X/274D/PCB). The PCB design plan undergoes a second check by the PCB Manufacturer known as a Design for Manufacture (DFM) check. A proper DFM check ensures that the PCB design fulfills, at minimum, the tolerances required for manufacture.
2. Cut Material
Cutting and laminating according to the size of the finished board or the size of the panel.The material need drying, the main purpose of drying is to remove the moisture in the board and prevent it from warping during processing. Generally, it is dried at 150℃ for 3-4 hours.
3.Buried Via Drilling
Many multi-layer PCB have buried via design, these buried via need drilling before lamination or after the first lamination. As the picture shown below, the buried via
L2-L3 need drilling before lamination.
4. Platting Copper
There is a process of chemical deposition of a very thin layer of copper on the hole walls before the plating process. Almost all PCB with 2 or more copper layers use plated through holes to connect the conductors between the layers. A good connection needs about 25 microns of copper on the walls of the holes. This thickness must be electroplated, but the walls of the holes are non-conductive glass cloth and resin. So the first step is to deposit a conductive layer over the hole walls. LDH tech use electrolysis copper, that is we deposit chemically a layer of copper about 1 micron thick over the walls of the hole (and incidentally across the whole panel). This is a multi-stage process as you see from the video with washing steps between the stages. LDH tech retreat the panel, then LDH tech seed the hole wall with micro-particles of palladium, and finally deposit the copper.
5. Inner Layer Dry Film
LDH tech image the outer layers in a clean room to make sure that no dust gets onto the panel surface where it could cause a short or open circuit on the finished PCB.
The panel is first coated with a layer of photosensitive film, the photo resist, which is hot-rolled onto the copper using a cut-sheet lamina tor. The laminated panels are collected by an automatic rack.The clean room uses yellow lighting as the photo resist is sensitive to UV light.
6. Inner Etching
We remove the unwanted copper using a powerful alkaline solution to dissolve (or etch away) the exposed copper from the inner layer. The process is carefully controlled to ensure that the finished conductor widths are exactly as designed. But designers should be aware that thicker copper foils need wider spaces between the tracks. The operator checks carefully that all the unwanted copper has been etched away.
Next we strip off the blue photo-resist which protected the copper image. So now we have the exact pattern required. The operator checks that all the photo-resist has been removed. You can see that Euro circuits put several different designs on one production panel. That way we can make small numbers of PCB cost-effectively.
7. Black Oxide/Brown Oxide
Black Oxide and Brown Oxide are two-way to roughness the inner layer core before lamination.
The outer layers of our multi-layer consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (prep-reg) and a thin copper foil.
The laminating operator need placed a copper foil and 2 sheets of prep-reg on the heavy steel baseplate. Then places the retreated core carefully over the alignment pins. Then adds 2 more sheets of prep-reg, another copper foil and an aluminum press plate.
9. PTH Drilling
LHD tech use air-driven spindles which can rotate up to 150,000 revolutions per minute. High speed drilling ensures clean hole walls to provide a secure base for good plating on the hole walls. Drilling is a slow process as each hole must be drilled individually. So depending on the drill size we drill a stack of one to three PCB panels together. We can drill holes down to 100 microns in diameter. To give you an idea of the size, the diameter of a human hair is about 150 microns. Drill change is fully automatic. The machine selects the drill to use from the drill rack, checks that it is the correct size, and then loads it into the drill head.
10. Plated-through hole
Through the chemical reaction, the hole is coated with a very thin layer of copper, about 2-3 um.
To ensure good conductivity through the holes we need to plate an average of 25 microns of copper on the hole walls. This means that we also plate 25 – 30 microns on the surface tracks. So if we start with a typical 17.5 micron copper foil it will be 40 – 42 microns after processing.
11. Outer Dry Film
12. Pattern Plating
The bare copper (that is, the copper that needs to be retained at the end) is plated to the finished copper thickness. Generally, the copper plating thickness is 18-25 um. At this time, the surface copper thickness and the copper thickness in the hole are plated together to reach the finished copper and holes copper thickness requirements.
The surface of the copper has been thickened with a layer of white metallic tin to protect the copper foil.
The dry film attached to the board is removed. At this time, the copper under the dry film will be exposed (it will be etched away), and the copper that needs to be retained will be protected by tin underneath.
13. Outer Etching
The tin protects the desired copper during this stage. By etching the line, the exposed copper will be etched away, while the copper protected by tin will remain.
The tin used to protect the copper is removed. At this time, the copper that needs to be preserved will be exposed. Up to now, all the outer circuit patterns have been completed.
Coat the entire panel with a liquid solder mask. The board is then exposed to high-intensity UV light. Solder mask is applied to achieve the following: Protect copper circuitry from oxidation, damage, and corrosion.
LDH tech prints the solder mask under customers’ requirements and do not have an extra charge.
15. Silk Screen
Silk screening is a vital step since this process is what prints important information onto the board. Then PCB finally passes onto the coating and surface finishing process.
16. Surface Treatment
This step is to enable protection of the surface and good solder ability. Common surface finishes include such as Electrolysis Nickel Immersion Gold(ENIG), ENEPIG, Hot Air Solder Leveling(HASL), Lead-Free HASL, Immersion Silver, Immersion Tin, OSP(anti-oxidation), etc.
Following the implementation of the ROHS directive in the EU, all products containing lead and bromine will not be allowed to enter the EU market; therefore, the traditional pewter process will be gradually replaced.
LDH tech defaults to the lead-free HASL for our customers If you need any other requirement, please send us with the orders.
Such as V-CUT, gold finger beveling will be done on Routing department.
V-CUT:This is the process of cutting the manufacturing panels into specific sizes and shapes based upon the customer design as defined within the Gerber data.
The method employed either centers on using 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.
Gold Finger:Primarily, PCB connection points are subject to constant plugging and unplugging owing to their nature of interconnecting PCB. Hence, without a strong contact edge, they are prone to wear-and-tear that can cause device malfunctioning. The act of plaiting the connectors with other metals (in this case gold) is done to enhance the durability of the edge connectors.
So far, the board production has been basically completed, but another important thing is to conduct electrical testing to ensure functionality. The main tests that are performed are the circuit continuity and isolation tests.
Basic electrical reliability testing, used for checking the integrity of the tracks and the through hole interconnections-checking to ensure there are now open circuits or no short circuits on the finished board. LDH tech using Flying Probe Testing for less than 3sqm orders, and Fixture Testing for bigger than 3sqm orders. We check each net to ensure that it is complete (no open circuits) and does not short to any other net, to ensure optimum performance and quality.
19. Final Visual Inspection(FQA、FQC)
This is the final step in PCB fabrication process. The professional quality control team will perform the final inspection of each PCB. Including: visual inspection, finished product size inspection, aperture hole number measurement, war-page measurement, etc. If the inspection is qualified, we will print a test report for customer reference.
20. Packing and Output
After inspection, the PCB are vacuum-sealed to keep out dirt and moisture. Securely boxed, sealed and shipped off to the customers all over the world by courier. LDH tech suggest DHL and FEDEX which are more convenient and fast - generally 1-4 Days so you can receive the board as soon as possible and start your project soon.