Tag Archives: turnkey pcb assembly

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What’s a HDI Printed Circuit Board?

HDI stands for High Density Interconnect. HDI PCBs have finer traces and trace spacing, laser drilled micro vias and higher connection pad density. Its two chief advantages are that it permits the use of fine pitch BGAs and it reduces the number of PCB layers required because the finer traces and smaller vias allow more circuitry in a smaller area.

Narrow trace widths mean higher circuit density

At PNC standard PC Board fabrication uses a minimum trace width of 5 mil, with a 5 mil space between traces (5/5mil) PNC’s HDI trace widths can be as narrow as 3 mil with 3 mil spacing.  These finer traces allow 160% more traces in the same real estate. 3/3mil spacing will also allow two traces to escape between pads of a standard BGA, meaning less PCB layers are needed to fan out the pins from the BGA.

Microvias are the enabling technology for HDI

Narrow trace widths used in HDI PC Board are a result of the gradual refinement of photolithography and etching technology.  Microvias on the other hand, are a revolutionary innovation driven by the development of high powered lasers that can be controlled accurately enough to ablate a 3 mil hole through the surface layer of copper and underlying laminate, without damaging the underlying layer of copper.

The minimum hole size for PNC’s laser drilled microvias are 3 mil and the minimum pad diameter for the microvia is 7 mil.  Pads for laser drilled holes can be smaller than for mechanically drilled holes because of the location accuracy of the laser drilled hole.  There is no mechanical deflection of the drill bit to account for.  The laser drilled holes can be fully copper filled and planarized flat, so they can be used as pads for fine pitched BGAs with 0.4mm or smaller spacing. Using microvias as pads allows the signal trace to fan out by going straight down and out to an inner layer of the printed circuit board.

The biggest limitation with microvias is the aspect ratio of the holes.  Where a drilled through hole can have a 10:1 depth to diameter aspect ratio, a laser drill can achieve no more than around a 1:1 aspect ratio.  This means that the smallest microvia can only connect two adjacent copper layers. A larger diameter microvia can penetrate two layers. To connect deeper layers, the designer must stack vias one directly atop another.

Laser drilling of the microvias changes the way PCBs are fabricated and gives the designer flexibility that they do not have with through hole vias.  In a standard drilled PCB, via holes are drilled and plated after the PCB fabrication stack-up is completed.  Because the microvias can only bridge two or three copper layers, the microvias must be drilled and plated at each lamination step.  This means that microvias can be fully buried between layers, stacked or staggered to allow the microvia to connect multiple layers of the stack up.

The major space saving advantage of the microvia technology is that vias can just connect traces that need to be connected, rather than taking up real estate all the way through the PCB the way a through hole via does.

The Printed Circuit Board designers at PNC take advantage of this by locating the power and ground layers at the top of the stack up.  Since all active components access power and ground, sometimes through multiple pins, having the power and ground layers directly below the component layer allows all those connections to be made directly by microvias.  This leaves the component layers and layers beneath the power and ground layers completely unobstructed for signal routing. This has the added advantage of reducing parasitic capacitance because it eliminates the circuit stubs caused by plated through holes.

Two sided boards are typically fabricated with a combination of through holes and microvias.   Though holes can be drilled just through the core, connecting the stacks on the top and bottom of the board from the lowest layer, or through holes can be drilled through the entire stack directly connecting the traces on the top and bottom component layers.

 

HDI PCBs are a necessity when using fine pitched BGAs, but they can also reduce cost on PCBs without fine pitched BGAs because of the reduced layer count.  On your next PCB design, talk to the experts at PNC.  They can help you determine if HDI technology is can reduce your PCB cost by reducing layer count and shrinking the PCB size.

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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.

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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.

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CONFORMAL COATING

What is Conformal Coating?

Conformal coating is protective chemical material coating applied after the final SMT PCB assembly or through-hole assembly process. Coatings are comprised of 5 different types, Acrylic, Epoxy, Urethane, Silicone and Parylene resins. Applying the coatings to a PCB board assembly can be done by hand spray, robotic spray, brush or dipping. The coating acts as an additional dielectric layer that provides protection due to environmental and mechanical stresses, such as thermal extremes, chemicals, dust, salt fog, abrasions, and moisture. In a PCB assembly that has close spacing of conductive pathways or close spacing of components the coating will help minimize dendrite growth over a period of time that causes shorting.

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Robotic Spray vs Traditional applications:

The method of application will depend on the customer requirements of the turnkey pcb assembly, but PNC prefers spray method for consistency in overall coating thickness. There are often select components on a pcb board assembly that will not be required to be coated. The traditional way is to block out or mask the selected components with tape. This is time consuming, costly and a possibility that the component can be damaged on the SMT assembly when removing. The robotic method can be programmed accurately to spray around the parts thus ensuring no component damage from masking and maintain a repeatable coating thickness.

When choosing a conformal coating for your turnkey pcb assembly, there are many manufacturers to choose from. The following are a few that we work with, Dymax, HumiSeal, Dow Corning, Hysol, Loctite, and Huntsman. Based on your application and environment of your PCB assembly, choose the Type of coating that fits the PCB board assembly, by visiting their web pages to find the strengths and weaknesses.

IPC J-STD-001D Conformal Coating thickness requirements

Type AR Acrylic Resin 0.03 – 0.13 mm (0.00118 – 0.00512 in)
Type ER Epoxy Resin 0.03 – 0.13 mm (0.00118 – 0.00512 in)
Type UR Urethane Resin 0.03 – 0.13 mm (0.00118 – 0.00512 in)
Type SR Silicone Resin 0.05 – 0.21 mm (0.00197 – 0.00827 in)
Type XY Parylene Resin 0.01 – 0.05 mm (0.000394 – 0.00197 in)

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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.