Rework isn’t rocket science, or is it?

ESUK Jul13 p18_19_20 SMS3Printed circuit boards today are both complex and costly, which means that although rework and repair may be awkward, it is also a necessity

However severely damaged a board may be, the customer might still need it and thanks the to the value of many boards, scrap is not an option. In fact, even less expensive assemblies often require repair, because just-in-time manufacturing and tightly controlled production runs leave little room for shortage.

Today’s PC boards are also more complex than ever before, with fine pitch components, ball grid arrays and fine line circuits making repair a challenge. Whether repairing surface mount pads or damaged internal circuitry, the technical knowledge and manual skills required are demanding. For a reliable outcome, each repair project must follow proven and well established procedures.

Setting the standard

Standards are set by the Institute for Interconnecting and Packaging Electronic Circuits (IPC), in the USA. They include: J-STD-001, which covers the requirements for soldered electrical and electronic assemblies; IPC-A-600, covering acceptability of printed boards; IPC-A-610, covering acceptability of electronic assemblies; and IPC-R-700, which covers modification, rework and repair of printed boards and assemblies.

Products are also divided into three different classes. Class one relates to general electronic products including consumer products, some computer products and computer peripherals and hardware. Dedicated service electronic products fall under class two. This includes communications equipment, sophisticated business machines, and instruments where high performance and extended life is required. Uninterrupted service is desired, but not critical and typically, the end use environment would not cause failures.

Class three products, on the other hand encompass high performance electronic products, including equipment for commercial and military products, where continued performance or performance-on-demand is critical. Equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh and the equipment must function when required.

A conformance level rating indicates how closely the repaired product will be to the original specifications. High level conformance most closely duplicates the physical characteristics of the original and probably complies with all the functional, environmental and serviceability factors. Medium level conformance may exhibit some variance and low conformance may vary significantly from the physical character of the original.

Naturally, class three products must use procedures rated high, unless it can be demonstrated that a lower level procedure will not adversely affect the product’s functional characteristics. Class two and class one products should use procedures rated high for assured safety and dependability, but medium or low level procedures can be used if it has been determined that they are suitable for the specific product’s functional characteristics.

Developing the skills

Looking at the repair operation, it is clear that the skills required are entirely different to those required for PC board manufacturing and assembly. Repair skills are more specific, necessitating increased manual dexterity, patience and a thorough understanding of the repair process. It is therefore a personnel issue and it is vital to have, not only have the proper training program, but also the right people.

Repair personnel cannot be part-timers or work on a rotational basis with other duties. They should be dedicated to the repair operation since for challenging procedures to be done reliably, they must be done repeatedly. Furthermore, some repair skills are so specific that they should be limited to certain individuals who demonstrate an affinity for the job.

Considerable supervision is required during training, with lots of individual help and attention. Regardless of who provides the training, the greatest cost and investment is in personnel.

ESUK Jul13 p18_19_20 SMS4Tools for the job

Repair is a highly labour intensive operation, relying more on individual operator skills than automation. The right equipment, however, will improve the outcome of any repair operation.

PC board repair requires a high degree of concentration and dexterity so a proper workstation that is ESD grounded with proper lighting, outlets and comfort is therefore essential. A high quality stereo microscope is also vital. Precision repair cannot be done without a microscope of this type available 100 per cent of the time, with halogen or fibre optic lighting featuring flexible goose-necks to direct the light.

Accurate soldering is vital, so technicians cannot get by with out-dated soldering tools. They need the latest soldering irons that are highly controlled, ergonomically designed and feature a wide assortment of small tips. They also require component removal tools, designed to handle today’s expanding variety of large and small components. These tools generally use either conductive heating or infra-red heating, depending on the application.

When possible you should preheat the entire PC board before SMT component removal. Preheat minimises thermal shock due to localised heating and speeds up the rework process. Most facilities have a curing/drying oven, but a preheating station for maintaining heat is often necessary. A hotplate-style pre-heater or infra-red heater will maintain the temperature of the board after it is taken out of the oven and can heat the board up from ambient.

Micro drilling and grinding tools may be necessary. Bulky, hand-held drilling and grinding tools are difficult to manipulate for detailed work, so a lightweight, high quality drilling tool is preferable. It may also be necessary to make precise holes, slots and grooves, so accurate depth control and high speed are a must.

Invest in materials

Circuits and surface mount pads can be replaced using liquid epoxy, but this can be messy and unreliable when replacing fine pitch pads. Pads are available with a dry film adhesive on the back. These replacement pads and circuits are heat-bonded to the board surface and are available in any pattern you might need.

Plating gold edge contacts or any metal surface is a serious business. The chemicals used are hazardous and must be handled properly and the power applied to the plating surfaces must be controlled accurately to expect reliable results. For many operations you will also need high strength, high temperature epoxies. A two-part epoxy offers the high strength, thermal resistance and durability that one-part and quick-setting epoxies do not have. It is also important to have masks or colouring agents, so that you can restore the cosmetic appearance of the board. It is best to cure epoxies in an oven if possible.

In-house or outsource?

It is a fact that far more printed circuit assemblies are damaged during the manufacturing process than they are in the field. Even though PC boards are more complex today than ever before, they are still repairable, so the primary question is whether to develop and maintain a full repair department in-house, or to contract the repair out.

Any contract manufacturer or OEMs keen to establish a good in-house repair operation should bear in mind five key requirements for reliability. These are: to implement documented standards, documented procedures, comprehensive training, up-to-date equipment and highly skilled technicians.

You must be prepared to make a real commitment in several key areas if you plan to complete repair work in-house, since repair encompasses much more than simply removing and re-attaching components. In reality, more damage can be done to a board from a botched repair than from most other causes, so unless these factors can be met, it may be better to contract the work out to a reputable repair facility.