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One fundamental realization that often escapes people is that as things become smaller, the things inside them also become smaller and closer together. This last fact — the closer together part — is what can be challenging for electronics assemblers. It significantly impacts placement equipment, solder technology and reflow profiles, and can cause a breakdown between the cleaning materials and the equipment that apply them.
As the devices and the pads that they connect get smaller, so too does the amount of solder paste on any one pad. Yet, it still must do its job of removing oxides and reflowing to create a successful, reliable intermetallic bond. That implies that the flux, which removes the oxides, must be a bit stronger, and in larger quantity relative to the paste volume or both to get the job done on today's lead-free metals.
More aggressive flux on the surface means that there is probably more aggressive flux to be removed from the surface after reflow, to ensure a long, effective lifetime for an electronics assembly. Of course, many electronics assemblies have relatively short anticipated life times and, therefore, these aggressive residues can remain behind, often encapsulated, without ill effect. However, more and more manufacturers and their customers are viewing the residues as problematic, therefore requiring removal, particularly prior to conformal coating.
It is somewhat obvious to conclude that leaving more aggressive flux residues on a surface may have adverse effects and that removing larger quantities of aggressive flux residues may be more challenging. However, it is not intuitively obvious that removing residues in and around smaller devices presents a greater challenge as well.
Let's look at that challenge in a bit more detail, and start with bottom termination components. These packages have bucked part of the trend toward miniaturization by actually growing in the X-Y axis, providing more I/O and more capability. In the process, the relative size of each I/O (or each solder bump) has continued to shrink at a rapid rate. These smaller solder bumps are smaller not only in the X/Y but also in height.
|Clean micro BGA. |
It is worth remembering, however, that cleaning is defined as removing an unwanted contaminant from a desired substrate, leaving it residue-free and unharmed by the cleaning process. That last part often is unspoken but a type of oath for cleaning technology. Think back to the ball grid array (BGA) that, when introduced, had a 4 mil gap between the lower side and the assembly. This was considered difficult for some technology, but now we often see gap heights of 2 mils and less. This has dramatically affected cleaning material design and selection. Add in a ground that is centered in the package to further impede the flow of cleaning and rinsing materials, and you have a major, but not insurmountable, challenge.
Likewise, the very dense designs commonly used today also impede fluid flow. There is one other, more significant effect that originated with dense designs: how does one reach peak reflow temperature everywhere on the assembly? This drives longer, occasionally hotter soak profiles that, unsurprisingly, make removing those residues even more difficult.
Thus we are left with harder-to-remove residues in places that are more difficult to reach. Fortunately, cleaning technology has advanced rapidly over the past 20 years. Leading material suppliers, much like their solder technology material brethren, have conducted tens of thousands of experiments to understand and refine the technology, making significant discoveries and more than a few breakthroughs along the way.
|Kyzen 1210 chip cap with 3 mil gap. |
Of particular note are the award-winning products that fall into the mild alkaline category of cleaning agents. As Aristotle taught — and the Romans adopted as Via Media — the Middle Way — by avoiding extremes, success most often is found in the gentle balance of moderation. Moderation in the land of cleaning means we are well-advised to have products that are aggressive enough to work; baby shampoo has its place although not as a flux remover. Likewise, choosing products that are not too aggressive is desirable since aggressive product can damage the assemblies that are being cleaned.
This same Via Media concept applies to designing the mechanical elements of the process. Critical mechanical process variables are time, temperature and mechanical energy. If we choose parameters that are too low, we will leave soil behind, as certainly as if we chose parameters that are too high, we may damage electronic assemblies.
How do you go about finding your own Via Media? First, start with the new edition of the IPC Cleaning Handbook. It was recently revised from beginning to end by a large committee of cleaning experts and is a wonderful resource that is only a mouse click and a few dollars away. Next, be sure to thoroughly evaluate your assembly process and consider how that will drive your cleaning process. Are you high volume and need a conveyorized inline machine? Or are you more of a specialty, work cell-oriented shop and a small footprint batch process is a better match? What soldering technology have you selected and why? Lead-free technology means higher reflow temperatures as well as soak profiles that often leave more difficult-to-remove residues that will drive your process design. In particular, lead-free organic acid (OA) materials often require the use of a cleaning material beyond water for complete soil removal. This will impact your machine selection criteria.
By now you are thinking, I can barely keep up with production — how can I possibly design and execute a proper DOE to understand my requirements in the proper detail? That is where your suppliers come in. World-class cleaning material suppliers have applications laboratories just for this purpose: use them. Go to those labs and learn more about cleaning as well as how your designs are driving your cleaning requirements. These visits often provide enormous benefits down the road where you can apply the lessons learned to your next big contract, resulting in an efficient, economical and easy-to-run process that meets your needs as well as those of your customers. If you do so, you will have found your own Via Media.
Contact: Kyzen Corporation, 430 Harding Industrial Drive, Nashville, TN 37211 615-831-0888 fax: 615-831-0889 E-mail: firstname.lastname@example.org Web: http://www.kyzen.com