The Role of Pre-Heating in Electronics Rework

This article is based on an original publication by PACE Worldwide.

Like any other industry, the field of electronics production is constantly evolving. Early on, rework and repair jobs were relatively simple, with single- or double-sided boards and thru-hole components. Essential equipment such as soldering irons easily had the power and heat capacity to accomplish the necessary tasks.

As the industry progressed, consumer demand began to increase for enhanced functionality with the introduction of multi-layered assemblies and larger, higher-density components. The same soldering irons and extractors that had performed so well in the past now struggled to meet the power requirements of everyday tasks. Assembly pre-heating became necessary for many applications and the basic hot plate evolved into a more sophisticated, temperature-controlled device.

Alongside combined conductive/convective heat systems, convective-only preheaters began to emerge. However, just as these were becoming widely used, surface mounted devices were introduced to the market, offering increased capability housed in a smaller design. This evolvement led to a smaller, lighter PCB assembly with less thermal mass - meaning that soldering irons and extractors could once again handle everyday tasks without use of a preheater.

Tiny and Complex

In recent years, the electronics industry has undergone incredible technological advancements. Tablets outperform what once required roomfuls of equipment, and smart phones act as handheld personal computers. Now highly complex and incredibly dense, current PCBs containing these technologies are as compact as an iPod.

Due to modern PCB complexity, value, sensitivity and size, standard repair and rework tasks must be performed as closely to original manufacturing specs as possible. This often involves pre-heating to ensure pre-accepted temperature settings are maintained.

With the shift to Pb-free solders, operators tend to increase set handpiece temperatures in an effort to compensate for increased solder melt temperatures. When coupled with a thermally dense PCB, the potential for damage is particularly elevated. Avoid the risk by pre-heating during the rework process to facilitate successful reflow at a safe, low-set temperature.

Proper Pre-Heating

Proper pre-heating promotes a successful overall process by ensuring temperatures are uniform across the board and components. Pre-heating maintains PCB stability during reflow while allowing for successful completion at lower temperatures to ensure safety of the PCB.

Regardless of the pre-heating method used, the resulting temperature of the PCB must be measured across multiple assembly locations to ensure homogenous temperatures throughout the board. Heat sink factors including ground planes, connectors and shielding can pull heat to create cooler areas which could lead to warping, twisting, and reflow problems.

To create a heat profile, be sure to measure topside temperatures at locations both close to and away from heat sinks as well as the working area. Once the temperature variation of all locations is equivalent to less than 3-5°C, the board is thoroughly, evenly warmed.

Options for underside preheating and assembly include conductive, convective and radiant methods.

  1. Conductive methods generally require a close proximity to the PCB and are not as easily controlled, often resulting in "hot spots" between contact points.
  2. Convective methods are more effective although air is a poor medium for heat transfer, leading to reduced effeciency.
  3. Radiant methods are both efficient and effective for use on large boards. Easily controlled with an average current draw, this heating source provides a reliable medium for heat transfer.

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