PCB Rework – Evolving Cleaning Methods

Smaller devices makes PCB rework harder.

Today’s manufacturers of printed circuit boards (PCBs) strive for quality and drive operation yields close to 99% and above – yet some boards will still fail functional tests when coming off the assembly line and need rework. With the increase of smaller, better PCBs for smartphones and tablets, PCBs can cost upwards of $200 each. Original equipment manufacturers (OEMs) must develop effective and efficient rework processes and save the damaged boards when possible to minimize scrapping PCBs and operation losses.

The value of reworking faulty boards is quite clear. With the development of more sophisticated boards and the continuing demand for smaller and smaller devices, performing manual rework has become increasingly difficult.

Previously thought of as a very specialized process used by few OEMs, contactless cleaning is quickly becoming mainstream and mandatory for most PCB manufacturers to stay competitive and meet customer demand.

When performing PCB rework, technicians generally follow three steps –

  1. Remove the non-working component
  2. Clean the leftover solder from the ball grid assembly pads
  3. Replace the non-working component with one that works

Today, both the first and third steps have been automated and can be completed on rework machines. The second step, however, continues to remain in the hands of a skilled technician because of the intricate, critical nature of properly repairing a damaged PCB. Boards can be cleaned using different kinds of tools—from a wicking braid to a soldering iron—along with specialized tips for detailed work. The skill level of the technician can determine whether a board comes through the process undamaged and how often.

One common way a board can be damaged is during manual cleaning. If the soldering iron dissipates for a moment from the pad, it can cause the pad to stick to the wick. As a result, the pad can chip or lift totally off when the technician pulls the tool away. If the pad is destroyed, the PCB becomes scrap.

To help ward of this king of damage, manufacturers are creating thicker PCBs with multiple inner layers of copper.

Open vias and solder resist damage

Unfortunately, the newer PCBs can cause heat to dissipate faster when using a handheld cleaning tool —making sticking challenges even more of a problem.

Additional issues like molten solder flowing into electrical connections or into vias can also occur, causing shorts in the board. Also, if solder is removed inconsistently from the pads, the components may not adhere well and portions of the solder resist may be accidently removed by the wick. The solder can then flow into electrical connections and cause bridges and shorts when it’s put back into the rework machine.

One of the worst case scenarios, in addition to a PCB becoming scrap, is the creation of pad craters within the fiberglass substrate. These cannot be seen during inspection or by X-ray. Pad craters can happen when an operator presses too hard with their soldering iron or rests an excessively hot solder tip for too long on a board.

In this kind of damage, the pad and the solder balls remain connected to one another; however, the pad is not fully anchored to the circuit board, leaving it vulnerable to just the smallest jolt. If the product goes to market and this happens, the result may cause customer dissatisfaction.

Tough cleaning woes

Two different types of solder are used in manufacturing, with melting points of 183° C and 302° C (361° F and 575° F) respectively. Manual cleaning can change the melting temperature of solder on pads and become too low to form a connection, causing a dry joint. Also, package-on-package (PoP) chips cannot be manually cleaned with assurance or without the threat of the package bottom melting.

Advanced chip technology also causes PCB cleaning challenges. Even though one company’s breakthrough technology makes it almost impossible for criminals to access data, it also makes components difficult to clean due to differing pad sizes and uneven solder volumes.

Ceramic ball grid arrays present another huge challenge. These highly specialized boards, along with the components used in aerospace, military and other high-reliability applications, have become extremely hard to successfully, manually rework, and usually have to be trashed if faulty.

Alternatives – Contactless cleaning

Contactless cleaning or scavenging can significantly reduce the challenges associated with manual cleaning, and can be a substitute method in many applications. If no contact is made with the pad or board, possible mechanical damage is greatly reduced. Instead, precision-controlled tips clean pads too small and too close together for technicians, while software and equipment continually control thermal profiles.

All-in-one designs

Contactless cleaning has generally been used on very large, high-end machines or as an optional, retrofitted add-on that allows for the cleaning function to become part of the removal and replacement steps on a single rework machine. Stand-alone contactless cleaning machines require a much lower capital investment, plus other added benefits like great throughput.

The standalone machines take only a single operator to run and can increase the speed of an assembly line operation if used next to a standard rework machine. They offer excellent flexible opportunities for manufacturers to easily add contactless capability to their existing operations. If they presently have a standard rework machine and no scavenger add-on, a contactless cleaning machine can be added quickly to work side-by-side with their current system. This is a useful route if suddenly the OEM needs a quick way to add capability because of increased customer demand.

As chip sizes shrink, and production struggles to keep up with an ever-expanding demand for smaller and more powerful electronics, rework processes must continue to develop.

From a Metcal article –

From the original article – The Evolution of Cleaning Methods,in PCB Rework by Robert Roush and Paul Wood, Metcal

Posted on by Barb N. in Printed circuit board rework, Technical Articles