Peter Marshall, KSM International
The process of solder paste deposition has changed little over the years. A stencil is placed in contact with a substrate and a squeegee or enclosed wiper blade passes over the stencil, depositing the paste through the apertures to make the print. The reduction in component size and advances in board design have lead to printing techniques progressively becoming more accurate and sophisticated. Vision systems used on in-line solutions are primarily used to accurately align the stencil to the board. After the printing procedure, the systems can then be used to examine selected aperture areas to check that a reliable image has been deposited. The print medium solder paste has also undergone continual product development using more precise metal particles, and advanced flux formulation to increase and improve stability and working life. In an effort to further improve print quality, some SMT lines now incorporate high-speed 3D pre-reflow inspection systems to check the printed deposit for size and volume. If defects are detected, it is easier to clean the board and start again at this stage, than rework expensive components from an assembled board with the joint already made. However, despite these extra measures, these lines still end up with random defects associated with the printing process.
In a bid to reduce the impact of these faults, smart engineers are now looking beyond the printer to explore where the random defects come from. Closer examination into the process has revealed that one critical area identified is the board or substrate, carrying foreign remains into the printing environment.
The first case is a QFP: the print definition doesn’t look too bad and unless a vision system has targeted that corner, the defect would not be picked up. In this case, a curly hair has entered the printer on the board sitting in the middle of the QFP. As the board comes into contact with the stencil the hair is squashed over the pads, a perfect print stroke is made and on separation the hair springs back taking with it particles of solder paste. Solder can now be seen attached to the hair; and following placement this solder will comfortably sit under the QFP, eventually forming a solder ball at the reflow stage. Such a solder ball is then free to roll around, eventually generating a short within the circuitry.
Further examination of the image shows a volume of paste missing from the pads. A small amount is still attached to the hair, but where is the rest? It is highly likely that it is now attached to the underside of the stencil about to contaminate the next board entering the printer. The pad structures from where the paste has been taken are also potential areas for a failure as although a joint may be made, the volume of solder is now below the required quantity.
The second example shows the printed pads of a BGA. In this case, the culprit is a small fiber of clothing. As the board enters into the printer, the fiber is sitting across the upper pads of the BGA, and while the board comes into contact with the stencil, the fiber is squashed and moved downwards, resulting in it covering the lower pad. Again the print process is executed perfectly, but as separation occurs the fiber returns to its natural form, dragging all the paste from the lower pad structure into the paste of the upper pad. The resulting print has one pad with insufficient paste to make a joint while the other has double the original design volume. As mentioned previously, unless programmed to look at this exact area, the vision system would not pick up the error. Undetected, the fault would result in a skilled operator requiring dedicated rework equipment to recover the suspected “faulty component” at the end of the line. The true fault almost certainly would not be identified.
These two examples illustrate very clearly how contamination can effect the overall quality of board assembly. Hairs and clothing fibers are just two of the random contaminants that sit around all SMT lines. These lines are generally not located in a cleanroom area, and the nature of glass epoxy PCBs (static charge) at this stage in the process lead them to attract loose surface contaminants. An average person loses between 50 to 100 hairs per day. Other causes of contamination include epoxy dust from sawed edges, loose fibers from glass fiber boards or boards individually wrapped in tissue paper. Whatever the cause, the impact will always affect the productivity of any line.
KSM has developed a range of low-cost, just-in-time solutions capable of removing this source of random defect from around the printer platform. Occupying less than 150mm of line space, the system utilizes two-step contact-cleaning technology that perfectly prepares the substrate, removing all loose surface contaminants down to 1 micron.
The first step employs twin, double-sided soft elastomer rollers that conform to the contours of the board and remove contamination from its surface. The elastomer rollers in turn contact accompanying adhesive rolls that collect and store the offending particles. Some printer-integrated systems employ compressed air jets directed under the stencil to clear debris or contamination. However, over time this has proved unreliable as the contamination remains within the printing environment. The adhesive rollers utilized in the KSM equipment are a result of many years in contamination removal sectors. They are formulated to allow only one-way transfer of particles from the elastomer to the adhesive, thus completely and safely removing them from the print environment.
The final stage of contamination removal utilizes a separate anti-static system designed to eliminate any risk of recontamination. The equipment is designed to ensure that operator intervention is minimal, resulting in no extra down time, compatibility with any printing platform, and no bulky peripherals.
Circa zwei Drittel aller fertigungstypischen Defekte in der Baugruppenfertigung lassen sich auf den Pastendruck und Lötprozess zurückführen. Soweit die Statistiken. Doch oft stellt sich bei näherem Hinsehen heraus, dass kleine Fasern oder Partikel, die sich auf der Leiterplatte vor dem Druckvorgang befanden, die Ursache für schlechte Ausbeuten sind. Ein kurzer Reinigungsvorgang vor dem Print liefert eine kostengünstige Lösung.
Deux tiers environ des défauts typiques de fabrication des sous-groupes sont dus à l’impression de la pâte et au brasage. C’est ce que disent les statistiques. Mais un examen détaillé révèle fréquemment la présence sur la carte imprimée, avant l’impression, de petites fibres ou particules responsables de ces défauts. Une solution économique consiste à effectuer un rapide nettoyage avant l’impression.
Circa due terzi di tutti i difetti di produzione tipicamente risultanti nella realizzazione di gruppi costruttivi sono attribuibili alla pressione della pasta e al processo di saldatura. Ciò in quanto alle statistiche. Ma molto spesso, in particolare quando si da un’occhiata ravvicinata, si scopre che la causa principale di uno sfruttamento poco proficuo sono piccole fibre o particelle, che si trovavano sul circuito stampato prima del procedimento di saldatura. Un breve procedimento di lavaggio prima della stampa offre una soluzione economica e conveniente.
