Seeed Fusion PCBA Series 1: What is PCB Assembly?

This blog series is designed to guide you through the entire Seeed Fusion PCBA process including the facilities and ordering process. To start the series, we will take you on a quick tour of the Seeed Assembly facilities to give you a general idea of what we can do. For large PCBA orders, we can use the same automated assembly line used for Seeed products. PCBA stands for PCB Assembly, and can refer to the finished populated PCB or the process of populating the PCB with components itself. There are 4 main stages for SMT assembly including paste application, automated component placement, reflow soldering and cleaning, inspection and testing if requested. Seeed also have automated equipment to solder through-hole components and trained technicians to solder BGA and fine pitch components.

Solder Paste Application

To prepare the boards for surface mount (SMT) soldering; a PCB stencil, solder paste and a paste applicator is needed. The stencil is a stainless-steel sheet in a metal frame that consists of small laser-cut holes where the SMT pads are located on the PCB. The size of the openings relative to the pads is important and specific to the type of solder paste used. If too much paste is applied then the solder may overflow and cause solder bridges to form. Too little and the connection between the component terminals and the pads will be weak.

Stencils are included in all Seeed Fusion PCBA orders and will be shipped to you with the populated PCBs. We will carefully control the size of these openings when they produce the stencil to match the lead-free solder paste used in the assembly facilities, ensuring the best wetting on the solder joints.

Close up on some stencil openings

During application, the stencil is locked in place in the automated paste printer and the plain printed circuit boards are fed in, one by one, and aligned using cameras. Lead-free solder paste is then applied to the surface and the squeegee blade is dragged across to evenly deposit the paste onto the desired areas. The resulting boards come out of the other end of the machine and are inspected for paste uniformity.

Inside the automated solder paste applicator

SMT Component Placement

The most advanced and maybe most exciting machine in the PCB assembly line, the Pick and Place machine suctions up SMT components from input feeders, tubes or trays and accurately positions them into their respective locations at lightning speeds. This machine can deal with different orientations, polarities, BGAs and exceptionally small components as small as 0603 packages. Using in-built cameras, the head can recognize components by extracting part shape and appearance, and the smooth flying head mechanism with 6 spindles allows part placement speeds of almost 30,000 components per hour. It is certainly the most amusing to watch as well.

Components are fed into the pick and place machine in feeders loaded with reels or tubes

This mechanical head can suction up 6 individual components at one time

The components are held in place onto the pads with the help of the wet solder paste and are inspected before transferring to the reflow oven.

Stuck but not soldered

Reflow Soldering

The reflow oven is a long machine in which the boards slowly travel through along a conveyor, passing through various zones at carefully controlled temperatures. Think of the toast machines at the hotel breakfast buffet but much bigger and more complex.

Unlike bread, the heating process to melt the solder is not simple and depends on the components and solder paste composition. Typically, the boards are passed through four main stages from pre-heating, thermal soak, reflow and cooling. The first two processes activate the solder flux, ensure that the components do not crack from thermal shock and reduces solder paste splatter. The reflow zone briefly raises the temperature above the melting point and liquifies the solder before solidifying it during the cooling period.

Seeed’s Lead-free solder reflow machine

10 different temperature zones precisely control the state of the solder paste and the boards

Now the boards have been baked, each board is carefully inspected for soldering defects such as tombstoning, misalignment and solder bridges. If any defects are found then they proceed to the re-work station for amendments.

In the first of the PCBA series, we covered the general SMT assembly process that many people will be more familiar with. But we are not quite finished. In the next segment, we will show you the assembly process for through-hole components at Seeed using selective soldering methods, and give more details on the testing methods available. Stay tuned!

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Tags: Assembly line, Assembly types, paste machine, PCBA Series, pick and place machine, reflow soldering, smt machine, solder paste

3 thoughts on “Seeed Fusion PCBA Series 1: What is PCB Assembly?”

thepaintedman says:

March 20, 2018 at 2:51 am

Thanks always for the updating the readings.
Pete Prossen says:

March 9, 2018 at 1:21 am

Thanks for the nice description of your fabrication facility. I’m looking forward to more of your articles, especially one involving the Bill of Materials. I’d like to know how to arrange for components that are only available from China, but are not in your standard library. Examples of such items are ESP-M1 or ESP-M2 finished modules that are based on the ESP8285 chip.
1. Liyan Gong says:
  
  March 29, 2018 at 11:30 am
  
  Hi Pete, thanks for your feedback! It just so happens that we have written a general article on the ordering process here:
  http://www.seeedstudio.com/blog/2018/03/23/seeed-pcba-series-3-how-to-get-pcb-assembly-quotes-in-seconds/
  But if you want parts not obtainable from our partner databases, then you can upload the BOM, add a purchase link for the component, add it to your cart and drop us a message to fusion@seeed.cc. We’ll check out the links and see if we can purchase them.