PCBs
easier to source through-hole devices that are compatible with breadboard platforms. Experienced distributors can advise on the packaging options open to a design team for prototyping and which support a transition to surface-mount components for final production.
Importance of packaging Packaging choices represent an important factor in determining whether it is better to order fully populated prototypes from an EMS provider or to perform some of the assembly in the design team’s own lab. A commonly used approach is to use tools such as those from Altium and Autodesk to design a PCB for a prototype and then assemble the necessary components for the design in the lab. To use this approach, the team relies on combination of design insight and access to low-cost lab tooling and test.
In contrast to the breadboard option, designers can choose to employ not just components in through-hole packages but those that are designed for surface-mount assembly. However, there are practical restrictions on what can be realistically assembled and soldered in a lab environment. This is simply because of the difference
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in accuracy between human hands and automated pick-and-place equipment which can position tiny components with sub- millimetre accuracy. To some extent, the surface tension of a hot solder, assuming it is deposited reasonably accurately on the surface of a PCB, will help to pull small discrete into place. However, it is clearly easier to mount by hand discretes that come in the larger surface- mount packages or their through-hole versions than 0402 or smaller surface-mount devices. Similarly, surface-mount devices with pins around the edge, such as quad flat- pack packages, will be easier to place and solder in a lab environment compared to those that use ball-grid array (BGA) because the engineer can see whether the pins are correctly aligned before soldering. Many surface-mount ICs come in a variety of packages so that the QFP variant can be used for low-run production and prototyping with the BGA or chipscale package variants used in production. Distributors with design-in support experience can advise on which components are suitable for a twin-track approach in which the prototype uses one form and the final PCB the lower-cost or more compact variant. Simulation tools will help identify any potential changes in signal integrity or I/O
routing that may be needed to accommodate the transition from prototype to production.
Benchtop tools to facilitate in-lab assembly
There are a variety of everyday benchtop tools, such as fine-tip soldering irons, to facilitate in-lab assembly. Today, the microscope is an essential aid to mounting fine-pitch devices on bare PCBs. The stereo-zoom microscope typically provides ring illumination to assist with the precision placement of components and to support post-reflow inspection. A solder-paste injection is another important tool. Typically, this uses compressed air to apply controlled amounts of solder paste to the PCB in a repeatable way. The high level of control over dispensing greatly eases the mounting of packages, such as QFP, with large numbers of closely spaced pins. For the solder-reflow operation itself, an engineer may use a reflow hotplate to reflow small portions of the board at a time. Alternatively, all the components may be mounted before transfer to a benchtop reflow oven.
As some level of soldering failures will be inevitable for all but the most practised engineers, test and rework tools will be vital.
Hot plates and hot-air tools assist with the removal of misaligned or failed components ready for a resoldering operation. For test, a multimeter is an important tool as it can be used to probe any of the visible pads and surface traces for when checking for connectivity. Farnell stocks a comprehensive range of lab tools made by specialists such as Metcal and Weller and has the in-house expertise to advise on their capabilities for prototype assembly and rework.
Bringing it all together
There are many options and trade-offs open to design teams when it comes to creating and using prototypes during product development. Experienced distributors can provide valuable advice on sourcing components suitable for lab assembly and the best form of delivery to EMS partners as well as how the package and supply options can be changed to suit final production. In addition, they can guide teams on the best tools to use in the lab when in-house assembly is the best direction to take. All that is needed now is the imagination and skill of an engineer to turn a product idea into reality.
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Components in Electronics
March 2022 37
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