Tag Archives: pcb assembly and soldering

Beyond PCB Assembly Services, Board Support Package Development

Beyond PCB Assembly Services, Board Support Package Development

What is a Board Support Package?

A board support package (BSP) is a collection of essential low level software applications configured for a specific microprocessor and its associated hardware. It supplies the drivers for all the hardware in the system andcontainsa bootloader to initialize the microprocessor and hardware prior to loading the operating system. The package may contain additional low level software to assist the developer in initializing the operating system. The BSP can also include a root file system, and a utility to configure the microprocessor and other hardware.By using PNC to develop the BSP, it will allow them to design around their circuit board fabrication capabilities and process.

Board Support Packages are specific to a family of microprocessors and to a specific operating system.  A typical BSP may contain drivers and initialization code for:

  • Initializing the microprocessor
  • The parallel and serial buses
  • The volatile and nonvolatile memory
  • The display and graphics card,
  • Digital and analog I/O
  • Camera, wireless modules, user input devices etc.

While a BSP for the hardware is the first requirement for developing a product with embedded software, this doesn’t mean that every company developing embedded software needs to develop their own BSP with the drivers for their specific hardware configuration. There are five reasons to let an outside BSP developer like PNC develop the BSP for your embedded application.

1. The BSP supplied by the microprocessor manufacture is an incomplete solution

The microprocessor manufacturer will typically supply a rudimentary BSP with their evaluation board.   This is because manufacturers know that making it easier for the developer to work with the microprocessor is helpful to being selected for the final design and pcb assembly process.   However, the manufacturer’s BSP may not have the drivers for the specific hardware in your design – the only way to ensure a BSP fully supports your hardware design is to have it customized for you.

2. Developing device drivers is a specialized skill

Developing the drivers and initialization code BSP requires detailed knowledge of the microprocessor and its peripheral hardware.  Most developers writing applications running on an OS do not have the requisite expertise to write the hardware driversunderneath that OS.  On the other hand, a group focused only on BSP development like the team at PNC obtains that expertise by working with many hardware platforms every year, and by developing robust tested reference code for common peripherals such as displays and USB ports.

3. A BSP is needed only once for a product

A BSP is needed near the beginning of an embedded software product to allow the developers to work with the target hardware instead of an evaluation PC board or emulation software. Once all drivers are debugged, however, the BSP rarely needs to be touched again except for occasional updates to address hardware end-of-life issues.  This is different than the product’s application, which may see multiple releases over the life of the product.  Since BSP updates are so infrequent it does not make sense for an organization to maintain that highly specialized expertise for the months or years between BSP updates.

4. The BSP and associated drivers are invisible to the customer

Application software that meets customer needs is a close collaboration between developers, product management, marketing, and sales.  Any time spent by the in-house team developing a BSP is time not spent developing features the customer will see and use.  Outsourcing the invisible aspects of the product like the BSP allows the development team to stay focused on the customer.

5. Outsourcing the BSP can accelerate product development

Handing off the BSP to an outside supplier like PNC means that the team’s developers are not tied down developing it internally.  The BSP supplier can develop the BSP incrementally starting with core functionality followed by drivers for some of the less critical hardware once the development team is ready for it.  The outside supplier also brings deep expertise to the driver development, meaning driver development takes less time, and works the first time. The most beneficial reason for PNC to develop your BSP is that they can also fabricate PCB’s as well having in house pcb assembly services.

Talk to the software team at PNC the next time you have a time critical embedded project.  Let PNC help you with your Board Support Package, device drivers, operating systems porting, or protocol stacks development.

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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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Printed Circuit Board thickness considerations and requirements

How Do You Select a Printed Circuit Board Thickness?

Selecting the correct PC Board thickness for your product requires balancing of three often competing aspects of the design: manufacturability, electrical performance and mechanical constraints. Modern PCB fabrication techniques at PNC give the PCB designer great flexibility in specifying the PCB lamination stack-up and gives them the option to design a PCB in a thickness other than the typical choices of .031”, 062” or .093”.

Manufacturing considerations in selecting a PCB thickness

To understand how to specify PCB thickness, it is important to understand how PCBs are fabricated. Most multilayer PCBS from 2 layers to 40 layers are constructed of these three basic materials.

  • Core – a fully cured fiberglass panel usually with copper foil on both sides. It is essentially a two-sided Printed Circuit Board. PNC stocks cores in a variety of materials and thicknesses, down to 3 mils thick.
  • Prepregs – fiberglass sheets impregnated with uncured epoxy resin. This resin will cure and harden when subjected to heat and pressure during the lamination process. It is the functional equivalent of double-sided tape. PNC also stocks prepregs in a variety of materials and thicknesses.
  • Copper foil – used to create conductive layers. Copper foil thickness is chosen by the amount of current the traces in each layer of the board will need to carry.

Multilayer boards can be manufactured in a variety of thicknesses by mixing standard core thicknesses with standard prepreg thicknesses. The lower limit of PCB thickness is set by the number of layers and the minimum available core and prepreg thicknesses. Copper foil thickness also plays a small role in overall PC Board thickness.

Providing you want to stay with the standard thicknesses, I’ll give you an example of PNC’s standard core thicknesses based off the core copper thickness. Let’s take an .062 and compare the stack-ups. See Figure  below:

Stack-up for an .062 multilayer:
062 thick PCB (002)

 

 

 

Notice to achieve the .062 overall thickness, the core thickness needs to be compensated based on the copper weight of the design. When using 1 oz copper we would use .038 core and .035 core for 2 oz. Also, the amount of pre preg would need to be adjusted as well to get to the thickness required. This is just one example when trying to determine overall thickness of your PCB.

The maximum PCB thickness is usually governed by something called the drill aspect ratio, which is the ratio of a drilled holes depth vs its diameter. When a hole is drilled, the deeper the holes becomes the harder it is to guide the drill accurately (because the drill deflects as it is pushed through the material) until you eventually reach a limit where you can no longer guarantee that the resulting drill hole will be centered in all the pads through the PCB. This is also referred to as drill wander in the PC Board fabrication world.

The aspect ratio for a through hole is the (board thickness) / (drill hole diameter).

Typically, the ratio is limited to approximately 10:1 This means that for a .062” thick board the smallest through-hole drill size is .006” which is the minimum drill size available at PNC. For a .093” thick PCB the smallest drill hole size will be .009” As board thickness increases, more complex via creation techniques such as laser drilled microvias are required to maintain the required pad density under components such as BGAs.

PCB thickness considerations for high speed circuits

One of the assumptions an electrical engineer makes when designing a circuit is that the PCB itself does not contribute any impedance to the circuit. However, in high-speed PCB design the integrity of the signals is definitely affected by the physical characteristics of the PCB.

Unfortunately for the designer the primary sources of parasitic capacitance and inductance of a PCB are affected by board thickness in conflicting ways, forcing the designer to carefully consider the trade-offs.

Vias

PCB vias can introduce both inductance and capacitance to the high-speed circuit. Parasitic inductance and capacitance of a via both increase proportional to board thickness.

Capacitance between layers
The capacitance between traces on different board layers or between traces and the ground plane are inversely proportional to the thickness of the dielectric between them. More space between layers will reduce parasitic capacitance and ensuring that all high-speed signal traces are the same distance from the ground plane will help impedance matching between them.

Crosstalk

Crosstalk between signal traces can be minimized by routing the PCB traces further apart and reducing the dielectric thickness between PCB trace and reference plane.

Mechanical Constraints

A PCB is ultimately a mechanical component of the product and is therefore subject to a variety of mechanical constraints. The PCB or PCBs must fit within the product envelope. This often requires that a PCB be as thin as possible. On the other hand, PCBs are subject to the torqueing forces from cables connected to the board edge and external connections. Preventing board failure during assembly or in use requires a circuit board fabrication thick enough to withstand these forces.

Finally, PCBs in the field are subjected to both vibration and shock. For example, large PCB panels can vibrate in several different modes when subjected to vehicle vibration, causing fatigue and eventual failure of solder joints. The rule of thumb is to get the resonance modes of a PCB to be 10X the input vibration. This requires designing thicker, stiffer PCBS and by careful placement and design of the PCB mounts.

These are the three most important considerations in selecting a PCB thickness. To optimize a PCB design, the PCB designer needs to understand all of these requirements and constrains on the PCB lamination stack-up, and have the flexibility to choose a PCB thickness that balances these often conflicting constraints. The team at PNC can help you design a PCB that meets your needs at a competitive cost and lead time.