Special software permits factory floor adjustments, potentially saving capital investments.

What is virtual manufacturing? The simple answer is a PC-based manufacturing simulation. The more complete answer is that virtual manufacturing is the process of designing a model of a real system and conducting experiments with this model for the purpose of understanding system behavior. Processes must be completely understood before implementation in order to “get it right the first time.” To achieve this, use of a virtual environment is essential for simulating individual manufacturing processes and the total manufacturing system. By driving compatibility between product design and assembly plant processes, virtual tools enable early optimization of cost, quality and time to help achieve integrated products, process and resource design, and affordability.

Moreover, manufacturing systems may be widely distributed geographically and linked in terms of material, information and knowledge flows. Virtual manufacturing is the only method that can encompass product, process, resources and plant to provide flexible and agile production.

Benefits of virtual manufacturing include:
Visualize the material flow through the manufacturing system.
Identify the bottlenecks of the system.
Understand the equipment and manpower utilization.
Optimize the system by virtually adding resources (equipment/manpower) to observe performance responses.
Perform cost analysis per manufacturing process.
Predict and eliminate on-the-job injuries as well as ensure manufacturing feasibility, part by part.
Assembly planning and validation.
Process simulation.
Virtual manufacturing has also been successfully implemented in the following areas:
Airport operations.
Urban traffic study and development.
Maintenance operations.
National economy study.
Waging military battles.
Material and warehouse distribution systems.

Figure 1 shows the manufacture of a double-sided assembly, modeled using simulation software.

1. Kitting area. Work orders are created, and PWBs and components are pulled from the stock room and kitted. Components are replenished.

2. Automated area. PWBs are labeled and routed through an automated screen printer. Solder paste is inspected and SMT components placed on the PWB using a chipshooter or a pick-and-place machine. AOI checks the component presence, component value and orientation. Missing components are replaced manually. PWBs are then passed through a reflow oven where solder paste is melted and component attachment takes place. Residual solder flux is removed using cleaning equipment. If components must be attached on the bottom-side, the process is repeated. Assembled SMT components are then inspected under x-ray for solder reflow quality. If any defects (missing solder, voids, shorts) are observed, the PWBs are reworked.

3. Manual area. Non-SMT components and some heavy SMT parts that need special attention are hand-soldered in this area. The PWB panel is depanelized into individual PWBs using a depanelizer. After mounting on a special casing to protect the bottom-side components, connectors and filters are hand-soldered, and the PWB is cleaned. Flying probe and continuity tests are performed on individual PWBs. Failed components are reworked or replaced at this station.

4. Conformal coating. PWBs are conformal coated, cured in the oven, and inspected. Any coating defects found in this area are returned to the manual area for recoating. Good parts are packed and the job order closed out.

Assumptions Before Simulation

During the manufacturing simulation, educated assumptions were made regarding the resource, equipment layout, equipment availability and process flow. Some important assumptions included:

Double-sided, hybrid PWBs will flow through the kitting area, automated area, the first manual area, coating area, and the second manual area. Individual PWBs are on a 2 x 2’ panel.
PWBs are transferred between different areas in batches of 16 PWBs.
Six operators will be needed in these positions: kitting, placement, inspection, assembly, test and coating.
Assembly time used in the simulation is based on previous experience.

Equipment resource was assumed to be dedicated to PWB assembly with no conflicts in resources. It is assumed that machine utilization is 100% with no downtime. Resource utilization (manpower) is 70%.

Processes such as solder paste printability, component placement, solder reflow, and conformal coating are assumed to be 98% defect-free.

Simulation report and analysis. We ran 95 iterations to get a 95% confidence level in the simulation model. The time spent in each area and manpower utilization was calculated. The average time spent to assemble a batch of 16 PWBs was determined to be 26 hrs. The most time is spent in the first manual area, where connectors hand-soldering and PWB cleaning took place.

Based on statistical analysis, with 95% confidence we can state that:

The time spent in the manual area accounts for approximately 50% of the total manufacturing time.
The assembly operator is the most utilized resource at 34% utilization, more than twice any other resource.
The system is underutilized, with most of the resources utilized less than 20% of capacity.

It can be concluded that with the current input parameters, the system is underutilized. Steps taken included:

The biggest bottleneck was hand soldering. With operator cross-training and additional hand-soldering stations, this bottleneck was eliminated and manpower utilization improved to 50%.

Equipment layout was modeled using simulation software to optimize product flow with minimal handling, thus saving unnecessary installation and moving costs.

Operator movement around the machines and workbenches was modeled to provide ergonomically designed workcells.

Kan ban storage for replenishing components and floor stock was strategically placed to optimize production flow.

The kitting operator was trained and certified to perform conformal coating. After optimizing the production line resource to five operators instead of six, the simulation was recalculated and showed an additional 10% improvement in manpower utilization. 

ACI Technologies Inc. (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.