PCBA or Printed Circuit Board Assembly is the process of getting the PCB components populated to the bare PCB, where Surface Mount Device or SMD parts land on the PCB surface and Through Hole Device or THD parts are inserted in the conductive holes.
Depending on the design, PCB may have components on a single side or double side with either SMD only or THD only or a mix of them. Assembly can be done manually for sure. Hobbyists or engineers may choose to grab soldering iron/station, the components, solder tin, flux, and start assembling their board.
Some would make it easier for themself by using a Stencil to put solder paste through it to the PCB pads, and later to place the components on their place manually. Moreover, the process of placing the component on the Solder pasted pads can be automated using Pick and Place PnP machines and getting or DIYing an entry-level reflow oven, talking about SMD assembly, and/or a soldering pot, talking about THD assembly.
When the development is not finished and production is not reached yet, the assembly can be done manually when assembly time is available and there is a need to cut assembly services/machines fees. Yet, automated assembly is a must during the production stage.
Some firms choose to build their in-house assembly line, but it’s more common to ask for a second party to assemble the PCB for you and ideally it should be the same PCB manufacture.
Assembling 10 PCBs of Arduino UNO as A Test
In this article, we are going to explore the PCBWay manufacturer service for PCBA. This will give an overview of the steps of this service and how to order it. This article targets people who never tried PCBA as a service before and people that usually do PCBA with other firms and like to see about PCBWay service. Although this is a sponsored content to promote PCBWay service, it will be showing real technical details and trying things in action as we are going to see in 2 parts.
The first part to give some background and introductions and in the second part will examine things in real using a real PCB and PCBA order.
At the time of writing this article, PCBWay accepts assembly for PCBs only manufactured by them, so you need to order PCB and select assembly service. To keep things practical, I chose to try the process before putting a real order. So I used one of the Arduino open-source designs, namely Arduino UNO R3, and made a dummy order to get familiar with their service.
First Step: Order the Service
Customers need to go PCB instant quote form first, then by scrolling down there is a PCB assembly service option. By selecting it, it will open another punch of fields to fill and options to select.
Customers need to specify a few things to get a quotation of PCBA costs.
Firstly, the customer can choose to supply PCBWay with his parts to assemble under the ‘Kitted or Consigned” option, or can supply some of the parts and rest will be supplied with PCBWay, or ask PCBWay to handle sourcing all components with “Turnkey” option.
In certain case, customer may have spare parts in their inventory and don’t want to buy new parts or maybe they will use some sensitive parts that need some papers to get, so they need to send these parts to PCBWay.
It is important to note that when you use Pick and Place machines, you need to provide some extra parts (called overages) and this have different reasons.
PnP machine feeders need a minimum amount inside the components tape. This is due to how the feeder works (the tool that holds component tape) . Let’s say you only use 2 smd capacitors with package 1206. If you use a tape containing exactly 2 capacitors, then the PnP feeder can’t hold the very short tape that contains only 2 caps. However, while PnP machines get new enhancements, they become more efficient and require less overages.
Moreover, PnP can mispick a part during operation, and using a vision inspection system it will reject some parts so the operator needs to have already some packup.
These rejected parts will be in a special bin, and can be loaded manually later, but if they are resistors or capacitors, it will be hard to identify them.
Unused and excessive parts supplied by customers or purchased by PCBWay will be packaged and returned to the customer with the assembled boards or can either ask PCBWay to keep them on their shelf for the next order.
FAQ page from PCBWay gives some numbers of how much overages are needed, summarized in the following table:
| Type | Package | Minimum | Example |
|---|---|---|---|
| Resistors, Capacitors, Diodes | 0603, 0805, 1206, 2225, SOT, SOD, MELF | 50 pieces and have to exceed the required quantity by 30 | assemble quantity 40 pieces – will need 70 pieces – this meets the minimum of 50 pieces plus 30 pieces over assemble quantity |
| Resistors, Capacitors, Diodes | 0201, 0402, miniMelf, miniature packages | 100 pieces have to exceed the required quantity by 50 pieces | assemble quantity 80 pieces – will need 130 pieces- this meets the minimum of 100 pieces plus 50 pieces over assemble quantity |
| Expensive parts | IC, BGA, QFP, Connectors, etc | 1-5 pieces based on the total quantity of assemblies |
This is important to know as this will be an additional cost when you order assembly with the “Trunky” option, and important to know if you want to supply PCBWay in “Kitted” or “Combo” option. The image below is an example of a part of BOM sent to PCBWay with overages added.
Now let see the second part of the order form, and it’s about specifying if you have components on one side or both sides and if you will do penalization or not.
Knowing that, if you choose to have 2 sides assembly, when your PCB has components in the top and bottom layer, this will increase the service fees, as the process of 1- solder paste printing 2- Pick and Place components 3- reflow soldering will be repeated as the diagram below shows
The last part of the form is to declare the number of parts used in your design. Customers can leave it empty and the review process will take handle of it.
Last part of the form is to declare the number of parts used in your design. Customers can leave it empty and the review process will take handle of it.
After that, you need to upload the design Gerber files for the PCB manufacturer, and we are not going to talk about it as it is not this article scoop. You have to upload a proper BOM for your design to have a final quotation for the assembly service fee and costs of purchasing the components including the overages as we explained before.
PCBWay provides a sample file of required BOM, and I re-used it when I sent the example Arduino UNO board Bill of material to examine the process.
Moreover, you can make your template but make sure to include the following information:
- Line#
- Quantity Per Part Number
- Reference Designator
- Part Number
- Part Description
- Package
- Type (Surface mount, Thru-hole or Hybrid)
- Manufacturers Name (only for Turn-key and Partial Turn-key orders)
- Manufacturers Part Number (only for Turn-key and Partial Turn-key orders)
- Distributors Part Number (only for Turn-key and Partial Turn-key orders)
Customers also need to provide a file used for assembly called Centroid. It is used for programming assembly machines to know the position and rotation of each component. The Centroid file includes the position and orientation of all the surface mount parts, which includes the reference designator, X and Y position, rotation and side of board (Top or Bottom). Only surface mounting parts are listed in the Centroid.
Second Step: Wait for Quotation and Review
Now, the customer needs to wait for the reviewer to review and quote the customer’s BOM and check if there is anything missed or need customer clarification.
Keep in mind that, the price you get using the quotation calculator is only an estimation for PCB manufacturing and assembly service fees. Assembly fee would be around $30 for low quantities for average complexity circuits .
When I sent the Arduino UNO bill of materials, I mistakenly did a copy and past of D1’s description to D2 and D3.
Then, I found an email from the reviewer asking for the problems I have in my BOM, which was a conflict in D2 description and out-of-stock of 2 parts and ask to set alternatives.
Alos the status of the order will be updated and mention that it needs customer action.
However, this doesn’t mean that all mistakes will be discovered at this stage, as I did an intentional other conflict and it was not discovered. However, it will be discovered during the assembly stage for sure. I put one of the parts manufacturer numbers different than the description with a different package too. During assembly, the operator will catch it and communicate with me for a correction.
It is the customer responsibility to make sure that his design has no critical issues that make it impossible to assemble. For example, I made another intentional mistake in the PCB by making 2 components close in a way that will make it impossible to assemble and made one of the capacitor pads overlap with another part Pad and this affects solderability during the overflow oven stage.
However, the reviewer asked about other issues, but yet the design passed the review. So make sure you review well your design.
When I asked Gloria from PCBWay about what happened, the answer was honest and direct: Customers should take the responsibility, and moreover, if the PCB needs a modification, the customer will mostly be charged again.
In the real world, no one does mass production without a prototype!
By the end, I got the BOM with prices and the total of producing 10 copies of Arduino R3 was:
| PCB x10 | $5 |
|---|---|
| Assemble X10 | $30 |
| BOM | $230 |
| Total | $255. Around $25 per piece. |
It is important to know that assembly for low quantities/ small-batch like my case will be an assembly by hand unless you have a special request to use the machine. Moreover, The overages mentioned in the last table is when the assembly is using PnP, which includes more losses than assembly manually. You can mention that the quotation for manually assembled orders do not include the overages we talked about.
Third Step: Doing the Payment and Waiting for the Manufacturing
At this stage, your Gerber files are reviewed and BOM is quoted and you’re ready to be charged.
After you do the payment, PCBWay will start the manufacturing process of the PCB and later do the assembly. Meanwhile, the components should be ordered and arrive later. BOM includes notes on the lead time needed for some parts to arrive.
So to calculate the turn time on a Turn-key order, you need to take into consideration the time needed to source all the parts. Before all parts are ready, PCBWay will fabricate the PCBs and start to make the stencil so that assembly starts immediately after they get the parts.
The Arduino UNO R3 bill of material is attached for reference.
However, this sounds reasonable as the time required to setup the machines (.i.e loading PnP’s feeders with components) for automated assembly will be too long compared to hand assembly. However, the rest process is similar.
A Closer Look of What PCBWay Do during Assembly
The assembly process may look as simple as printing PCB, then placing components and solder paste and finally enter the reflow oven. This is what I tried to explain in the diagram below.
It’s way more complicated than that and a lot of machines are involved to make your PCB assembled.
At each stage there are various testing method will be applied to the assembled boards before the final shipment:
- Visual inspection: general quality check especially before the board enters the reflow oven, to make sure no failure errors happen from PnP. Post the reflow oven to make sure that all components “seems” solderedand no rework is required.
- X-ray Inspection: checks for BGAs, QFN, and bare circuit boards, because solder joints are hard to be visually inspected without X-ray and pins may be under the body of these chips.
- AOI Testing: checks for solder paste, 0201 components, missing components, and polarity.
- ICT (In-Circuit Test) and Functional test (you can ask to follow your test procedures) can be done optimally after the assembly to make sure that the circuit is electrically connected, working, and functional.
To see in action these steps behind the scene, check Scotty from “Strange Parts” Youtube channel who made a tour inside the PCBWay factory:
To know more about PCBWay assembly service, please check the following Web pages.
In the next part, I will send a real order for a PCB designed specially to test the PCBA service.
