PCB Manufacturing Process is steps of printed circuit boards manufacturing . Rayming is one of PCB Board manufacturer in china , Have 10 years experience on PCB board manufacturing . PCB Capability 1 – 56 Layers .
At RayMing, we take intrinsic steps to ensure that the quality of our work matches and exceeds the expectations of our customers. The RayMing team utilizes the latest in PCB board manufacturing technology and equipment to meet the demand for quality. Our dedication to providing quality PCB products and services have helped us earn the trust and respect of our customers.
We constantly work towards creating innovative solutions, and we spare no expense in acquiring the latest in PCB board manufacturing technologies and an experienced workforce. This places us in a strong position to provide top notch services, from PCB manufacturing to assembly ,testing and delivery.
PCB Board Manufacturing Steps
First :PCB Film Generation
Films for all of the copper and soldermask layers are made from photographically exposed mylar. We generate these films from your design files, creating an exact (1:1) film representation of your design. When Gerber files are submitted, each individual Gerber file represents one layer of the PCB Board .
Second :PCB Select Raw Material
Industry standard 1.6mm thick FR-4 laminate copper clad on two sides. Panels will be sized to accommodate many boards.
3rd :PCB Drill Holes
Through holes required for your PCB design are created from your submitted files, using NC drill machines and carbide drill bits.
4th : PCB Electroless Copper
In order for the through holes to electrically connect to different layers of the PCB, a thin layer of copper is chemically deposited into the through holes. This copper will later be thickened through electrolytic copper plating (step 6th).
5th :PCB Apply Photo-resist and Image
To transfer the PCB design from the electronic CAD data to the physical circuit board, we start by applying photosensitive photo-resist to the panel,covering the entire board area. Then the copper layer film image (step first ) is placed over the board, a high intensity UV light source exposes the uncovered portion of the photo-resist. Then we chemically develop the board (remove the unexposed photo-resist from the panel) creating the pads and traces.
6th : PCB Pattern Plate
This step is an electrochemical process which builds copper thickness into the holes and onto the surface of the PCB. Once the copper thickness is built up on the circuitry and in the holes, we plate and additional layer of tin to the exposed surface. This tin will protect the plated copper during the etching process (step 7th ) and be later removed.
7th:PCB Strip & PCB Etch
This process takes place in multiple steps. The first is to chemically remove (strip) the photo-resist from the panel. Then the newly exposed copper is chemically removed (etched) from the panel. The tin applied in step 6 protects the desired copper circuitry from being etched. At this point, the fundamental circuitry of the PCB is defined. Finally, the protective layer of tin is chemically removed (stripped) to expose the copper circuitry.
8th :PCB Solder mask
Next we coat the entire panel with a liquid soldermask layer. Using films and high intensity UV light (similar to step 5th) we expose the solderable areas of the PCB. The main function of the soldermask is to protect the majority of the copper circuitry from oxidation, damage and corrosion as well as maintain isolation of the circuits during assembly.
9th: PCB Legend (silkscreen)
Next we print the reference designators, logos, and other information contained in the electronic files onto the panel. This process is very similar to an inkjet printing process, but designed specifically for PCBs
10th : PCB Surface Finish
The final surface finish is then applied to the panels. This surface finish (tin / lead solder or immersion silver,gold plating ) is used to protect the copper (solderable surfaces) from oxidation as well as act as a site for soldering of electronic components to the PCB.
11th: PCB Fabrication
Finally , but not least, we route the perimeter of the PCB from the larger panel using NC equipment. The PCB board is now complete, and is quickly shipped to you.
This is single side PCB and double sided PCB board manufacturing process , Multi-layer PCB board manufacturing will be more complicate . need press lamination .
After 11 PCB manufacturing steps , We will do 100% Electronic E-test for your PCB board .
RayMing will be your one-stop source for all types of PCBs, Printed Circuit board manufacturing, and Full Turnkey PCB Assembly. We specialize in high layer count PCBs, Engineering PCB Prototypes, and the full range of Electronics Manufacturing Services. Every PCB is built to the highest quality standards, including Flex PCB and Rigid Flex PCB boards and Circuit Board Assemblies. Our Electronic Assembly Service is unmatched anywhere in speed, quality, and workmanship. From bare circuit boards to box build and final assembly, RayMing PCB is your premier source, with the most competitive pricing in the industry and a commitment to total customer satisfaction.
The PCB is made of a substrate that is a FR-4 material and copper pathways imposed all over the board that routes electrical current or signals throughout the board.
Prior to the PCB design, the electronic circuit designer must go to the PCB fabrication shop and get complete knowledge about the capacity and constraints of the PCB fabrication facility. This is very important because many PCB designers are unaware of the PCB fabrication facility constraints and when they send the design files to PCB fabrication shop / facility then they are returned back and asked for changes to meet the capacity / constraints of PCB fabrication process. However, if a circuit designer works in a company where there is no in house PCB fabrication shop and that company outsource the jobs to foreign country PCB fabrication factory then the designer must contact the fabricator online and ask for the constraints or specifications like max / min copper thickness, max number of layers, smallest hole diameter and maximum size of PCB panel.
In this article we will be focusing on the PCB manufacturing process, so this article will be beneficial for t he circuit designers to get a comprehensive know how about PCB manufacturing process step by step and avoid the design errors as discussed.
Manufacturing Process From Design to Final PCB
Process 1# : PCB Design and GERBER file
The circuit designer draws schematics in CAD software that will be used to layout the PCB design. The designer must coordinate with the PCB manufacturer about the software used to layout the PCB design so that there will be no compatibility issues. The most popular CAD PCB design software is Altium Designer, Eagle, ORCAD and Mentor PADS.
After the PCB design has been accepted for fabrication, the designer will generate a file from the design that is accepted by the PCB manufacturers. This file is called “GERBER file”. The Gerber file is the standard file used by majority PCB fabricators to show the components of PCB layout like copper tracking layers, and solder mask etc. The Gerber file is the 2D vector image file. The extended Gerber delivers perfect output.
The software has the user/designer defined algorithm that has key elements like track width, board edge spacing, trace and hole spacing and hole size. This algorithm is run by the designer to check for any errors in the design. After the design is verified, it is sent to PCB fabrication house where a DFM check is performed. The DFM (Design for Manufacturing) check is used to assure the minimum tolerance in PCB design.
Process 2 # : GERBER to Photo Film
The special printers used to print the PCB photo film is called “plotter”. These plotters will print circuit boards on the films. These films are used to image the PCB. Plotters are very accurate in printing technology to give a highly detailed film of PCB design.
The plastic sheet come out from the plotter is the PCB printed with black ink. In case inner layer, the black ink represents the conductive copper track while the blank portion is non-conductive part. On the other hand, for outer layer, the black ink will be etched away and blank area is for copper. These films should be stored properly to avoid unnecessary contacts or fingerprints.
Each layer has its own film sheet. Solder mask has a separate film. All these films must be aligned together to draw a PCB alignment. This PCB alignment is achieved by adjusting the table on which the film sheets are fit, and after a small calibration of table the optimum alignment is attained. The films must have registration holes to sit accurately upon each other. The registration pins will fit in the registration holes.
Process 3 # : Inner layer printing: Photo resists and Copper
Now these photo films are printed on copper foil. The basic structure of PCB is made of a laminate board. The core material is epoxy resin and glass fiber known as substrate material. Laminate receives the copper that construct the PCB.pcb Substrate gives a strong platform for the PCB. Copper is covered on both sides. The process involves removing away the copper to show up the design from the films.
The decontaminated environment is very important to clean the PCB from copper laminate. It must be ensured that no dust particle lie on the PCB otherwise it will cause circuit to short or open
Now the photo resist film is applied. The photo resist is made of photo sensitive chemical which will harden when Ultra Violet radiation is applied. It must be ensured that the photo films and photo resist films must match exactly upon each other.
These photo film and photo resist film are settled on the laminate board by holding pins. Now UV radiation is applied. The black ink on the photo film will block the UV light thus preventing the copper underneath it and not hardening the photo resist under the black ink traces. The clear area will pass the UV light thus hardening the excess photo resist which will be removed.
The board is then washed with alkaline solution to remove excess photo resist. The board will now dry.
The PCB now is available with resist covering the copper used to make tracks of circuit. If the board is two layers pcb then it will go for drilling otherwise for multilayer the more steps to come.
Process 4 # : Removing the Unwanted Copper
A powerful copper solvent solution is used to remove the excess copper just as alkaline solution removed the excess photo resist. The copper that is under the hardened photo resist will not be removed.
Now the hardened photo resist will be removed that protected the desired copper. This is achieved by washing off the PCB with another solvent.
Process 5 #: Layer Alignment and Optical Inspection
After all the layers are ready from above mentioned steps, they are then aligned upon each other. This can be done by punching registration holes as mentioned in previous step. The technician will place all layers in the machine known as “optical punch”. This machine will punch the holes accurately.
Layers ones placed and error occurred cannot be reversed.
The automatic optical inspection machine will detect any defects using laser and compare the digital image with Gerber file.
Process 6: Layer-up and Bonding
At this stage all of the layers including the outer layer will be bonded together one upon each other. All the layers will be stacked upon substrate.
The outer layer is made of fiber glass “pre-impregnated” with epoxy resin called prepreg. Top and bottom of the substrate will be covered with thin copper layer having the copper trace etching.
A heavy steel table with metallic clamps is used for bonding/pressing layers. The layers are tightly secured with the table to avoid movement during alignment.
A prepreg layer is fit over alignment table, then the substrate layer fits upon it, then copper sheet is placed. More prepreg sheets are placed similarly and finally aluminum foil complete the stack.
A computer will automate the process of bond press, heating the stack and cooling at controlled rate.
Now the technician will remove the packing pins and pressure plates to unpack the stack.
Process 7: Drilling
Now it is time to bore the holes in stack PCB. The precision drill achieves 100 microns diameter holes, with great accuracy. This drill is air driven drill has a spindle speed around 300K RPM. But even with this speed the drilling process takes time because each hole takes its time to be bored perfectly. The x-ray based identifier spots the drill locations accurately.
The drill files are also generated by PCB designer that is given to PCB fabrication house at earlier stage. This drill file determines the micro movement of drill and spots where to bore holes. These holes will now become vias and holes after plating.
Process 8: Plating and Copper Deposition
After carefully cleaning, the PCB panel is now treated with chemical deposition. During this a thin layer (1 micron thick) of copper is deposited over the surface of the panel. The copper flows into the drilled holes. The walls of the holes are entirely plated with copper. The whole process of dipping and removal is computer controlled
Process 9 #: Outer Layer Imaging
Same as for the inner layers, the photo resist is applied to the outer layer, the prepreg panel and black ink film joined together is now blasted with UV rays in yellow room. The photo resist hardens. The panel now goes through the machine to wash away hardened resist protected by black ink opacity.
Process 10: Plating of Outer Layer:
Electroplate panel with thin layer of copper. After initial copper plating, the panel gets tin plating, which allows removal of all the copper left on the board. The tin prevents the part of the panel required to remain enclosed with copper during the etching stage. Etching eliminated undesired copper from the panel.
Process 11: Etching
The unwanted copper and copper under the residual resist layer will be removed. Chemical are used to clean the surplus copper. On the other hand tin covers the desired copper. It now finally results in proper connections and tracks
Process 12: Solder Mask Application
The panels are cleaned and epoxy solder mask ink will cover the panel. The UV radiation is imposed on board that goes through the solder mask photo film. Covered sections stay unhardened and will be removed. The board is now put in oven to heal solder mask.
Process 13: Surface Finish
The HASL (Hot Air Solder Leveling) gives extra ability to the PCB for solder. The RayPCB provides immersion gold and immersion silver HASL. The HASL gives uniform pads. This leads to surface finish.
Process 14 # : Silkscreen
The PCB is in its final stage receives ink-jet printing/writing on surface. This is used to indicate important information related to the PCB.
Process 15 #: Electrical Test
Final stage is electrical testing of final PCB. The automatic process validates the functionality of PCB to match the original design. At RayPCB, we offer Flying Probe Testing or Nails Bed testing.
Full Introduction about PCB Manufacturing Process
Printed circuit boards (PCBs) form the basis of all modern electronics. They provide the mechanical structure to mount and interconnect electronic components. The tracks etched from copper sheets on PCBs replace individual component-to-component wiring. This article will provide a comprehensive step-by-step overview of the PCB fabrication process.
We’ll cover PCB manufacturing starting from the raw materials, through prototyping and volume production, all the way to final testing and delivery to the customer. Understanding the complete flow enables designing optimized boards and managing the manufacturing cycle smoothly.
PCB Manufacturing Process Overview
PCB fabrication involves many steps that can be grouped into the following key stages:
1. Design – Creating schematics and layout for the PCB.
2. Prototyping – Low volume initial boards for testing before full production.
3. Production Preparation – Data and component readiness for volume manufacturing.
4. Fabrication – Actual PCB manufacturing using processes like etching, drilling.
5. Assembly – Populating boards with components either manually or using automation.
6. Testing – Validating board functionality using electrical testing and inspection.
7. Delivery – Handling, packing and shipping finished boards to the customer.
We’ll now look at each of these stages in detail, explaining the typical flow of the PCB production cycle.
1. Design Phase
The design process involves two major steps:
1.1 Schematic Capture
This involves creating a schematic diagram representing the logical connectivity between electronic components. Each component becomes a symbol, while conductors represent wires between them.
Engineers create schematics using Computer Aided Design (CAD) software tools like Altium, Eagle, KiCad etc. The schematic captures the circuit logically. It is annotated with component names, values and the like.
1.2 PCB Layout
The schematic is the basis for laying out a physical PCB to mount and connect the components. This layout accounts for electrical considerations like high-speed signals, RF design, thermal management and also mechanical aspects like clearances, rigidity and manufacturability.
Engineers use PCB layout software to define circuit traces, pads, drill holes, cutouts and other features that translate the schematic into a physical board. The completed layout contains all information needed to fabricate the bare board.
2. Prototyping Stage
Before committing to full-scale production, prototypes help validate the design. This initial small-volume fabrication verifies the PCB layout and may reveal aspects needing improvement in the design.
Prototyping uses the same fabrication techniques as volume production, just on a smaller scale. This preliminary build also allows testing the selected electronic components on the board. Prototype quantities typically range from 5 to 25 boards.
If prototypes work as intended, the design progresses to preparing for volume production. Else, the issues noted are resolved by modifying the schematics or layout and building revised prototypes.
3. Production Preparation Stage
After prototyping and design validation, the PCB moves into preparation for volume fabrication. This involves three major steps:
3.1 Generate Fabrication Data
The PCB layout file contains the board geometry definitions. This must be converted into machine-readable files used to drive fabrication. Commonly used formats are:
- Gerber – Standard file format containing 2D vector image data for PCB layers
- Excellon – Used for numerical control (NC) machine drill files that define drill sizes and hole coordinates.
- IPC-D-356 Netlist – Specifies electrical connectivity details between pads for testing assembled boards.
- ODB++ – Unified layered manufacturing data format combining all required fabrication and assembly data.
Modern PCB software can export data in these formats compatible with fabrication equipment. Additional files like assembly drawings, fabrication notes, drill tables, and component lists may supplement the main data files.
3.2 Verify Design Rules
The PCB manufacturer will examine the fabrication data to ensure the layout complies with capabilities and constraints of their processes and equipment. This Design for Manufacturability (DFM) analysis confirms adherence to:
- Minimum trace width and spacing – Matching equipment resolution and tolerances
- Minimum hole size – Compatible with fabrication drill sizes
- Board thickness – Within stackup capabilities
- Copper thickness – Usable with available copper foil types
- Clearances – Keeping high-voltage areas safely apart
Any violations of manufacturing rules found are reported back to the customer to modify the design before production. This pre-checks help avoid costly mid-production design fixes.
3.3 Procure Components
In addition to the bare boards, volume production requires procuring sufficient quantities of the required electronic components. This includes:
- Sourcing – Finding suppliers and negotiating quotes for component purchase.
- Quality assurance – Vetting and validating component samples to ensure reliable, robust supply.
- Procurement – Placing orders with vendors and coordinating delivery into manufacturing.
- Inventory – Storing incoming components safely in warehouses ready for PCB assembly.
Components may be sourced locally or imported depending on costs, quality and lead times. Inventory levels must account for production volumes, timelines, as well as potential risks of shortage or delays.
4. Fabrication Stage
With design data validated and components secured, fabrication of the bare PCBs at volume can begin. PCB fabrication involves creating the layered board structure, adding conductive copper traces and holes to mount and connect the components.
The sequence of PCB fabrication steps includes:
4.1 Substrate Selection
The substrate forms the base insulating material upon which the layers of circuitry are built. The commonly used basic material in most PCBs is FR-4 fiberglass, but many other options exist:
| Material | Properties | Applications |
|---|---|---|
| FR-4 | Low cost, good flexibility | General purpose printed circuit boards |
| CEM-1 | High performance, low loss | High speed digital, telecom |
| Polyimide | Extremely flexible | Flex PCBs |
| Rogers RO4350B | High frequency, stable dielectric | Microwave, radar, telecom circuits |
| PTFE (Teflon) | Excellent chemical resistance | Industrial electronics |
The choice of PCB substrate depends on operating environment, performance requirements, production volumes and cost considerations.
4.2 Creating Layer Stackup
PCBs can consist of a single conductive layer or multiple layers separated by insulating substrates bonded together under heat and pressure. The fabrication process stacks layers of substrates and copper foil to build the required layer count:
- The core substrate like FR-4 forms the central layer with copper foils on either side.
- Prepregs made of partly cured cores sandwich additional copper layers.
- Lamination under high pressure and temperature fully cures the layers into an integrated board.
The sequence of substrates, foil and process steps builds up a multilayer board with internal copper layers interleaved by prepreg bonding material.
4.3 Etching Copper Layers
The raw copper foils laminated onto the substrates are etched to create the conductive traces and pads. This is done by:
- Cleaning the copper surfaces
- Applying an etch-resistant mask material to define the required conductive pattern. Common masking options:
- Dry Film – Photosensitive epoxy layers
- Liquid Photoimageable Solder Mask – Liquid polymers exposing like photoresist
- Direct Etching – Directly laser/mechanically ablating unwanted copper
- Immersing boards in etchant chemicals which dissolve the unmasked copper
- Stripping off the mask, leaving behind the desired copper layer circuitry
Repeat masking, etching and stripping multiple layers builds up the conductive pattern through the PCB stackup, connected by plated through holes.
4.4 Drilling Holes
Holes mechanically drill through pads and vias in the PCB to carry signals between layers and mount components. Drilling is done by:
- CNC machines using small diameter drill bits for through-hole and vias. Larger bits drill cutouts for connectors etc.
- Accurate alignment and precision drilling ensures vias connect pads on different layers.
- Deburring clears up the holes’ inner walls after drilling.
- Depth control results in repeatable uniform hole depths across panels.
4.5 Plating and Hole Wall Preparation
The hole walls provide connectivity between layers so must be made conductive. This is done by:
- Electroless Plating – Coats holes with initial thin copper layer
- Electroplating – Thicker copper plating to desired thickness
- Pillar Plating fills holes with solid copper for stronger multilayer bonding
- Panel Plating applies pattern plating over holes and traces
- Hole Wall Preparation – Roughens hole walls to improve multilayer bonding
This metallization makes the drilled vias and through holes electrically conductive between layers.
4.6 Solder Masking and Finishing
Solder masking coats the PCB selectively to protect traces leaving pads exposed for component terminals. Additional finishes enhance solderability or corrosion resistance.
- LPI Mask – UV-exposed liquid solder mask for solderable layer patterning
- Legend Printing – Screen/inkjet prints legends, markings and identifications
- Immersion Tin – Coats exposed copper with solderable tin layer
- Immersion Silver – Silver finish for best conductivity and solderability
- Organic Solderability Preservative – OSP organic coating prevents oxidation and tarnishing
This concludes the fabrication of the bare multilayer PCB with the required conductive and insulating areas.
5. Assembly Stage
In PCB assembly, electronic components are mounted and soldered onto the fabricated bare boards. This is done through:
5.1 Manual Assembly
For prototypes and small volumes, components are placed and soldered onto boards manually. This uses operators and hand tools for:
- Paste application – Dispensing solder paste precisely on pads
- Component placement – Positioning components using tweezers/pick and place tools
- Reflow soldering – Heating boards in oven to melt paste and attach components
- Cleaning – Removing flux residues after soldering
- Inspection – Visual checking for defects using a microscope
Though slow and labor-intensive, manual assembly allows building small quantities where setting up automation is uneconomical.
5.2 Automated Assembly
For volume production, dedicated assembly machinery automates the placement and soldering steps for faster throughput and repeatable quality:
SMT Assembly
- High speed pick and place machines precisely place tiny surface mount devices onto paste-printed pads across the PCB panel.
- Reflow ovens solder all devices simultaneously. Conveyor belt furnaces with computerized temperature control profiles ensure uniform soldering.
Through Hole Assembly
- Components with wire leads are inserted into their plated through-holes either manually or using automated insertion machines
- Wave soldering passes bottom side of boards over molten solder, bonding leads
- Selective soldering machines solder splashes individual through-hole joints
Automated assembly with computer-controlled machines enables mass production of PCBs with rapid assembly rates and minimal defects.
6. Testing Stage
Once assembly is completed, every board must pass through rigorous in-circuit and functional testing to validate quality before shipment.
6.1 In-Circuit Testing
This checks the integrity of electrical connections between components assembled on the board:
- Tests use test probes to touch soldered pads/leads while supplying currents/signals and measuring responses
- Detects short circuits, open circuits, wrong components, misplacements
- Executes both continuity tests and functional logic tests
6.2 Functional Testing
The fully assembled board is tested as a functional unit:
- Exercises comprehensive test suites to verify intended functionality
- Confirms system-level performance – clocks, power, inputs/outputs
- Stresses boards under voltage, temperature or frequency margins
These tests validate assembly correctness and PCB functionality before delivery. Debugging and rework rectify any failing boards found.
7. Delivery Stage
The finished boards are finally packed and shipped:
- Panels broken into individual PCB units
- Packaging in ESD-safe tubes, trays, or reels depending on product type
- Programming any serial numbers or identifiers into onboard memory
- Attaching labels
- Careful packing into shipping boxes, pallets for transportation
- Generating packing slips/documents and invoices
- Final delivery to customer via road/air freight
This completes the PCB manufacturing process flow starting from design through volume production and testing culminating in delivery to the end-customer. Automation and digital systems integrate the fabrication, assembly and test processes seamlessly. By understanding the complete cycle, PCB designers and product teams can streamline development and manufacturing. Electronics companies rely on the robust world-class infrastructure for PCB fabrication and assembly available globally to enable innovating and bringing their products to life.
Key Process Parameters at Each Stage
Here we summarize the key process parameters, capabilities and options at each major stage of PCB manufacturing:
| Stage | Key Process Capabilities and Parameters |
|---|---|
| Design | PCB Layout software, Design rules/constraints |
| Prototyping | Low volume fabrication, Quickturn options |
| Pre-production | Data processing, DFM analysis |
| Fabrication | Layer count, Laminate materials, Copper weights, Trace/space tolerances, Hole sizes, Plating options |
| Assembly | Manual assembly, SMT, Through-hole assembly, Automation options |
| Testing | ICT test, Functional validation, Debugging/rework |
| Delivery | Volume breakdown, Packaging methods, Lead time |
Understanding these factors help design optimized boards accounting for capabilities, cost and tradeoffs. Selecting the right options at each stage ensure efficient manufacturing.
Conclusion
The PCB fabrication, assembly and testing ecosystem enables going from electronic circuit design all the way to volume production of complex, dense and high-performance boards. Advances like HDI technology support multilayer boards with microvias enabling electronics miniaturization.
Automation continues to improve product quality and yields for competitive low-cost manufacturing globally. With growing demand and capabilities, PCB technologies will continue evolving, underpinning progress of the electronics industry.
Frequently Asked Questions
Here are some common FAQs regarding the PCB manufacturing process:
What are the key differences between PCB prototyping vs volume production?
Prototyping uses same fabrication techniques as volume production but on smaller scale. It allows quick fabrication of 5-25 boards for design validation before largescale manufacturing.
What data does the PCB Design software generate for manufacturing?
PCB design software generates Gerber, Excellon, ODB++, netlist and other files containing circuit geometry, drill holes, layer stackup, connectivity etc. needed for fabrication and assembly.
How are very fine traces and spaces achieved in PCBs?
Advances in lithographic processes like direct imaging enable trace resolution down to 5 um linewidth and spacing needed for high density interconnect boards.
What is HDI technology in PCBs?
HDI uses microvias of around 0.15mm diameter and thinner trace layers to interconnect across multiple fine line multilayer boards. It enables dense component mounting and circuit routing.
How is component soldering done manually vs in automated assembly?
Manual soldering uses hand tools for paste printing, pick and place and soldering components. Automated assembly uses machines for faster and repeatable SMT and through-hole soldering.
