PCB assembly is a critical step in producing reliable, high-performance electronic devices. When you’ve finished your design files, the assembly begins, turning your concept into a fully functioning circuit board. Precision-assembled PCBs are the basis of every modern electronic product – whether it’s a smartphone, computer, medical device, or industrial machine. In this article, we will break down the whole PCB assembly process step by step, covering everything from the assembly technologies used to the final quality control checks. Let’s dive right in.
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What Is PCB Assembly?
PCB assembly is a manufacturing process where electronic components are soldered onto a printed circuit board, creating a functional electronic system. This process can be done by machine or manually. However, unlike PCB fabrication which involves building the board itself, PCB assembly focuses on populating the board with resistor, capacitor, integrated circuit, connector and so on. To ensure that each board can meet exact performance specifications, precision, state-of-the-art technology and expert skill are all needed to carry out this process.
Types of PCB Assembly Technologies
1. Surface Mount Technology (SMT) Assembly
SMT assembly is the most popular assembly technology in the electronics industry today. It refers to mounting electronic components directly on the surface of the board through solder paste by using specialized equipment. SMT allows for:
- Smaller, more compact board designs
- Higher component density
- Faster assembly processes
- Improved performance for high-frequency applications
2. Through-Hole Technology (THT) Assembly
The second is through-hole technology, which involves inserting component leads into pre-drilled holes in the PCB and then soldering them on the other side. While gradually being replaced by SMT, THT remains crucial for:
- Components needing high mechanical strength
- Power electronics
- Military and aerospace applications
3. Mixed Technology Assembly
This kind of PCB assembly technique combines both SMT and THT techniques. This approach is ideal for complex electronic systems requiring:
- Advanced component placement
- Varied component types
- Optimal performance and reliability
- Design and manufacturing flexibility
PCB Assembly Process: Step-by-Step Breakdown
Step 1: Apply solder paste
First, put the PCB stencil over PCB pads. Then apply the solder paste and spread it evenly. After removal of the stencil, the solder paste will cover evenly to the area where the components have to be placed. The process is usually done with automated equipment to improve the speed and accuracy.
Step 2: Place components
In the second step, we need to use an automatic pick-and-place machine to accurately place the required components in the position where the solder paste is applied.
Step 3: Reflow soldering
In this step, we need to put those boards with components into the reflow oven to melt the solder paste, and when the solder paste is all melted and cooling down, the components stick to the board and form electric connections.
Step 4: Initial inspection
We then carry out initial inspection. The use of various inspection methods like visual inspection, AOI, X-ray inspection will help us to know if these PCBAs have any problem like components misalignment, poor solder joints, etc.
Step 5: Insert the plated through-hole component
For some larger and heavier components that need to withstand greater mechanical stress, they need to be assembled by through-hole technology. Simply insert the leads of these components into the pre-drilled holes and then connect them with manual soldering or wave soldering.
Step 6: Perform functional test
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Finally, test the functions of these assembled PCBs. Simulate the actual use conditions including voltage, signal and current, and see if they can operate normally and achieve the expected performance.
Soldering Methods in Printed Circuit Board Assembly
1. Wave Soldering
In wave soldering, the bottom side of the PCB gets passed over the wave of molten solder. The wave then touches exposed metal pads on which it forms the solder joints. It is a fast and efficient method of large scale production, especially good for soldering through-hole components and allows for multiple connections to be done at the same time.
2. Hand Soldering
Hand soldering involves technicians using solder iron to apply solder to the leads of individual components on the board. It is featured with high precision and great control, well suited for a small production run, rework or repairs. And it is also very helpful when dealing with fine tuning or complicated components.
3. Reflow Soldering
Reflow soldering has become the perfect solution for modern printed circuit board assembly. In this case, the component pads are covered with solder paste and the board is then heated in a reflow oven. The heat melts the solder paste, attaching the components to the board firmly as it cools down.
4. Selective Soldering
Selective soldering can solder specific area of PCB, which offers a high level of precision. It is ideally used where PCB has both surface mount and through holes components. They are also used when focused heat in certain areas is required.
Below we have listed a table to compare these 4 PCB soldering technologies from different aspects:
Testing and Inspection Methods in PCB Assembly
It ensures that we assemble the PCB the way it should operate. This testing method tests the board in the simulated real world conditions to ensure that the board is designed correctly and that it will operate correctly on the intended environment.
In-Circuit Testing (ICT) can test individual components assembled on the PCB, checking their functionality in circuit. One of its advantages is that it permits easy check without the need to take apart the board.
- Automated Optical Inspection (AOI)
AOI allows technicians to inspect the PCB with high-resolution imaging systems. It can find issues such as bad component placement, broken solder joints or potential problems. The defect detection process is automated and more efficient compared to manual inspection.
X-ray inspection is performed when solder joints are not visible at the surface of the board and there is coverage over joints or inside multilayer PCBs. This makes it possible to check internal structure by means of this non invasive technique, inspecting hidden defects that cannot be detected by simple visual checks.
Key Factors Affecting PCB Assembly Costs
PCB assembly costs can be affected by several factors, including:
- Board Complexity: The cost of assembly tends to increase if the PCB design is complex such as multi layer PCBs, or high density interconnect (HDI) boards
- Component Types and Quantities: Customization, high-end components or large quantities of components can lead to more expensive solutions. And more precise handling may be required for special components such as BGAs or fine pitch devices, which also increase costs.
- Assembly Technology: For high volume production, SMT is usually less expensive; while for more specialized designs, THT or mixed technologies are used.
- Production Volume: As a rule, economy of scale cuts costs at higher production volumes. It could be more expensive for fabricating prototypes or low-volume production runs.
- Material Specifications: The cost would also be affected by the choice of materials, such as the PCB substrate or surface finish. All premium materials normally lead to higher production costs.
In conclusion, the PCB assembly is a vital process and contains several stages, from component placement to functional testing. Knowing the assembly technologies and processes will help you make decisions about your circuit board assembly projects.
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LoveLaika
How do you contract out PCB assembly?
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on: December 09, , 01:47:37 am »
Typically, for prototyping, I design the PCBs, order them from OSHpark, and assemble them myself. It's fine for my designs and the current quantity, though it is time consuming. However, I'm soon reaching a point where it will take too long for a very large quantity of boards. So, I wanted to ask: how would you go about contracting out PCB design and/or assembly? Depending on the vendor, can you separate the two actions, or do you typically get one company to build and assemble your PCBs? I guess in general, what should I look for, and what mistakes should I avoid?
Etesla
Re: How do you contract out PCB assembly?
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Reply #1 on: December 09, , 03:09:45 am »
So two things.
#1, my preferred method these days, is to use JLCPCB's SMT assembly service. The big downside there is they only do single sided assembly and you have to choose components from their predefined (but fairly vast) component library. They'll make your boards using the standard drop a .zip of your gerbers into their website type process, then you just check the box for enabling SMT assembly. From there you include 2 additional files. One is a pick and place data file, the other is a BOM. The formats for both of these files, as well as instructions for exporting specifically the pick and place files from various PCB design softwares are provided on their website. There are also a ton of youtube videos on it. If you mess it up they'll you and tell you how to fix it. I usually only have a couple components to hand solder that they don't have in their library, but almost all jellybean stuff is there. For reference I just got 5 four layer boards completely assembled and shipped with like a 1.5 week lead time for 37 bucks. Those boards have like 11 IC's and about 90 components total for scale. You really can't beat that. They can do higher layer boards as well, you just pay more for it.
#2, have you experimented with ordering a stencil along with you PCB, placing the components by hand, and reflowing the board in your toaster oven? That was my old favorite way to do things. Probably cuts assembly time down by 4 or 5x VS hand soldering. You can do 2 sided boards this way too if you're careful about things.
At work we have boards assembled by a variety of different board houses, but the prices there come out at like 100-500 dollars a board with like a week lead time of course depending on a lot of factors, so if you got that kind of dough there are plenty of stateside options, but I don't see the point in that for personal projects...
rpiloverbd
Re: How do you contract out PCB assembly?
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Reply #2 on: December 09, , 11:32:02 am »
In Bangladesh (where I live), there are not so many PCB printers/manufacturers. Technologies like electroplated vias are only available to a very few high-tech industries. There are some manufacturers who print PCB in both low and bulk quantities. Such as Taru projuktee, Techshopbd etc. Most of the time, hobbyists in my country print and assemble PCB by themselves. They just buy bare CCB, ferric chloride etc and print the traces with clothes iron. This manual method saves a lot of money.
« Last Edit: December 09, , 12:48:31 pm by rpiloverbd »
Psi
Re: How do you contract out PCB assembly?
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Reply #3 on: December 09, , 01:21:01 pm »
I've only ever used PCBway for assembly. So I cant really say if they are good or bad vs the norm.
But they seem to work ok.
If you use any china assembly service make sure you specify on the BOM if any parts are critical and give then a list of authorized sources like digikey/mouser etc.. (you can ask for their receipts copies if you are paranoid).
Otherwise they will try to source parts from companies somewhere in-between official and gray market, and you may have issues with parts that don't perform to spec.
I've not had any problems since I started giving a list of authorized sources. But sometimes you see some silkscreen on a part that is a little misaligned and you do start to wonder.
« Last Edit: December 09, , 01:22:36 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
LoveLaika
Re: How do you contract out PCB assembly?
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Reply #4 on: December 20, , 05:26:26 pm »
So two things.
#1, my preferred method these days, is to use JLCPCB's SMT assembly service. The big downside there is they only do single sided assembly and you have to choose components from their predefined (but fairly vast) component library. They'll make your boards using the standard drop a .zip of your gerbers into their website type process, then you just check the box for enabling SMT assembly. From there you include 2 additional files. One is a pick and place data file, the other is a BOM. The formats for both of these files, as well as instructions for exporting specifically the pick and place files from various PCB design softwares are provided on their website. There are also a ton of youtube videos on it. If you mess it up they'll you and tell you how to fix it. I usually only have a couple components to hand solder that they don't have in their library, but almost all jellybean stuff is there. For reference I just got 5 four layer boards completely assembled and shipped with like a 1.5 week lead time for 37 bucks. Those boards have like 11 IC's and about 90 components total for scale. You really can't beat that. They can do higher layer boards as well, you just pay more for it.
#2, have you experimented with ordering a stencil along with you PCB, placing the components by hand, and reflowing the board in your toaster oven? That was my old favorite way to do things. Probably cuts assembly time down by 4 or 5x VS hand soldering. You can do 2 sided boards this way too if you're careful about things.
At work we have boards assembled by a variety of different board houses, but the prices there come out at like 100-500 dollars a board with like a week lead time of course depending on a lot of factors, so if you got that kind of dough there are plenty of stateside options, but I don't see the point in that for personal projects...
Thanks for the reply. Sorry for not reaching back sooner. .....my hypothetical board is double-sided, but I can always make most of it single-sided, so that shouldn't be an issue other than rerouting some things. Is that typical, only having single-sided assembly? I suppose that dual-sided assembly would be harder. I never used JLCPCB as they are Chinese in origin, and we prefer US manufacturers, but I might go for it if the quality is good and the price is reasonable. Compared to something like Advanced Circuits, they certainly seem cheaper (given the price of the boards I had done by them in the past). For their component library, do they pick the parts from specific retailers like Digikey and Mouser?
2. For complex boards, I did order a stencil to use with my PCB oven, but at times, it's difficult. It's mainly for one-side for BGA components, and even then, there's still some issues. I think mainly it's my technique and solder being used. Nevertheless, the stencils were for small board batches, like less than 10, and I only used it for one side. I never used my oven for both sides of a board. I was concerned that my sensitive BGA components wouldn't handle the oven a second time around, and I was concerned about them falling off during the process. Once I had one side done, I just a used a pre-heater and a hot-air gun to finish up the rest.
Siwastaja
Re: How do you contract out PCB assembly?
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Reply #5 on: December 21, , 08:59:21 am »
If you have no strong reason for 2-sided load, just do 1-sided assembly.
Of course with high density integration or large application processors, SoCs or FPGAs in BGA packages, you pretty much need MLCCs on the bottom side. For a QFP / QFN design, put everything on top unless very space constrained.
Note there's a difference, if you just place small bypass caps on bottom side, they don't need the glue step, they just run the board through P&P and reflow twice and the surface tension of the melting solder keeps the bottom side parts in place while they reflow the top side. Large parts on the bottom need to be glued down. All this affects the cost.
I have decent experience with PCBWay's assembly service and will be using them in future as well. I have also sourced parts myself and sent them to China. Shipping is of course quite expensive.
« Last Edit: December 21, , 09:00:56 am by Siwastaja »
T_guttata
Re: How do you contract out PCB assembly?
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Reply #6 on: December 21, , 09:13:08 pm »
I'm also interested in that topic. I started doing electronics only some months ago for a specific project. I have ordered some PCBs from JLCPCB and I can't complain. I needed several iterations to get the circuit to work and I will probably need some more, so I'm glad it's cheap.
I usually ordered a PCB and a stencil as well. The stencil is actually more expensive than the PCB itself.
Now, I would also like to order some fully assembled PCBs in the future, but honestly I don't know yet how to do that. I will have some parts, which probably have to be sourced from digikey. But I don't know yet how to use components , if they are not in the database of JLCPCB. I made some custom components, but how will JLCPCB actually do the identification of the part?
Further: how can the price be estimated? What's the minimum PCB quantity to get reasonable prices?
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