DfA is the best solution for grappling with varying machine tolerances.

When it comes to effective component placement, machine tolerances, height restrictions and fixture limitations require close attention because they pose assembly issues and hinder process flow. Equipment involved here includes pick-and-place machines, flying probe and ICT testers, wave solder machines, reflow ovens, AOI and other associated gear. Each piece of equipment is designed with different tolerances; plus, similar equipment made by different OEMs has diverse tolerances.

Tolerances, height and space restrictions, and fixture limitations have become increasingly important. Today’s IBM PC-compatible motherboards, for example, are about one-third the size they were a few years ago. PCB area is shrinking, and more components are packed into smaller PCB real estates. This growing complexity usually leaves no option but to place components at the PCB edge, thus increasing the probability of tolerance issues.

Surface mount placement machines, like other associated assembly equipment, use conveyors to move PCBs from one side of the machine to the other. PCBs moving through placement machines on these conveyors usually have about a 0.50" of rail space clearance on either side as the PCB moves through these machines (Figure 1).


Take, for example, a 60-pin SMT edge-mounted connector. If it extends past that 0.50" tolerance, the PCB may incur damage and subsequent failure. On the other hand, if technicians spot the problem ahead of time, they can remove that PCB from the pick-and-place line. One solution is loading this connector as a second operation after all the SMT components are first machine-loaded on the board. This means adding unnecessary manual loading, which delays the assembly operation and adds cost.

Also, a wide variety of fixture limitations are related to height, space and materials. Plus, there’s an added set of issues dealing with multi-depth fixtures (MDFs). An MDF protects components of different heights (Figure 2). For example, a tall and heat-sensitive component in the first operation must be protected from the high temperature of the wave solder machine; then an MDF is used to protect it. An MDF is milled at two different depth levels to incorporate dissimilar component heights.


In addition to height, space, and material fixture issues, MDFs can easily prove ineffective if they are not milled to the exact requirements for proper depth. Mistakes can thus result in lost assembly time and incur additional cost.

Basic fixtures, on the other hand, vary in size and type (Figure 3). They include SMT machine fixtures, wave soldering fixtures, re-soldering or testing fixtures. Sometimes a fixture is used for PCB panelization. For example, a fixture is used because a particular PCB is small and needs to be panelized with multiple boards on the same panel. This type of fixture is used to minimize assembly run time, thereby reducing cost (Figure 4). Also, a fixture must be used to support a thin PCB or flex circuit loaded with heavy components to prevent bending during assembly. Another example: fixtures used to protect bottom-side SMT components during reflow.


As stated, fixture limitations can be countless, but they generally fall under the categories of height, space and materials.

An added word of caution as it relates to panelization: Each PCB must have sufficient clearance from one another in terms of width and height. For instance, on an 8" x 10" panel, a small PCB includes an edge connector. Sufficient room must be allocated for that connector for the required gap between the end of the connector and the start of the adjoining PCB.

Limitations also crop up as a result of incorrect material use. On one hand, a certain material may be overly difficult to mill, thus creating the probability of fixture inaccuracies. On the other, the wrong material can be inadvertently selected for Pb-free components subjected to temperatures 20˚ to 30˚C higher than traditional SnPb processes, hence creating assembly stoppages or delays.

In the area of height restrictions, advanced PCB designs sometimes use tall vertically mounted heatsinks, which can pose assembly issues (Figure 5). If the part is 0.50" to 1.0" taller than what the equipment can handle, a flying probe tester cannot be used. This height restriction prevents technicians from testing certain PCB areas. Here, there’s no choice but to go to a second or third operation and mount that particular heatsink or other tall device at the end of the process.


There are also space restrictions from component to component. Components must be properly spaced to ensure the correct amount of flux and solder is applied to a joint. Placed too close, there may not be sufficient area for test point placement, or it could interfere with other components’ peripheries.

Extra Assembly Cost

Exposing poorly designed boards to assembly equipment and its tolerances and limitations results from either inattentive or untrained technicians and operators, or little to no design for assembly (DfA) planning. Consequently, components are not properly placed on the PCB, or components or PCB damage results because of contact with equipment boundaries. Either way, those oversights incur lost assembly time and associated cost.

Borderline components can also face problems. For example, if a PCB with a 60-pin SMT edge connector requires 0.75" of space on either side of the placement machine rail, whereas the machine only has 0.50", there is a 50:50 chance this connector will be properly placed on the board. Now, reliability is questionable, since a percentage of the 60 SMT pins will probably not be correctly soldered on the board. This assembly problem can be detected at the QC stage, and measures will be taken to correct it – but with delays and added costs.

If and when these types of issues are detected, rework needs to be performed and hand placement required. This extra work can induce errors and require more QC and AOI time – and even more cost. Hence, poor design significantly increases cost. It is not acceptable to allow a questionable design to enter or go through assembly, especially if production quantities are in the hundreds.

Other additional costs can come from creating the wrong fixture based on errors made in milling or using incorrect material. Each fixture costs anywhere from $1,000 to $2,000 apiece, and if the wrong fixture is produced, the cost to create the right one doubles.

DfA Solutions

Together, the OEM and EMS provider can institute several steps to create a more effective assembly environment. DfA is the first step toward minimizing and eliminating issues machine tolerances and limitations pose. Applying sound DfA practices provides a basis for interaction necessary between the OEM and EMS provider. The objective is to exchange valuable design information and get OEM feedback to iron out issues at the outset.

As a vital part of DfA, the EMS provider should always encourage the OEM to perform a first article check. It not only provides a proof of concept, but also verifies that there are no glaring component placement issues and sends up red flags if there are.

The start of a project focuses on critical placement, and by its very name, tells the EMS provider that a certain placement is critical and cannot be changed under any circumstances. While placement, per se, cannot be changed, the component can; this is where DfA expertise proves invaluable to an OEM.

DfA or layout engineers can suggest equally effective replacement components to create a smoother assembly process. For instance, a connector with an unacceptable 1" protruding length from the edge of the board can be substituted with another connector that protrudes only 0.50" from the edge. As a result, the replacement can easily process through all assembly equipment without incurring special and costly hand soldering.

Noncritical placement is also subject to change on a design that can be optimized. In many cases, the DfA expert can recommend certain components be moved around on the PCB to make it more assembly-friendly. At times, the remedy may be to reorient component placement. For example, vertically assembling heatsinks compared to horizontally mounting them results in increased surface area for improved heat dissipation.

Another way an OEM can benefit is to make an initial nonrecurring engineering (NRE) investment at the planning and layout stages. A modest NRE like this provides the necessary DfA expertise to help ensure a 25 to 30% higher assembly throughput on monthly production quantities of 500 to 1,000, for example.

By ignoring this initial planning stage, assembly issues and related increase in costs can result. For instance, an EMS provider receives a PCB assembly project with three questionable connector placements. During assembly, the project requires transferring those three connectors from one PCB location to another. Taking the necessary action to resolve this problem includes layout changes, PCB fabrication and assembly drawing changes, all costly and time-consuming. Overall extra cost in this instance can range from a few hundred dollars to thousands, depending on the complexity of the project.

Conversely, upfront NRE costs to adjust such a design range from $2,000 to $5,000, miniscule compared to the inordinate costs poor designs can incur during assembly. For example, saving the cost of 20 to 30 minutes per PCB for hand-loading will more than recoup the initial NRE outlay.

Zulki Khan is president and founder of NexLogic Technologies Inc. (nexlogic.com); zk@nexlogic.com.