Aspects to evaluate when considering customization.

Versatility, or the ability to address different requirements with the same method or hardware, is a critical differentiator for most production facilities. Flexibility is a key to competitive advantage. One of the key components of a surface mount line is the printer. It is the first piece of equipment any product will see. When purchasing screen-printing equipment, consider the process requirements that dictate the machine’s versatility. In many instances the same printer is used for both traditional solder paste printing and non-solder paste printing applications, such as adhesives for bottom-side component attachment. In such instances, some custom application evaluations require process investigation, while others require investigation into the equipment’s capability. Below are key factors to consider when purchasing a printer for use in a variety of applications.

Frame construction. The skeleton of a living creature is what holds the outer body together and provides strength and stability. The function of a printer frame is very similar. It provides stability for various movements that take place during a print operation and acts as a shock absorber for vibration. The base frame should be made of rigid steel or other high-strength materials to provide stability and precision. This is critical to accuracy.

Motion control. As Jim Montague of Control Design says:

Juggling is all about keeping appointments. Visual, tactile and other sensory data help jugglers’ brains tell their hands where to go, when to be on time, and how to catch and throw when they get there. Practice literally creates and strengthens neural pathways, which combine many separate tasks in the novice’s head into one or two fluid moves in the expert’s mind. Motion control engineers managing multiple axes or servo-based applications know these jobs only too well, and few more so than surface-mount printer users. Consequently, they understand that better information and faster networking means greater motion accuracy and efficiency, and so they continually demand it from their machines and builders.

Simply put, the printer motion control system should be able to run multiple components simultaneously. The motion control technology commonly known as CAN (controller area network) permits true multitasking operations (i.e., juggling). Fast motion control communication should be a goal for any SMT equipment.

Vision. When processing a board, the first thing an engineer wants is the ability not only to see the board but to distinguish pads from background noise. The vision system’s primary function is to communicate the pad location to the stencil alignment routine, thus enabling the stencil to put down paste at the right location. Important components of a vision system to evaluate include camera type (digital vs. analog), lens type, lighting type (white, IR, UV), etc.

Inspection. The printing operation is believed to introduce 50 to 80% of SMT line defects (a highly contested topic, to be sure). Since there are several value-added steps beyond printing, it is highly beneficial to catch these defects early. One way to prevent defect propagation through the SMT line is to inspect the board for print defects before it begins the next step, usually placement. Inspection becomes more critical with the use of 0201 and 01005 components.

There are two ways to achieve inspection: an external SPI (solder paste inspection) system or an internal printer inspection system. An external SPI can be cost-prohibitive and takes up floor space. In-printer inspection offers a “two-for-one” deal. Attributes to look for in an in-printer inspection system:

• Can it see any/all board surface finishes?
• Can it detect solder bridging?
• Can it inspect angular devices?
• Can it copy or step-and-repeat devices previously taught?
• Does it use accept/fail criteria to the device level?
• Image acquisition speed or inspection speed?
• Can it sort print errors to the pad level?
• Are the data sufficient for troubleshooting defects?

Accuracy and repeatability. Determining the true capability of the process equipment can be daunting. There is no standard requiring the equipment manufacturer to report equipment/process capability in a specific manner. Hence, it has become a contest of statistical gamesmanship. Some report equipment/process capability as accuracy of +/-N, while others report it as a repeatability or process capability index (Cp, Cpk).

The correct way to approach this is a sound statistical approach toward understanding process capabilities in conjunction with confidence interval. A printer’s ability to perform according to the product design and process capability indices (Cp, Cpk, etc.) is the right approach. Don’t be taken by claims, for example, of a “printing process capability of +/-10 µm”; hold out for the process capability index. Most likely, the Cpk for the former process is less than 1.0. To a knowledgeable engineer, a claimed Cpk of 2 @ +/-25 µm is a much more capable process. Many process engineers are becoming aware of, particularly through Six Sigma programs, the ability of statistical tools in determining process capabilities and subsequently improving them.

Board handling. The wider the board handling capability, the more flexible the process. When evaluating handling, look for: transport track system; centernest configurations; vacuum hold-down options; support tooling for thin boards; board stop sensor; prestaging.

Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.

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