Key Takeaways

• What are the types of assembly processes?

• What is assembly process planning?

• Understanding the importance of design for assembly (DFA).

• Guidelines to optimize DFA process planning.

Lost or very lost

I am not sure how this rumor got started, but apparently some folks are going around saying that engineers are not good at taking directions. In fact, they say if you go on a road trip never let an engineer take the wheel. Otherwise, before you reach your destination you are likely to get either lost or very lost. Now, I do not agree with these assertions; however, I must admit that I seldom use GPS. The ironic thing is that engineers actually excel at giving directions.

One example of this prowess is in designing PCB layouts, which are the directions or blueprints for building electronic circuit boards. For these designs, the materials, dimensions, components and drill holes must be precisely specified for contract manufacturers (CMs) to construct the boards. And most engineers know that manufacturability improves significantly when the CM design for manufacturing (DFM) rules are followed. It naturally follows then that PCB design for assembly process planning should lead to a better outcome during the assembly stage of manufacture.  

Before seeking to validate this statement, let’s clearly define the PCB assembly process types and what planning for them involves.  

Types of PCB Assembly Processes

PCB assembly or PCBA is the final stage of circuit board manufacturing, which also includes fabrication. As such, assembly provides the last opportunity to inspect your board and make any corrections prior to shipping. However, the primary function of this stage is to securely mount the components, which is accomplished through soldering. And the components used define the assembly process type, as shown below.

PCB Assembly Process Types

• Through-hole

Through-hole technology (THT) refers to the mounting and attachment of through-hole components utilizing vias that extend from the top to the bottom of the board. 

• Surface mount

Surface mount technology (SMT) is the assembly process for securing surface mount components, which may utilize fanouts or vias for connection to other board elements.

• Mixed 

When the design includes through-hole and surface mount components, which is very common, the assembly process is mixed and both THT and SMT soldering techniques are employed.

For each of the assembly process types to be successful, certain requirements must be met. This includes board fabrication that falls with the constraints of your CM’s DFM and planning for the assembly process that will be used for your PCBA.

Planning for the Assembly Process

PCB assembly depends upon the accuracy of the design data supplied to your CM. The specifications and selections determined are used to set up and program the equipment that comprises the mostly automated assembly process. This planning is not trivial. In fact, there are many aspects of the assembly plan that may lead to performance problems or lead to the failure of the board assembly, as listed below. 

_{* Edge components affect the depanelization of your boards. These components would have to be mounted after board separation, which may incur additional costs.}

_{** Many CMs will not manufacture boards with this via type or not guarantee the work.}

The list of potential problems and failures shown in the table is not exhaustive. However, it does serve to clearly indicate why it is important to plan for your board’s assembly during design. 

Design for Assembly Process Planning Optimization Guidelines 

As discussed above, in order to create the assembly process, plan design attributes must be quantified. The constraints within which these attribute specifications must fall are determined by the equipment and processes that your CM uses. Collectively, these are known as design for assembly (DFA) guidelines. To ensure the best assembly process for your boards, the incorporation of these guidelines into the design stage of PCBA development should be optimized. 

The targeting of the assembly process during design begins with the schematic. Specifically, with the selection of components and their packages. A component’s package defines its footprint which along with its circuit functionality influences placement on the board. During board layout, a number of other assembly process attributes are defined. For example, the spacing between adjacent component packages, as shown in the figure below. 

OrCAD Assembly Process Planning Tool

The figure above illustrates the best method of applying DFA, which is by integration into your PCBA design tool.  

Cadence’s PCB Design and Analysis software, OrCAD includes a DFM checker that makes PCB assembly process planning a simple and straightforward task. Additionally, Celsius can be used to analyze thermal properties to ensure the soldering process will have no negative effects on the board.  

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.