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Why printed circuit board manufacturers use plated slots ?

Printed Circuit Board Plated Slots

Slots are holes, which are either plated or non- plated-through. So, plated slots are holes plated in copper. And we use them for electrical connections on the PCB. A through slot is the one that goes to the entire depth of the board, starting from one end to another. Whereas a PCB outline or edge also has plating called side plating.

printed circuit board manufacturers use plated slots for components packaging, however, non-plated slots are also in practice. But, PCB with multiple layers have only plated-through slots.

Why Prefer Plated Slots?

PCB assembly involves different designs and components, and you would often see round holes to accommodate the round components or square as well. A round hole suits a PCB with through-holes. However, certain components are compatible with the blade or rectangular leads, so the round or square holes are not ideal in that case, and this is where a plated slot works.

When the pin size becomes large, the rectangular connectors work with plated-through slots rather than round holes. Like, they are ideal for the DC Jacks. Though you can use the round holes for small connectors and rectangular pins, plated slots are better because they take less space on the PCB, unlike the round holes.

Difference between Plated And Non-Plated Slots

Plated slots have copper plating in the circuit layer, whereas it opens in the solder mask.
On the other hand, non-plated slots don’t have copper plating in the circuit layer, besides, it does not open in the solder mask.

Design of Plated Slots

There is a specific way to show plated holes on the Gerber, like:

    • You have to put the copper pads on the upper and lower solder mask.
    • Then you insert a milling slot in the mechanical layer.

Designers prefer a mechanical layer to put slots in the Gerber and it involves two possible options to do it.;

Use of Flashes Having the Right Size Slot

Drawing a slot with a 0.50 mm thick line, as it helps designers to check it visually to ensure that there is enough tolerance between copper and the PCB edge. Remember that a line’s center is considered the edge of the slot.

Then you have to join the slots with the PCB outline into the Gerber, and it should be parallel to the copper layer. However, the copper layer should also have a PC Board outline to stay on the safer side.
The mechanical layer in this case has different names, depending on the system. It also depends on milling as it should be there. However, you can use another layer in the absence of a mechanical layer. Use the README file to avoid any doubts regarding the right file.

Don’t always define slots in a legend or a copper layer because they could be misinterpreted. Show large slots in the legend or copper layer, however, the outline should be precise. Don’t forget to write the text SLOT in the center.

Creating a Drill File

You can also define the plated slots through drill files as it is a precise way, however not all CAD software allows this option. But, defining through the drill files involves the X and Y or the slot width and length, instead of a complete row of holes that overlap.

Understanding A Small Slot In The PCB

The size of the smallest slot varies, depending on the type of the PCB, like if it’s flex, rigid, or rigid-flex. The width of the smallest slot is 0.50 mm for a rigid and flex-rigid PCB, and the length is often 1.0 mm.

These sizes are so due to more thickness of the PCB, besides, the slots are created through the NC milling that is mechanical. As, the grooving cutter of NC is 0.50 mm and its length is twice the width, like 1.0 mm.

On the other hand, the thickness of the flex PCB is less, and you can make slots through a laser machine. Designers prefer the smallest slot because the bigger slot’s length becomes more which is twice its width.

Milling Of the Cutouts Or Slots

You have to use the round NC grooving bit for the milling of slots in the rigid PCB, and it’s just like the CNC machine. However, the inner corners of the slots are made round instead of sharp. Whereas the PCB edge is created to the center of the border.

Which Industries Can Use the Plated Slots?

Mostly the thick or multilayer PCBs have plated slots, and such boards are ideal for different industries, including aerospace, consumer electronics, computer, and telecommunications. As these slots don’t take much space, they are cost-effective in terms of making. A board with both plated slots and round slots is also ideal for multiple electrical connections.

Frequently Asked Questions

What Is A PCB?

PCB stands for a printed circuit board having different electrical components, holes, and other features. Various industries use PCB boards to provide the electrical signals to run different devices or electronics. A PCB board can be embedded, single or double-layered, or can have many layers like up to 60 plus.

What Is The Definition Of A Plated Slot?

A plated slot is a hole in the PCB with copper plating. It is not round, so it is ideal to accommodate the leads with rectangular pins. You can use such a slot for electrical connections and component packaging
The slot goes throughout the circuit board that’s why we call it a plated-through slot.

What Is Edge Plating?

If plating is done on the edges of a PCB, we call it edge plating. Besides, you can also call it the side plating of a circuit board. It goes from the upper layer to the lower layer of the PCB, extending to an edge of the perimeter.

Which Circuit Board Should Have Plated Slots?

Generally, the multilayered PCBs have slots with plating also known as plated-through slots. However, there are also non-plated slots, depending on the PCB design and its application.

Do I Need A Lot Of Space For Plated-Through Slots?

Plated slots are mostly rectangular, hence they don’t take much space on the PCB like the round slots. So, such slots are ideal when you run short of space on the PCB, and where you need to use the rectangular Jacks.

What is a Copper Layer?

PCB has different layers of which one is a lamination made of copper foil, and it is attached to the circuit board with some adhesive. The copper layer is essential for a two-sided PCB, including copper on both sides. But, PCB boards with more than 60 layers of copper are available by different companies.

What is Solder Mask Layer?

The green color on the circuit board is a solder mask and its surface is called the solder mask layer. The purpose of the solder mask is to cover the exposed copper to prevent users from the electric shock upon contact. Though it is mostly green, other colors are also available.

Final Thoughts

PCB manufacturers use plate slots for components packaging, however, the non-plated slots are also in practice. But, PCB with multiple layers has only the plated-through slots. Certain components are compatible with the blade or rectangular leads, so the round or square holes are not ideal in that case. This is where a plated slot works.

Plated slots have copper plating in the circuit layer, whereas it opens in the solder mask. Non-plated slots don’t have copper plating in the circuit layer, besides it does not open in the solder mask.

The size of the smallest slot varies, depending on the type of PCB, like if it’s flex, rigid, or rigid-flex. The width of the smallest slot is 0.50mm for rigid and rigid-flex PCB and the length is often 1.0 mm. Engineers use cad software to make Gerber files for different types of slots including plated through and non-plated through slots.

Would like to know best practices for Plates Slots or printed circuit board assembly? Email us at sales@pnconline.com

PCB assembly Pre-Reflow FAI

First article inspection (FAI) prior to SMT assembly is a design verification methodology that provides a reported verification and validation of details of a product on the shopfloor per its manufacturing procedure and requirements. There are various ways to perform FAI, from both supplier’s and customer’s side, making it a very dynamic process. This means that each organization can tailor its FAI method to benefit itself and consequently, its customer, yet maintain rigid performance standards at the same time. FAI involves qualitative and quantitative measurement. FAI is also highly effective since it can potentially fulfill process validation requirements of quality management systems like ISO9001 or AS9100.

In the PCBA manufacturing industry, FAI can be effectively employed in validating materials for manufacture, underlying technologies, manufacturing processes used, packaging, and equipment. It can also be applied to a batch of a given sample-size from a mass-production instead of just the first sample, as the name might suggest. At PNC, strict adherence to our manufacturing standards helps in production with better yield but at the same time, facilitating dynamic validation techniques in our manufacturing process allows us to reduce lead time. The focus of FAI in PNC assembly lies in validating the pcb assembly before reflowing so that the SMT team can make necessary adjustments for the next batch, saving time and effort during rework. They are also responsible for validating the correct loading of the right component in its allotted slot per the assembly program. This extra step helps in validating the placements of the components and improves the turnout rate for a successful production.

All aspects of reflow also must be amenable to improve solder performance and the same translates to our guideline where only the most recent batch of solder paste (with most activity) is permitted for use, which is validated by FAI. Apart from pre-reflow FAI, post-reflow X-Ray also helps validate the solder performance based on the reflow profile which can then be adjusted accordingly so that all components are successfully soldered. This can be similarly implemented at the rest of the printed circuit board assembly stages as well up to testing. But there is a necessity to establish a constant groundwork or point of reference in such a dynamic process to give each validation at a particular stage, the perspective of what changes were made before. This is achieved by using a single piece of documentation used to validate at every stage, wherever applicable, and that document reports any changes made to the processes or product, to the next stage.


PNC employs the use of AEGIS software to combine SMT assembly guidelines and inspection requirements into a single document (internally referred to as AEGIS). The AEGIS is used to report every single FAI validation to different stages of assembly. PNC’s FAI process for SMT starts with thorough solder paste FAI & its validation, which will be detailed in another post. For this post, let us consider pre-reflow FAI and highlight its validation process since it is the most crucial stage. The procedure is as follows:
1. The SMT team confirms the correct allocation of components as given in the assembly program. This is done by comparing each component with its description, measuring component value wherever applicable, and checking for physical marking on ICs. This helps in validating that the right component has been placed in its respective position on the board.
2. The next step involves checking for the polarity of components, wherever applicable. This is a two-step process. First, the supply angle of a component in the reel needs to be checked and second, the placement of that very component on the PCB needs to be verified.
3. Now, once the first board is assembled, the pcb assembly is put through FAI, where the placements of all components on the board are checked, any necessary placements that remain are placed manually and polarities of applicable components are checked and changed as per what is given in the AEGIS. The same changes are made in the assembly program to avoid the same occurrence in the rest of the batch. Components that are designated as DNP (Do Not Place) are also checked and finally, the solder paste information such as solder type, lot number, date of manufacture, and expiry are checked to ensure that the right solder paste has been used.
4. All these checks translate to notes, remarks, and checks on the AEGIS document, which can then be referred at later stages up to final inspection. If the job in consideration is a repeat job, it can be optimized to avoid any errors made in the first batch of production.
5. The board is then sent through reflow. Once reflowed, the board is extensively inspected under high magnification camera for quality of component placement, solder joints etc. yielded by SMT process.
6. Each section in the AEGIS is meant for FAI by a different team performing a different operation.

PNC has been able to reduce its lead time and increase customer satisfaction significantly and our personalized and successful FAI is a big factor contributing towards it. Further development to the FAI process is underway as much as it is needed to achieve better production yield over time for all the different types of PC Board assembly that are assembled at PNC.


Ensuring a successful Turnkey PCB Assembly project

There are many detailed factors involved when pursuing the right company for your electronic or PCB assembly needs. These factors can be broken down into two distinct areas, customer communication and supplier contract review. Either the customer or the supplier cannot afford time lost if there is a misunderstanding or lack of data to efficiently and effectively produce a quality product on time. Time spent up front makes for a smooth and efficient transition through the quoting and manufacturing process.

Customer communication

A majority of communications for a request for quote, RFQ’s, in today’s industry are via email. The email needs to contain the required data files and be clear and concise in regard to quantities and delivery dates, along with any details that are not stated on the fabrication/assembly drawings. Since we are talking about Printed Circuit Board Assembly Turnkey projects, let’s break this down further with the required data files for PCB and PCBA.

PCB data files:

1- Fab drawing with build details such as material type, thickness, Copper weight, Tg rating, IPC-A-600 Class, Stack-up, Drill Chart, LPI & silk screen color, Serialization, Panelization array, MIL Spec, final finish and type(RoHs/Non RoHs) etc.
2- Complete set of gerber files.
3- Drill files.
4- IPC-356 Netlist for electrical testing.
5- Read me file for additional information not stated in fabrication drawing or email.

PCB Assembly data files:

1- BOM with manufacturers part number/description and alternates if applicable or DNP’s.
2- Assembly drawing with build details, Solder paste requirements, torque specs, IPC-A-610 Class, DNP’s, serialization, etc.
3- Pick & Place file.
4- ICT or Probe testing if applicable.
5- Functional test procedure if applicable.
6- Read me file additional information not stated in fabrication drawing or email.
If all the required information and data files are complete, we have successfully met the first half of the RFQ process. With this in mind, it’s up to us to compile this information in our contract review process. Let’s take a look at what is processed on our end to complete the RFQ cycle.

Supplier Contract review

All incoming turnkey projects are given an internal number for uniqueness especially for part numbers that has been revised. They are stored in a secure file folder based on two groups of data. ITAR data is stored separately than non-ITAR data. Once the customers data is stored and secure, engineering is notified to start to contract review process for the PCB and PCBA data sets.

Contract review for PCB:

1- Gerber files are imported and overlaid into correct layer structure.
2- Drill files are imported and overlaid against the gerbers.
3- If there is no IPC-356 Net list file, we extract the net from the gerber.
4- The gerbers are ran through a design rule check for manufacturability.
5- Fab drawing is reviewed by engineering for manufacturing capability.
6- If any discrepancies are determined, customer is notified immediately, If no discrepancies, engineering hands off the internal contract review check sheet to customer service.

Contract review for PCB Assembly:

1- BOM is scrubbed to ensure all parts are identified by manufacturer and P/N.
2- BOM parts stock research from approved vendor list.
3- Assembly drawing reviewed by engineering for assembly capability.
4- Pick & Place file review.
5- Review for testing if applicable.
6- If any discrepancies are determined, customer is notified immediately, If no discrepancies, engineering notifies customer service.
7- Quoting team is notified to officially create the quote and send to customer.
Customer communication and supplier contract review is a relatively simple step in order to achieve and ensure a successful assembly turnkey project. Adhering to the steps above can make for a great partnership.