Issue link: https://iconnect007.uberflip.com/i/869081
September 2017 • SMT Magazine 67 Neighboring Device Damage As the density of components on PCBs has increased, the need to protect neigh- boring temperature sensitive components near the BGA such as ceramic capacitors, crystals and plastic-bodied components such as connectors has increased. These devices must be protected from exposure to heat during the rework process. If they are not, they can become damaged show - ing either visible or 'hidden' damage. In addition, these components' long-term reliability may be impacted by this heat exposure, even if their ap- pearance does not show it. While these compo- nents may be able to withstand the peak tempera- ture of 260°C (as defined in J-STD-002), there may be medium and long-term impacts to its reliabil- ity if the IMC layer grows too thick. Neighboring underfilled parts may have the material 'ooze out' as the softening point temperature is below the liquidus temperature of lead-free solder. There are a variety of shielding options in and around the BGA rework area. The in- creased liquidus temperature of lead-free solder has driven processing temperatures into areas where sensitive components have significant body temperature and time limitations (IPC J- STD-075 is the standard for maximum time/ temperature exposure for all non-semiconduc- tor devices). The intermetallic layer thickness, which to some degree represents the mechani- cal strength of the solder joint, can become too large with extended exposure to times above liquidus. If it becomes too large it can make the solder joint brittle. This in turn can impact the reliability of the solder joint. To prevent these negative impacts on neigh- boring devices in the BGA rework area, prop- er thermal shielding is needed. A recent study (Wettermann, 2015) indicated that the shield- ing effectiveness of 'historical' materials, such as Kapton™ tape and stainless steel, are not as effective as more modern materials such as shielding clay gel (Figure 5) and ceramic non- woven ceramic materials. To fully protect a de- vice from thermal damage, the study pointed out that the clay gel material is over two times as effective as Kapton tape as a thermal shield at a close distance, and three to four times as ef- fective at further distances. The ceramic non- woven fiber material is nearly as effective as the gel in terms of its heat withstand properties, but does not need to be cleaned off after use. Trending devices, such as smartphones and tablets, use underfill so that the BGA pack- age can withstand drop testing requirements without damaging the solder joints. The chal- lenge for a rework technician is the pliable na- ture of the underfill. This typically means the tack properties, even when above the softening point, make for a mess underneath the BGA. Even if the underfilled BGA can be pried off the PCB, the mechanical force exerted can poten- tially damage the device or board. Underfill Rework The softening point of the underfill is less than that of the reflow temperature Figure 5: Clay gel protects and shields devices from thermal damage. Figure 4: PCB support on BGA rework system. TOP 5 BGA REWORK CHALLENGES TO OVERCOME