The PCB assembly process (PCBA)
The PCB Assembly (PCBA) process is so much more than simply soldering parts. What customers want are circuits that work first time out of the box. Just good soldering is not enough. Many advanced skills are needed to achieve high-quality levels, all requiring the right methods, machines and processes. It is a long learning curve, and we have been refining our assembly procedures to achieve the very best results in contract electronics manufacturing (CEM) for over 60 years.
It is not practical to briefly explain every element of the PCB assembly process here, but set out below are some of the most pertinent.
The PCB assembly process (PCBA)
At contract review, we check all the details of your order and especially your Bill of Materials (BOM). This is the list of parts generated by the originating electronic design CAD software (e.g. Altium). It will list the component designator, the manufacturer and manufacturer part number, quantity and description.
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With our bespoke automated software, we will identify out of stock or obsolete parts and suggest replacements. We check that we have understood all the requirements of the build and that we have the capability to meet your expectations both as regards delivery time and technical considerations.
Before buying, each line of the BOM must be updated with the package type and size (0201, SOIC, QFP, etc.), the technology upon which it is mounted (reels, singles, trays, etc.), and the quantity adjusted for a safety margin of spares. Immediately prior to placing orders, the updated BOM will be checked against the customer’s original BOM for any transcription error, and automatically run against our major online supplier databases to check that stocks are still available. Once all queries have been resolved, orders are placed by automated software.
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Any unobtainable items will be manually resourced, or we will ask customers for alternatives. The BOM is mapped into the costing and buying software largely without rekeying any data, in order to minimise potential for error. Consideration is made for the appropriate step & repeat required for the PCB panel, taking account of safe breakout tab positioning, and the need for adequate rigidity of the panel. Orders are placed with the PCB factory detailing necessary technical requirements including orientation, step & repeat, fiducial pads and holes required for automatic line-up on the SMD placement machines.
The PCB factory sends the laser cut solder paste tooling department instructions to design and cut a suitable solder paste screen to perfectly match the PCB panel. Aperture sizes and shapes in the solder paste screen are adjusted to print the appropriate volume of solder paste according to the component footprint.
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Our failure mode analysis studies show that optimum paste screen design is the single most influential vector in achieving consistently high-quality solder joints.
Component deliveries are booked into the goods received department, where components are checked for quality, quantity and correctness. Bar-coded labels are printed and applied to every component bag for identification. Spare components are separated out, and exact quantities are issued and placed into an antistatic kit-box for issue to the shop-floor.
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Shortages and errors are chased and fixed respectively. The issue of exact component quantities into the kits ensure that every part must be accounted for on the shop-floor, whilst any losses can be made up from spares kept in the stores for up to 1 month.
The CAM programming department creates the surface mount placement machine programme whilst concurrently physically checking that each component fits the actual SMD footprint on the PCB, and has the correct rotation.
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This detailed analysis of the assembly process is combined with finalising production planning and releasing the documentation to production. Planning, scheduling and tracking is managed under computer control.
According to production planning schedules, SMD machine operators mount the loaded carts onto an SMD machine, finalise the set-up and run first-offs. These are buddy checked using AOI machines which sequentially match each component with the SMD placement programme location coordinate and a digital image of that component on the first-off with an image from an independent master database. This identifies any misplacements.
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In the event of a defect, programmes are fixed, first offs are re-run and rechecked until the build is faultless and production can start. In the course of a production run, the SMD operator monitors all aspects of the machine assembly process checking the integrity of solder paste printing and component placement, visually inspecting every assembly prior to solder paste reflow. QR code labels are applied to every circuit either by machine feeder or by hand for identification and traceability.
Find out more about our SMT Service <a href=”/printed-circuit-board-assembly/surface-mount-technology” target=”_blank”>here</a>
Reflow will be either by multi-section hot air convection or by vapour phase reflow machine. Hot air convection reflow is a conveyorised process, good for medium batch sizes, but requiring careful selection and monitoring of temperature profiles. Reflow temperature profiles are regularly checked with calibrated monitoring equipment.
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Some electronic assemblies are more suitable for vapour phase reflow which is a batch process, and extremely tolerant of assemblies with large thermal masses, and it has the benefit of a wide process window.
After reflow, racks of assemblies are automatically scanned with a 3D automatic optical inspection (AOI) machine both optically, saving a high-resolution image, and metrologically, saving a complete matrix of height measurements. The data is saved against the unique QR code of the circuit and all programming and inspection is done against this data on multiple off-line inspection workstations.
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We will inspect for presence or absence of correct part, positioning, rotation, presence and quality of solder fillet, lifted legs etc. 3D AOI systems are 10 times more effective than old style 2D AOI’s and are a key enabler in the achievement of high-quality small batches where the assembler does not have the opportunity to test or power up circuits.
Hand insertion of leaded and through hole components follows. These parts are soldered using automated selective soldering machines, which are really CNC controlled mini wave solder machines.
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Selective soldering has the advantage that it does not disturb adjacent surface mount components that have been already reflowed. It is many times faster than hand soldering, applying machine consistent heat and solder to every joint, completely enveloping each component leg with solder. Quality and repeatability are unsurpassed. In the case of very small batches, components requiring through-hole assembly may be hand soldered.
Inspection of solder joints hidden underneath components such as BGA’s and QFP’s is by high resolution X-ray inspection machine. All visible solder joints are scanned by a conveyorised 3D AOI machine and inspected at off-line workstations. Each circuit is identified by individual QR code label.
Before despatch all assemblies are visually inspected. When required, assemblies are subjected to electrical testing as per customer test procedures.
Finally, for safe despatch, finished assemblies are carefully packed in antistatic bubble wrap.
All components and production processes used to make your electronic design are fully traceable for every circuit produced. Comprehensive anti-static precautions are maintained throughout the production process.