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Layout and Tracing Rules for Box Build Assembly

Quality of electronic products largely depends on assembly technologies. Box build assembly refers to a process during which according to design files, work procedure and technologies, multiple components and accessories are assembled and fixed on certain positions of circuit boards or enclosures so that an integrated system is generated. Then, after testing and inspection, those systems will become final products and after packaging they can be distributed to sales offices around the world.

Among modules of mass manufactured electronic products, technology files are actually composed by files indicating technological features of each stage of the whole manufacturing procedure. The stages can be summarized into assembly preparation, sub assembly and box build assembly.
a. Assembly Preparation - refers to a series of preparations for subassemblies and box build assemblies in terms of material, technology and manufacturing organization.
b. Manufacturing Organization Preparation - based on technology files, workflow and assembly approaches will be determined with workflow operation and engineering staff distributed.
c. Assembly Tools and Equipment Preparation - Commonly-applied manual tools in the process of electronic product assembly play an important role in the whole assembly procedure.

Leading Attributes of Box Build Assembly

a. Electrical attribute of box build assembly lies in circuit soldering of components assembled on printed circuit boards (PCB) as a backplane. Structural attribute of box build assembly lies in mechanical integration and enclosure assembly in which assembly is implemented in a sequence from inside to outside through the approach of component fixation.

b. Box build assembly technology is comprised by multiple technologies such as component quality inspection and lead forming technologies, lead wires and harness processing technologies, soldering technologies, assembly technologies etc.

c. Assembly quality is inspected through visual inspection and hand feel rather than quantitative analysis. For example, soldering quality is usually judged by visual inspection whereas quality of assembly of rotary knob and dial is inspected by hand feel.

Box Build Assembly Approaches

From the perspective of assembly principles, box build assembly features three approaches:

a. Function Approach - refers to the process in which electronic products are divided into a few sections according to functional modules and each component, also called functional component, is functionally and structurally relatively complete, which can be independently assembled and inspected. Different functional component performs so differently in terms of structure, volume, connection spec and assembly spec that uniform regulations are difficult to be generated. The main advantage of this approach is that assembly density of the whole system can be reduced. Therefore, it is generally applied on products whose functions depend on discrete components or devices applying vacuum electron tubes.

b. Component Approach - refers to multiple components manufacturing whose figure spec and assembly spec feature uniform specifications. Its merit lies in electrical assembly unity and assembly standardization improvement and is usually applied to get devices assembled such as integrators.

c. Functional Component Approach - combines advantages of both function approach and component approach, it is capable of manufacturing components with integrated functions and standardized structure size. This approach works best on micro circuits.

Box Build Assembly Layout and Tracing

• Layout

Layout principle of box build assembly indicates that technical index of products should be ensured to be achieved with the following detailed rules; structural demands should be met; layout should be convenient; layout should be beneficial for thermal dissipation, inspection and rework. Detailed box build assembly layout rules are listed as the following:

a. Power. Power should be placed at the bottom of devices. Power supplying working energy to the whole operation is usually composed by power transformer, rectifier tube, electrolytic capacitor and pass element all of which feature relatively huge volume and weight and generate much heat. Therefore, power should be placed at the bottom of devices. Furthermore, certain distance should be maintained between high voltage section and low voltage section; high voltage terminal and high voltage wire should be insulated from enclosure or frame and far from other wires and ground wires; switches should be assembled on powers with a high voltage of 1KV at least. Moreover, control mechanism of power should be connected with enclosure that should be suitably connected with ground.

b. Control Mechanism and Indicating Instrument. Control mechanism and indicating instrument should be assembled on board for the convenience of operation, monitoring and rework.

c. Components. Components here refer to those that tend to suffer from fault or breakdown that should be arranged at places where components are easy to be maintained or modified like breaker or electrolytic capacitor. Vacuum tube should be inserted or pulled with few efforts. Those test points that demand frequent inspections should be reasonably arranged so that they are easy to be obtained.

d. High-power Components. High-power components tend to generate much heat when working so they should be arranged at places where heat can be easily dissipated in the whole mechanism. For example, high-power transistors are usually assembled at the external side of backplane along with radiator. Fans or thermostat devices should be added when necessary.

e. High-frequency Circuit. Apart from general component layout rules, high-frequency circuits have to conform to the following requirement:

1).Isolation measures should be implemented between high-frequency circuits and low-frequency circuits when they share the same base or are located at the same circuit board. Unit circuit should be applied as a shielding structure with high shielding performance and adjustment. Shielding of vacuum tube should be independently carried out.
2). Metal components shouldn't be installed near unknown shielding wires because they may reduce inductance value of foil and quality factor. Sufficient distance should be maintained when they have to be installed.
3). High-frequency and high-potential components and concerning connection wires should be arranged at places far from enclosure or shielding wall in order to reduce distributed capacitance. Silvering hard-drawn bare copper wires should be applied when connection wires are not so long so that position tend to be unchanged, distributed parameters stable and dielectric loss relatively low.
4). To avoid extra parasitic coupling, components in high-frequency circuits should be fixed by their own structure rather than external fixation parts.

• Tracing

a. Ground. Once metal base is applied, it's optimal to set thick copper wires as ground wires at the bottom of base. Ground wires of PCB generally depends on large-area layout to be arranged at the edge of board with components connected to ground nearby or with all ground points connected to the ground at a single point. High-frequency ground wires generally rely on flat copper wires to decrease impedance control of ground wires.

b. Harnesses. Harnesses should be fixed close to device base or on the frame. Leads in high-frequency circuits should be mixed into harnesses prior to shielding. High-frequency wires led from different return paths shouldn't be placed in the same harness or parallel arranged. Instead, they can be vertically intercrossed.

c. Leads and connecting wires. Component leads or connecting wires should be as short and straight as possible. However, they can't be pulled too tightly since sufficient flexibility should be kept for debugging and maintenance.

Connecting wires in high-frequency circuits should keep their diameters and length as small as possible. Insulating materials shouldn't be applied with high dielectric constant or dielectric loss. If leads have to be placed in parallel, distance between them should be enlarged as much as possible.

Your design won't become optimal realities unless a qualified and reliable box build assembler is depended. Prepare your design files based on the rules discussed in this article and deliver them to a capable assembler enabling to convert your design into genuine products.

Helpful Resources:
Box Build Assembly Technology Introduction
Printed Circuit Board Assembly Inspection Methods
Some Handy Methods in Evaluating SMT Assembler's Capabilities
6 Effective Ways to Cut PCB Assembly Cost Without Sacrificing Quality
Full Feature PCB Manufacturing Service from PCBCart - Multiple Value-added options
Advanced PCB Assembly Service from PCBCart - Start from 1 piece

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