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Economic Assembly of Small SMT Batch Sizes
December 31, 1969 |Estimated reading time: 6 minutes
For SMD prototyping or small-batch production, equipment requirements are different compared to those of mass production. Solutions used are mostly manual, semi-automatic, or fully-automatic equipment with high flexibility. This article explains possible solutions for the economic assembly of prototypes and small to mid-size batches of SMT prints.
By Adrian Schärli
It is important to invest in the best equipment that allows products to be developed in the required quality and quantity, and with equipment that brings the best benefit. In the market segment of prototyping and small to mid-size batch production, it is not high speed that matters. Production machines must be flexible enough to allow fast changeovers between products. Feeder capacity must be large enough to reduce changeover times, and must be able to fulfill requirements regarding application range and quality. For prototyping, machinery must be easy to operate and setup must be capable of producing series sizes with acceptable quality.
Technical requirements placed on tools depend on the application range. High-quality assembly of BGAs or fine-pitch components requires more advanced optical tools than placing standard surface mount devices (SMDs). Current production equipment can fulfill all requirements on a modular base. For small to mid-size companies, it is important to invest in a system that offers the required precision and quality, as well as the possibility to upgrade equipment to meet future needs.
Manual Prototyping
Most small companies begin SMD assembly with a manual production line. While it is easy to understand that a manual pick-and-place and a small oven are indispensable for the production process, deciding between dispensing and printing solder paste may be more difficult.
Dispensing is flexible, and therefore, it is preferred for prototyping. However, the technical limitation of a manual time-pressure dispenser is reached at a dot-size of about 0.8 mm. If the component’s lead pitch is smaller, as with fine-pitch ICs, then only the printing process can offer reliable results. Customers who are faced with small batch sizes and fine pitches draw a complete line of solder paste over the pads. This solves the problem, but the process will require some rework and cannot provide the same quality as printing.
The printing process becomes more economical above a certain batch size. To decide which technology should be used for planned production, manufacturing costs can be compared and calculated as follows:
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These calculations are also explained in Figure 1. The break-even point is the number of prints of one type where the costs of both production processes are equal. When the total number is larger, the printing process is more economical and a stencil should be made. The dispensing process should be used when the series size is less than the break-even point.
Figure 1. Break-even point analysis comparing the printing and dispensing processes.
A complete manual production package should consist of a printer, a manual pick-and-place system (preferably with a dispenser option), and a batch reflow oven. A modular production package offers all necessary features for the complete production of SMD boards. A high-precision manual printer can be equipped with a guided double-squeegee for fine-pitch printing. The pick-and-place system should allow a fast, precise operation and the capability to place fine-pitch components and BGAs. For prototyping, the placement head should have the capability of being equipped with a microprocessor-controlled, time-pressure dispenser. The batch oven should offer preset profiles for a fast process setup and a high-quality soldering process.
Semi-automatic Production
If more than just a few prototypes must be produced, or if working with a high-complexity board, a semi-automatic production system is desired. Some process steps can be controlled using a motor or microprocessor.
A semi-automatic production package should consist of a screen/stencil printer, a pick-and-place system, and a conveyor reflow oven. The aim is to increase the production speed as well as the stability of product quality. On the printing process, the operator must place the print at the correct position; the motorized squeegee will then deliver a process that is repeatable and under control.
With manual assembly, missing components, misplacements, and wrong component orientations can occur. These errors can be eliminated with a semi-automatic system. Linear measurement systems in both axes can accurately control the position of the placement head. The placement plan is stored in the connected computer, which guides the operator to the pick-and-place position. This allows speed and accuracy because no searching time is lost. Component orientation is clearly indicated to ensure correct placement.
Placement data may be imported directly from the computer-aided design (CAD) system. This enables fast programming, avoids errors, and can be beneficial when producing a single prototype with high complexity. A semi-automatic production package can allow up to twice the production speed of a manual system and can avoid rework-and-repair costs by eliminating assembly errors.
Deciding Factors
The bigger the series, and the more important process control and stability are, the greater the need for automation. It may also be a company strategy to invest in automatic systems, for example, to establish a computer integrated manufacturing (CIM) process, or to install the required production capacity for future growth. Regardless, a decision must not be made without economic proof. Production equipment is a key factor to success. Too little capacity or too large of an investment may have drastic consequences. Comparing production costs with different solutions illustrates whether a manual, semi-automatic, or automatic solution is ideal for the production. The common procedure is to make a list of all possible costs (floor space, operator costs, current, air); however, this generally does not work because some costs cannot be separated correctly, some are not known, and some are forgotten. Using a more general estimation with the most important cost factors for the comparison is helpful: investment, personnel, throughput, rework, and expected volume. In most cases, this analysis will clarify if a manual, semi-automatic, or automatic tool is the most economical solution. This comparison is shown in Figure 2. It is clear that a manual solution is preferred for small volumes. For a larger series, a semi-automatic system will be more economical because of higher throughput and lower rework costs. An automatic system shows its advantages in higher throughput and lower personnel and rework costs.
Figure 2. Comparison of production costs with different production types at different annual production volumes.
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Automatic Prototyping and High-mix Production
Production-speed increases experienced using an automatic production system compared to manual placement are significant. Placement rate and reliability may also increase, and cost and rework time can be saved.
Important requirements placed on an automatic pick-and-place system for high-mix production include high flexibility, high quality, fast programming, short changeover time, and universality. For small companies, additional factors of limited floor space and modularity to upgrade the machine with growing demands should be considered. One solution is an automatic production package that includes a programmable automatic printer, fully automatic pick-and-place system (with universal laser-centering and optional vision for fine-pitch and BGA components), automatic dispensing system for glue or solder paste dispensing or encapsulation, and a conveyor reflow oven with preset reflow and curing program recommendations.
An automatic solution can be economical, even at relatively low-production quantities. Saved operator costs, increased placement speed, and quality are key advantages compared to manual placement. That, combined with lower scrap and rework costs, can lead to lower variable manufacturing costs.
Conclusion
The production of SMD prototypes or a small to mid-size series requires flexible equipment. Solutions are available for manual, semi-automatic, or fully-automatic operation. To fulfill growing technical requirements, the equipment should be both modular and upgradeable. To decide if a manual, semi-automatic, or automatic production line should be used, a break-even analysis focusing on key figures - investment, personnel, throughput, volume, and rework - is helpful. This can show which solution is beneficial, and which equipment is required for future growth.
Adrian Schärli, technical advisor, ESSEMTEC, may be contacted at (856) 218-1131; e-mail: sales@essemtec-usa.com.