Mixed-technology designs are prime for waste elimination. Here’s how.

Taiichi Ohno developed the concept of the seven wastes (muda) in manufacturing as part of the Toyota Production System (TPS), the foundation for Lean manufacturing philosophy. They are:

1. Waste of overproducing (no immediate need for product being produced).
2. Waste of waiting (idle time between operations).
3. Waste of transport (product moving more than necessary).
4. Waste of processing (doing more than what has been agreed upon).
5. Waste of inventory (excess above what was required).
6. Waste of motion (any motion not necessary outside of production).
7. Waste of defects (producing defects requiring rework).

In eliminating waste or muda, it is as important to look at what occurs before the manufacturing process as it is the manufacturing process itself. Preventing defect opportunities at the product development stage is far more effective than process corrections or added inspection once defects appear. Standardizing production to eliminate the wastes of motion, waiting or processing also often involves making recommendations in the design process.

SigmaTron International’s engineering and manufacturing engineering teams work closely with its customer base and production facilities on design for manufacturability (DfM) recommendations that eliminate waste opportunities, while improving throughput whenever possible. That said, sometimes product cost or functionality constraints result in less than optimum manufacturability. That can be exacerbated when a PCB designer works from the perspective of designing to the limits of their design tools, rather than to the limits of manufacturing processes.

For this reason, scheduling design reviews among the customer’s design team and EMS provider’s engineering and manufacturing engineering personnel prior to any preliminary approval testing is critical.

A recent DfM project on a high-density, mixed-technology single-sided printed circuit board assembly (PCBA) highlights the challenge of balancing cost, functionality and manufacturability choices. In a perfect world, mixed-technology PCBAs would be an increasingly rare occurrence. In the contract manufacturing world, customer requirements related to cost, preferred PCB layer count and robustness of the product often drive a need to optimize designs that have inherent inefficiencies.  

Efficient processing and minimization of variation and wait time heavily depend on automation strategy. In this design, DfM recommendations that facilitated better use of automation included:

• Changes that ensured adequate clearances to protect already placed SMT components when using automated through-hole insertion equipment.
• Panelization recommendations to best align with production flow standardized handling systems.
• Evaluating component selection for ease of placement.

In terms of eliminating defect opportunities, recommendations focused on:

• Eliminating potential solder-related issues caused in part by the requirement to use both reflow- and wave-solder processes.
• Evaluating component selection relative to compatibility with required conformal-coating processes.

A strong focus on PCB layout goes a long way toward eliminating solder-related defects when multiple soldering processes must be used. For example, in this project, solder bridging was addressed by increasing spacing between adjacent pads, adding solder thieves and printing a solder screen dam around through-hole pads. Component packaging changes were also suggested to improve solder joint quality.

Shielding of SMT parts during reflow was proposed to eliminate the wastes of overprocessing and defects. Design modifications also included better clustering of through-hole parts, to enable an optimized pallet design that protected SMT parts during wave solder, while providing good solderability for the through-hole parts. A key concern addressed was the elimination of shadowing and insufficient solder that could result if SMT parts were intermixed with leaded components.

In making its DfM recommendation, the manufacturing engineering team utilizes a five-level, color-coded form that helps prioritize the criticality of each recommendation. The five levels are:

• Red/critical: a major process/assembly issue.
• Orange/hot: yield improvement suggestion.
• Yellow/warm: minor concern.
• Green/cool: no immediate concern.
• Blue/ignore: no action required.

The color codes apply to both open recommendations and closed recommendations, so once an orange/hot item is closed, it may be coded as green/cool or blue/ignore.

Finding a balance between design constraints imposed by product requirements and best practices in DfM can be challenging. Taking the time to find that middle ground helps eliminate many forms of waste, however. Helping customer design teams understand the impact of their initial choices via clearly prioritized recommendations also helps “Lean the process.” In this particular example, close coordination among the engineering team, the manufacturing engineering team and the customer helped achieve the best of both worlds.

Allen Abell is corporate director of quality and compliance at SigmaTron International (sigmatronintl.com); allen.abell@sigmatronintl.com.