Michael Brianda, general manager, DEK GmbH
Experience has shown that the stencil, as the most fundamental element of any pre-assembly process, not only defines the print-performance (and soldering) parameters, but also exerts a powerful influence on subsequent assembly stages. By implication, a „bargain-basement“ attitude to stencil procurement will not deliver the consistently highend of line yield that electronic manufacturers must achieve in order to survive in competitive markets. This is true in terms of assembly-production quality as well as the necessary application support in case of trouble.
As the cornerstone of the assembly process, the real cost of the stencil lies not in its price but in its performance at the point of use. Many factors contribute to this aspect, including quality of materials and knowledge of design, the supply chain and service network that delivers the stencil to the right place at the right time. In addition, the supplier’s investment in understanding the needs of its customers on an individual basis and the realization of the appropriate solution, and customer-support service mentality including printing process and set-up knowledge – all is of high importance.
Selecting quality materials should be the first concern of a stencil manufacturer considering the ongoing process towards further miniaturization and the demand for zero defect production. Materials make a relatively easy target for cost-reduction campaigns. For example, at DEK we are very particular about the materials we specify for manufacturing laser-cut stencils, ensuring uniform thickness by only taking steel cut from the center of the roll. Cheaper materials may be available, of course, but the saving would be false if the result were to be a less-than-optimal product. For a quality and reliability-conscious supplier, it is always important to put product performance before an attractive price tag. Indeed, focusing exclusively on just the price, either of the raw materials or the finished stencil, draws attention away from the true sources of cost in any manufacturing operation. These can include late delivery, poor production yield, inappropriate quality, excessive rework with high costs, imperfect reliability leading to frequent customer field service calls (high costs and loss of reputation), and an increased end-of-line inspection burden.
Manufacturers running SMT lines have to be very flexible in their production environment. They want to take advantage of knowledgeable international suppliers who take ownership for their local production. One-stop shopping becomes increasingly important. The full range of screens and stencils offered by DEK includes all different frame sizes as well as frameless systems to meet all printing requirements. There are emulsion screens optimized for electronic applications or other industrial materials, and laser-cut stencils optimized for paste or adhesive processes, as well as plastic stencils for adhesive deposition to meet all squeegee or direct imaging printing process requirements (this includes, of course, stencils optimized for the proprietary PumpPrinting process. PumpPrinting, when combined with ProFlow DirEKt Imaging (based on a closed print head), has emerged as the fastest and most repeatable solution to adhesive deposition in electronic applications.
Further options include additive stencils, where depositing nickel, to enable greater deposit height, thickens localized areas of the stencil. We also have the facility to generate recessed stencils where the thickness is reduced to accommodate the paste requirements of special component types, as well as electroformed stencils, pin-in-paste stencils for through-hole assembly, mini-rework stencils and inspection templates. This investment in manufacturing capability spans the available technologies, and allows stencil characteristics to be closely matched to each customer’s process, thereby ensuring the optimum yield in every case.
Of course, the accuracy of the basic data supplied by the customer describing the desired deposition pattern and printing processes to be used has a deep impact on the results. DEK’s European design centers are all equipped to accept customer Gerber data or the scan plot of a PCB as a minimum, but the company was also the very first stencil producer to invest and develop in Valor design and manufacturing software that uses the intelligent ODB++ CAD/CAM data exchange format. ODB++ combines CAD/EDA, assembly and PCB fabrication knowledge in one single and unified database. This eliminates the ambiguities inherent in the more simplistic Gerber data format, helping stencil designers to identify and fix errors quickly and easily.
The pan-European strategy to guarantee rapid delivery of high-quality stencils and emulsion screens extends throughout the continent. Technical centers in the U.K., Germany, France, Netherlands and Hungary are each equipped to generate laser-cut stencils to identical standards of accuracy, to ensure rapid turnaround on customer orders regardless of location. The operation managers meet frequently to exchange information concerning customer requirements, product optimization as well as upcoming manufacturing technologies.
In operation, yield is the primary factor that governs the return on the user’s investment in a stencil. Given that a well-designed stencil will always give a good process yield and repeatable results, taking the time at the beginning of the stencil design to understand the target process and desired result delivers immeasurable value. We in DEK are very much following the principle of „people want to buy from people“. A high-value stencil service, therefore, will not focus on slavishly fulfilling customer orders to the letter, but on generating a solution that genuinely enables the customer to reach the desired goal. A vast amount of learning and technical awareness is required if the finished product is to be the optimal solution.
The value of printing-process knowledge is brought to bear on each stencil design as soon as the customer product information and CAD design data is received. The stencil designers verify the data against the customer’s technical „standard“ expectations, to ensure that the stencil will deliver what the customer wants. This preview adds significant value, especially in cases where the customers’ engineering department has not generated the order. In some cases, the customer site that will actually be assembling the board may not have seen the data before the stencil arrives. It is also important to verify that none of the desired pads will cause problems, and check for other errors, which can enter the data unnoticed via a number of routes. The specialists’ skill here play a big part in achieving the optimum stencil and print solution in every case.
Even after applying these basic checks, some suppliers may be prepared to begin stencil manufacture immediately, but it is helpful to take the time to analyze the manufacturing process the customer will be running before embarking on production. It is then possible to develop a set of design rules that can be applied to accommodate the different processes and their requirements on the shop floor, and eliminate variability in subsequent stencils.
Taking extra care at these early stages of stencil production means that printing problems encountered by the users during their assembly process are rarely attributable to the stencil, and can usually be resolved quickly by small machine or process adjustments. This is a big advantage, because manufacturers cannot afford to delay production while the stencil supplier resolves design faults that should have been eliminated before committing to manufacture.
In some cases, particularly when board assembly or stencil design are especially complex, it is beneficial to test the completed stencil on the target process, before delivery to the customer. This seems very difficult for most stencil manufacturers to achieve. However, as a producer of a comprehensive range of automatic and semi-automated printing platforms, DEK enjoys ready access to equipment capable of replicating the entire customer process in-house. It’s part of the daily work of the stencil CAD designers and printing application engineers to develop process solutions on behalf of customers. As a result, users can be assured of receiving stencils that play their full part in maximizing end-of-line yield, straight out of the box.
Even post-delivery, we see our role as a stencil provider in terms of ongoing knowledge transfers, working continuously with customers to build understanding and enhance process yield. The engineers frequently carry out process audits at customers’ sites, by invitation. In this way, it is possible not only to refine stencil design for future reference, but also to fine-tune a process where it is necessary to extract the best performance from the stencil and the procedure, even if the delivery has to be done within a few hours. And it is not uncommon to sometimes find out that delivered data of PCBs, pads and tracks don’t exactly match the requirements in special cases.
In a nutshell: in the printing process the screen or stencil plays a pivotal role in determining the yield to be achieved. Users seek to drive up yield, thereby maximizing their return-on-investment in equipment, services and consumables such as solder paste, adhesive or conductive ink. There-fore we have to consider the stencil as a highly engineered part employed in the process chain. The true value of a screen or stencil can be measured in terms of rigorous design, compatibility with all types of printers, accurate production, timely delivery and ability to integrate perfectly with the process from the very first print.
In der Oberflächenmontage (SMT) kommt dem Druck der Lotpaste entscheidende Bedeutung in der Kette aller folgender Prozessschritte der Baugruppenfertigung zu. Ist der Pastendruck ungenau oder fehlerhaft, dann kann auch der auf das Genaueste kontrollierte Bestück- und Lötprozess keine besseren Ergebnisse erzielen. Der erste Schritt setzt also die Maßstäbe, wobei es sich hier um ein wohlabgewogenes System aus Schablone, Paste, Drucker und natürlich Leiterplatte handelt. Wir untersuchen hier die relevanten Einflussgrößen auf Kostenbertrachtungen in der Besorgung von Schablonen und Drucksieben. Nicht immer ist das Teil mit dem niedrigsten Einkaufspreis im Prozess die tatsächlich kostengünstige Lösung.
Dans le montage des composants en surface (SMT), la pression de la pâte à braser joue un rôle décisif dans la succession des opérations consécutives de fabrication des circuits. Si la pression est imprécise ou erronée, un processus d’implantation et de sou-dage, aussi bien contrôlé soit-il, ne pourra donner de bons résultats, le maillon le plus faible limitant les autres. La première étape est donc déterminante pour que le système comprenant le pochoir, la pâte, l’imprimante et bien sûr la carte imprimée soit homogène. Nous étudions ici les facteurs décisifs vus sous l’angle des coûts dans l’approvisionnement de pochoirs et de caches. L’élément le moins cher du processus ne représente pas toujours la solution la plus économique.
Nel montaggio superficiale (SMT) la pressione della pasta per saldare assume un significato sempre più importante nella catena di tutti i successivi processi della produzione di componenti costruttivi. Se la pressione della pasta non è esatta o è erronea, neanche il migliore processo di saldatura controllato con la massima precisione è in grado di raggiungere risultati migliori. Il primo passo pone dunque i criteri determinanti, infatti qui si tratta di un sistema sicuramente ponderato e consistente di modello, pasta, stampante e naturalmente del circuito stampato. Qui analizziamo le grandezze d’influsso rilevanti sulle considerazioni dei costi nell’approvvigionamento di modelli e crivelli da stampa. Non sempre il componente disponibile al prezzo d’acquisto più stracciato è la soluzione realmente più economica nel processo.
