12 Flexible Printed Circuit Boards
13.11 Adhesive for Mixed Technology Assembly
In many of today’s circuit board designs, some surface mount components are attached to the bottom side of the board. Through-hole components as well as large surface mount components are attached to the top side of the board. When surface mount and through hole components are combined, the board forms a mixed technology assembly.
With mixed technology assemblies, bottom side surface mount components require adhesive application so that they do not fall off during the subsequent component placement and wave soldering operations.
13.11.1 Requirements of Adhesive
The adhesive should be selected to meet the following basic requirements:
a Hold the component in the given place during the cure process a Maintain that orientation through the wave soldering operations a Must have adequate adhesion to different surfaces
a Must not be affected by exposure to the environment of solder flux and wave soldering.
a It must be chemically inert throughout the life of assembly.
Epoxies and Acrylics are commonly used as adhesives for SMT assembly. Adhesives must be stored in a cool, dry and dark location.
Epoxies provide good insulation resistance, high bond strength and low curing temperature. A disadvantage of epoxies is that defective components are difficult to remove during repair. They are typically single-component heat-curing systems and are available in a range of formulations for special requirements.
Acrylic adhesives have fast curing time, high peak profile of dispensed dots and good temperature stability. They are usually cured by applying UV and IR energy. The main disadvantage of these adhesives are lower shear strength and the relatively high temperature required for heat curing.
Acrylic epoxy adhesives represent a considerable improvement on simple epoxy resin or acrylic based adhesives. Acrylated epoxies give a combination of the favourable properties of each type.
Their viscosity/temperature stability is excellent enabling them to be stored at room temperature for up to one year. The bond strength is good without being so strong as to make rework difficult.
Acrylated epoxy based adhesives can be rapidly cured by heat or UV, or a combination of both.
Harris (1991) describes the various adhesive types, paying particular attention to acrylated epoxy based products.
13.11.2 Application of Adhesive
Adhesive can be applied by many different techniques. Most common methods are the syringe dispensing and the stencil printing method.
13.11.2.1 Syringe Dispensing
Adhesive is often dispensed by nozzles on the pick-and-place machine immediately prior to placing the component. For each component being glued, an appropriate adhesive dot is dispensed according to the space available and the size of the component. There are many different types of mechanisms used to force the adhesive through the nozzle and onto the PCB, but mainly all are air-driven. Each system has advantages and disadvantages. For example, one system may be easier to clean and another type may have better repeatability of dot size. Today’s ultra high-speed dispensers have a dispensing rate of 100 000 dots per hour. The main advantage of syringe dispensing is the flexibility of operation.
Soldering, Assembly and Re-working Techniques 503
13.11.2.2 Stencil Printing
In this method, a squeegee pushes the adhesive down into the apertures and ensures full aperture fill and contact to the board by stroking across the stencil. Proper alignment is a must to ensure the correct locations for the application of adhesive on the circuit board. The important parameters for stencil printing are homogeneity of speed, squeegee speed, pressure, snap-off distance and speed of board separation. The board must be kept flat during printing. The following points may be noted:
a Squeegee speed strongly depends on the viscosity. The lower the viscosity the higher the speed. For example, an adhesive with a low viscosity, “runs” at about 20 cm per second and a high viscosity adhesive is printed at about 1.3 cm per second.
a Pressure: Adhesives with higher viscosity need higher pressure than one with lower viscosity.
A rule of thumb is to have sufficient pressure so that the stencil is wiped clean of adhesive with each printing stroke.
a The adhesive used in printing should be thixotropic, that means its viscosity should drop during the application process. This ensures that it will flow onto the board properly.
Stencils for adhesive printing are usually stainless steel. However, plastic stencils are becoming more common now. The advantage of plastic stencils is their flexibility that reduces the need for periodic cleaning of the stencil like stencils of stainless steel. However, plastic stencils are not as durable as the one of stainless steel.
Metal stencils tend to have a longer useful life than mesh screens, which can lose their resilience and shape after much use, compromising print accuracy. Stencils have the potential for up to 50,000 prints compared with a life of 5000 for the mesh versions. Stencils are virtually the only way to produce so-called fine pitch prints (defined as any thing below 20 mil) and have been proven in applications down to 12 mil.
The adhesive for stencil printing must be designed for exposure at room temperature and ambient humidity. Good adhesive printing results are seen with a hard polyurethane squeegee or with one out of metal.
While stencil printers can be made to deposit adhesives on PCBs, the quality of the depositions is often inconsistent. This is because the reheology of solder paste for stencil printing is totally different from that of SMD epoxy. Solder has a slippery surface because of the properties of its lead and flux.
Epoxy adhesive, on the other hand, is sticky and stringy.
The positional accuracy in a dispensing system is data-driven. Each point of deposit is measured from the datum position in the CAD system. Any rotational offset or expansion/contraction of the PCB pattern is compensated, point by point, using fiducial correction. Duck (1996) points out that stencil printing is not based on positional correction but, instead, on a “best-fit” algorithm. A stencil cannot be changed to meet the dimensional variations of the PCB material caused by fluctuations in board fabrication. To overcome this problem, dispensing systems compensate for fluctuation in board thickness and board warp by utilizing support pins and vacuum supports.
The rotary pump (also known as auger screw pump) is not accurate enough for precision dispensing on ever shrinking package designs. A linear pump, shown in Figure 13.23(c), which is a true positive displacement piston pump overcomes these limitations. The pump is not affected by fluid viscosity, simply pressure, needle size or fluid/pump temperature. The pump’s servo drive mechanism allows programmable shot sizes and flow rates.
The metal step after a job is completed is to remove and properly dispose off all unconsumed materials. Next clean and remove adhesive from the squeegee, check the squeegee blades for nicks. Then clean immediately the stencil. The adhesive will eventually cure at room temperature, making it more difficult to remove later.