Rework is essential in the electronics manufacturing industry as an effort to cost saving and reducing wastage. Moreover, PCBA reworking is vital in case the subject component is not available in the market, which happens many times in recent markets of low supply. If the extent of damage on PCB is low then reworking is an efficient solution. However, it comes with few challenges.
Reworking on BGA (Ball Grid Array) is known as area array rework. There are a variety of high temperature prone BGA components that might get damaged while reworking on PCB. In some cases, reworking can also soften up or reflow components in proximity. Some components might restrict the peak reflow temperatures as well as the time above liquidus. The long-term reliability of these components impacted by such exposure to heat, even though they don’t show any sign of immediate damage. On top of that, high temperature requirements of lead-free soldering and sensitive nature of BGA components make it difficult to design rework procedures.
While reworking on PCB, the subject components are often desoldered with the application of a hot gun. During such a process, heat is not only transferred to components of interest but also to adjacent components. The greater the surface area of copper in PCB, The greater the heat transfer. Since it is an essential property of the material to conduct heat, it is not possible to eliminate it completely. However, there are some ways to restrict it to some extent.
Some ways around that are to introduce tighter thermal profiles and extreme precision during reworking PCBs. In the following article, we will be looking into using a heat shield on adjacent components while reworking PCBAs. Basically, a heat shield helps to reduce temperature difference of top and bottom side of the PCBs while practicing rework with a hot air gun., thereby minimizing exposure of heat transfer to neighboring components or solder joints. During rework, because of the reaction between copper pads and Tin of the solder paste, an intermetallic compound (IMC) forms. IMC has a negative impact on solder joint’s characteristics such as its reliability, quality, and strength.
Heat shields can be divided by various types of material and shapes. Each has diverse characteristics and properties.
Material of The Heat Sink
The property of material that will prevent heat from spreading to the other side is known as radiant heat. Major source of heat is infrared waves that travel at the speed of light. This infrared wave gets transmitted, absorbed, or reflected when they come across any material. Other than these, heat can be transferred to PCB through convection. Rate of heat transfer depends upon the thermal conductivity of the material. Any material with lower thermal conductivity will be very resistant to the heat transfer process. Color of surface also contributes towards thermal conductivity of material; White surface is known to be less conductive of heat whereas black surface is known to be more conductive.
In the given table, some of the standard materials with their thermal conductivity in Watt/m K are given at 330 °C.
|Material||Stainless Steel 304||Copper||Kapton tape||Heat Shield Gel||Ceramic|
|Thermal conductivity in Watt/m K||18||379||0.37||1.5||0.14|
Heat shields made of stainless steel were used due to its reflectivity, emissivity, thermal conductivity, and specific heat capacity. Moreover, steel is also durable, mouldable, and relatively cheaper. Stainless steel heat shields are designed to shield a component from absorbing excessive heat either by dissipating, reflecting or simply absorbing the heat. The high reflectivity and low emissivity of the Stainless surface ensure that it both absorbs and re-emits little infrared radiation. The relatively low conductivity of stainless steel 304 makes it easily able to resist changes in temperature.
Copper tape as copper shield is flexible and can be easily mounted on PCB. Although it is relatively inexpensive, copper tape is not an ideal option to be used as a heat shield; If used for too long, the copper heat shield may lose its properties.
Because of the ability of Kapton to maintain excellent physical, electrical, and mechanical properties over a wide temperature range, it is a popular approach for masking areas on a PCB. Kapton tap can be flexibly applied by cutting it to desired size and shape. Since it can be configured easily in and around parts, it has become a favorable option among PCB rework and repair technicians. Despite these advantages, poor thermal insulation is a major disadvantage of Kapton tape.
Heat Shield Gel
Heat shield gel is most used during welding in the automotive industry because of its high heat resistivity. The gel contains mainly deionized water and clay and is classified as non-toxic material. After heat is applied, the water in the gel begins to evaporate until a layer of clay is formed. This clay is not as effective of a shield as the wet product and should be reapplied once the water burns off.
Ceramic non-woven shields. These specialty ceramic fiber non-woven shields, previously relegated to aerospace, nuclear energy and high temperature processing industrial environments can now offer users these same characteristics offer users low thermal conductivity and low biopersistence, meaning that if the fibers are inhaled, they’re eliminated from the body within days. This material can be used at 1,100°C continuously with excursions to 1650°C. This ceramic fiber material offers several advantages including the properties of high-temperature stability, low thermal conductivity, high heat reflectance and the ability to be easily wrapped, cut to shape. Since these materials are compressible, they can be easily fit in between components making it an ideal material for shielding. Combined with metal shields for adding structure this material is highly effective at shielding nearby components from higher temperature rework/reflow.
Shape of The Heat Shield
Various kinds of heat shield can be designed based on a diverse shape of heat shield; U-shaped heat shield, square-shaped heat shield, individual heat shield for adjacent components.
There are several ways to examine the effectiveness of the heat shield placement location for rework and how impactful it is, to save adjacent solder joints and components from getting mechanically and thermally damaged. There are several methods to analyze these.
- Examining thermal distribution by Infrared thermography camera.
- Temperature measurement using thermocouple wire (TC wires)
- Dye and pull test
Result of all these tests is analyzed together to develop and understand the relation between various types of heat shield and change in damage to the neighboring components and their solder joints during rework.
As per these tests, Heat shield gel is known to be the best performing heat shield material. More testing needs to be done as the residue left from the heat shield gel and subsequent cleaning affects performance. The closest performing material to heat shield gel was the ceramic fiber pads. These have been used in the electronics industry for years and are known to be safe. The next closest performing materials in order were the copper tape, sheet metal, and kapton tape. The copper tape performed well and was able to adhere easily to surfaces. The sheet metal lacked the ability to completely seal a board area as air could easily travel underneath the metal. Kapton tape simply lacked thickness and thermal resistance and was found to be the least effective material.