For SMD prototyping or the production of small batches, the requirements for production equipment are different compared to that for mass production. The solutions used are mostly manual, semiautomatic or fully automatic equipment with high flexibility. A short changeover and set-up time is more important than speed during production.
Requirements for a high mix/low volume production
In the market segment of prototyping and the production of small batch size, high speed is not the highest priority. The production machines of interest must first of all be flexible to allow fast set-up and changeover from one product to another. The feeder capacity must be large so as to minimize the number of changeovers and it must be able to fulfill the requirements regarding application range and quality. For prototyping, the machinery must be easy to operate and set up, and must offer possibilities to produce a series of assemblies with acceptable quality.
In a production line for SMT assemblies, the technical requirements placed on the equipment depend on the application range. The high quality assembly of BGA or fine pitch components requires, of course, more advanced optical tools than the placement of standard SMD components. Today’s state-of-the-art production equipment can fulfill all requirements on a modular base. Especially for small- to mid-size companies, it is important to invest in a system which offers the required precision and quality, and the possibility to upgrade the equipmentwhen the demands grow.
Manual prototyping
Most small companies start their SMD assembly experience with a manual production line. While it is easy to understand that a manual pick and place and a small oven are indispensable for such a production process, the decision between dispensing or printing of solder paste may be harder to make.
Dispensing is very flexible and therefore the preferred solution for prototyping, but the technical limitation of a manual time-pressure dispenser is reached at a dot size of about 0.8mm (0603 chip components). While more advanced dispensing valves such as the Piezo-Flow Valve from Essemtec can reach much smaller dots, however, fine pitch dispensing can never reach the speed and reliabilityof stencil printing. Customers faced with the dilemma of very small batch sizes and fine pitch QFP usually draw acomplete line of solder paste over all the pads. This process,however, will require rework and cannot provide the samequality as printing.
The printing process becomes more economical above a certain batch size. To decide which technology should be used for a planned production, the manufacturing costs can be estimated using formula 1 and 2.
The result of the two calculations can best be explained graphically (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 bigger, then the printing process is more economical and a stencil should be made. The dispensing process should be used when the series size is smaller than break-even.
A complete modular manual production package consist of a screen/stencil printer, a manual pick and place system (preferably with dispenser option) and a batch reflow oven (figure 2) which offers all necessary features for the complete production of SMD boards. In this package, the high precision manual printer can be equipped with a guided double squeegee for fine-pitch printing, while the pick and place system offers the capability to place finepitch components and BGA. For prototyping, the placement head should have the capability of being equipped with a microprocessor-controlled time-pressure dispenser. The batch oven can offer preset profiles for a fast process setup and a high quality soldering process similar to large ovens.
Semi-automatic production
If more than just some prototypes must be produced or if the board is of high complexity then a semi-automatic production system will be the chosen solution. In such a system, some process steps, like the speed of the squeegee on a printer or the pickup and placement position on the assembly machine, can easily be controlled by a motor or a microprocessor.
A semi-automatic production package (figure 3) consists of a screen and stencil printer with motorized squeegee, a pick and place system with computer control and a conveyor or batch reflow oven. The aim is to increase production speed and also stability of product quality. To avoid huge rework costs and produce high quality continuously on the printing process, operator influence and control over all process parameters, such as squeegee pressure, angle and speed, must be reduced. The operator only places the board at the correct position, and the motorized squeegee will then deliver a process which is repeatable and therefore under control.
When assembling manually, missing components, misplacements and wrong component orientations can occur. These are errors which can be eliminated with a semiautomatic system such as the EXPERT-SA from Essemtec. Linear measurement systems in both axes control the position of the placement head accurately while the placement plan is stored in the connected computer, which guides the operator to the pick and placement position.
This guidance allows for speed and accuracy because no search time is needed as the orientation of the component is clearly indicated to ensure correct placement. Placement data may be imported directly from the CAD system, enabling fast programming and avoiding errors. This solution can also show its benefits when a single prototype of high complexity must be produced.
A semi-automatic production package can allow up to twice the production speed of a manual system and can also avoid a lot of repair and rework costs by eliminating assembly errors. It is usually chosen for the production of small series batches and SMD boards with a lot of components.
Automatic prototyping and high mix production
Production speed increases experienced using an automatic production line compared to manual production are significant. Placement rate and reliability may also increase, and cost and rework time can be saved.
Important requirements placed in automatic production equipment for high mix production include high flexibility, excellent quality, fast programming, quick set-up and changeover and the ability to place all kind of components. For small companies, additional factors of limited floor space and having the modularity to upgrade the machine with growing demands should be considered.
A possible solution is an automatic, standalone production package (figure 4) that includes a programmable automatic printer, a fully automatic pick and place system (with universal laser-centering and vision for BGA and QFP), an automatic dispensing system for glue or solder paste dispensing or encapsulation, and a conveyor reflow ovenwith preset reflow and curing program recommendations.
This automatic solution is economical even at relatively low production quantities. The saved operator costs, increased placement speed and quality are strong advantages compared to manual production. Together with the lower scrap and rework costs, it leads to much lower variable manufacturing costs.
Deciding factors
The larger the batch series and the more important the process control and stability, the higher will be the need for automation. It may also be a company’s strategy to invest in automatic systems, for example, to set up a CIM (computer integrated manufacturing) process or to install the required production capacity for future growth. Regardless of the reasons, no decision should be taken without first having economical proof. As production equipment is a key success factor, too small a capacity or too big an investment may have drastic consequences.
Comparison of production costs with different solutions shows whether a manual, a semi-automatic or an automatic solution is ideal for a planned production. The common procedure is to make a list of all possible costs like floor space, operator costs, electrical supply, air etc. Unfortunately, this generally does not work because either some costs cannot be separated correctly, they are not known or nobody has time to list them all. Additionally, the future is uncertain in detail. A more general calculation model with only the most important cost factors is the better approach, taking into account depreciation, personal cost, throughput, rework and expected volume. This analysis will in most cases clarify whether a manual, semiautomatic or an automatictool is the most economical solution (figure 5).
The production of SMD prototypes or small- to mid-size batch series requires above all flexible equipment. While solutions are available for manual, semiautomatic or fully automatic operation, in order to fulfill growing technical requirements, the equipment should be both modular and upgradeable.
