FEATURE CONTRACT MANUFACTURING Bonding and Sealing
Adhesives are used on PCBs for a variety of purposes but no one solution fits all. Eamonn Redmond, sales manager of Inseto, summarises which chemistries and curing processes should be employed, where and why?
B
efore discussing adhesive types, it is worth reminding readers that a PCB
substrate is itself an adhesive; essentially cured epoxy resin layers sandwiching copper tracks and fill planes. Not all substrate chemistries are the same though and surface finishes can vary considerably. Both factors will affect how well an adhesive adheres. The same material and surface finish considerations should then be applied to the components you wish to seal and/or bond to the substrate. The end application will, of course, have
a direct bearing on adhesive selection too. Is the objective to seal and prevent moisture ingress? Is it to make components more resilient to shock and vibration? Does the board need to flex at all?
As for production, it is important to appreciate the temperatures to which the adhesive might be exposed, and it is crucial to factor in curing times. So, let’s look at where and why adhesives are typically employed.
COMPONENT FIXING PRE-REFLOW It is often necessary to tack small SMDs, such as resistors and capacitors, into place to prevent their movement during reflow, as even a minute drift away from the solder paste pads will compromise quality. The best type of adhesive to use here is a heat-cured, one-part epoxy. These offer high thermal resistance, as they need to withstand three reflow passes (the industry norm) at circa 220o
C.
Placement of the adhesive is typically by automated dispenser before the board goes into the pick-and-place machine. A consideration here though is the time between the two activities. If it is likely to be a long time, you may need to assess the epoxy’s sensitivity to moisture. Regarding storage, the latest one-part
epoxies to join the market require a temperature of less than 10o can be as low as -40o vary from 60 to 150o
typically be less than an hour. C and some
C. Curing temperatures C and curing time will
coils have a relatively higher mass than most electronic components, and bonding can protect against the effects of shock and vibration. A two-part epoxy would be best to use, applied manually before soldering. There are no major curing issues, as the soldered joints will hold the coil in place while the adhesive cures (which is typically viscous and won’t run) at room temperature. Storage pre-use is typically at room temperature too.
Figure 1:
Choke coils are bonded into place to reinforce the electrical connection (solder joints) and protect against the effects of shock and vibration
SOCKET SECURING AND PIN COATING/SEALING This is done mainly to prevent moisture
ingress and to reduce the risk of dendritic growth. There are two options available for securing/bonding pins/sockets. The first is to use a light-cured epoxy. These are popular as they cure very quickly in the presence of a high intensity light- source, making them ideal for high volume, rapid-turnaround production runs.
Figure 2:
Two-part epoxies are ideal for casting because of the high volumes often required and room temperature curing. (Photo courtesy of DELO)
The second option is to use a dual-curing (but again one-part) epoxy. High intensity light can be used for a rapid, initial cure, so that the board can be handled. This is then followed by heat curing - some of which can be time spent in the solder reflow oven – and will cure any epoxy ‘shadow zones’ (i.e. areas the light could not penetrate).
COIL FIXING The objective here is primarily to provide additional strength (See figure 1). Choke
FERRITE BONDING Here, the bond might be ferrite-to-PCB or ferrite-to-ferrite (e.g. the two halves of a transformer integral to the board). Because at least one surface is metal, the best adhesive to use would be a one-part anaerobic, a common use for which is thread locking. Curing is at room temperature in the absence of oxygen, so any visible adhesive (i.e. at the edge of the bond) will remain viscous/liquid and should be removed. However, recent anaerobics to join the market can also be UV cured, meaning the excess adhesive can be treated.
CASTING This is typically for circuitry intended for use in harsh environments; to protect the PCB from moisture and contamination (See figure 2). The adhesive-of-choice would be a two-part epoxy, as they are relatively low cost and a significant volume might be required for some applications. Pre-use storage and curing (which takes about 24 hours) is at room temperature In summary, there is no single adhesive
chemistry that can be used for the applications discussed above, no more than there is really an ‘all-purpose’ glue for DIY jobs around home. For further advice, it is recommended that you visit Inseto’s Knowledge Base:
https://www.inseto.co.uk/knowledge.php where many of the hidden costs associated with adhesives are also covered.
Inseto
www.inseto.co.uk T: 01264 334505
14 OCTOBER 2018 | ELECTRONICS / ELECTRONICS
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