PCB Tech - BGA for lead-free soldering of circuit boards

BGA for lead-free soldering of circuit boards

1. BGA for lead-free soldering
(1), the lead-free BGA specification remains the same

As explained in the previous article, the basic design of lead-free soldering BGA can still follow the current leaded method, and it is best to use NSMD for PCB soldering pads to reduce the accumulation of stress. However, due to the higher soldering temperature of the tin-silver-copper SAC, it is necessary to carefully consider the better placement of the heat-resistant BGA on the PCB. Generally, in the welding of various types of large plates, the edge of the plate is about 5-15 degree Celsius higher than that of the plate. Generally, large BGA not only absorbs water easily, but also suffers great thermal stress impact at high temperature. It is best not to place these large parts on the side of the board. Once the large BGA has to be placed on the side of the board due to wiring difficulties, the welding conditions must be more strictly controlled to avoid damage.

(Two), lead-free BGA must pay more attention to operation and inspection

In order to make it easier to increase the temperature more smoothly, the heating zone of the Reflow unit must be more than those with lead. If possible, it is better to switch to hot nitrogen for fusion welding, for OSP, the yield will be better. In order to reduce the damage of BGA, when conducting trial welding of the first product (First Artic1e), the temperature sensing wire (Thermocouple) of the Profi1er full-scale heat detector must be installed at or near the bottom of the body of the BGA., In order to know whether the heating of the large part has exceeded the temperature limit, usually the temperature of the body is 5 degree Celsius higher than that of the ball.

In order to better understand the appearance and solder joints of the lead-free ball pins, a special side-view microscope can be used to inspect the surface features of the outer SAC ball pins.

(3) Responses during the transition period

As for the transition period from BGA to full lead-free (such as the follow-up maintenance of a large machine that has been sold), if the three members first only change the solder paste to lead-free SAC, it is generally called "forward matching"; if the ball is changed first When the pin is changed to LF, it is called "reverse matching" and other two half-set lead-free soldering. However, these two expedient practices will inevitably lead to the consequences of "lead pollution", and the troubles of interface cracking are prone to occur.

Now take the semi-lead-free one that only replaces the ball but not the paste as an example. When the paste is completely melted during the soldering process but the ball is not yet fully melted, the lead in the paste will diffuse into the lead-free ball, and Concentrated at the boundaries of each character (Grain Boundaries), resulting in structural inhomogeneity and instability of the overall solder joints. The following figure illustrates this phenomenon.

BGA lead-free soldering often has two major shortcomings; one is that the SAC pins are not fully melted, which not only makes the self-alignment performance worse, but also hides the crisis of interface cracking. The second is that the SAC ball is not completely melted, so that when the ball collapses (Ball Collapse) is not enough, cracks (Open) between the ball and the solder paste are often caused.

2. Repair after BGA/CSP assembly
Once a small number of defects are found in the assembled PCBA, of course, the expensive assembly board cannot be scrapped; instead, necessary rework and replacement must be performed. At this time, the defective BGA/CSP components must be disassembled from the PCB surface, and the new parts of good quality must be re-soldered in place for shipment. Such highly difficult Repair or Rework must of course use sophisticated professional tools and familiar techniques to avoid heavy losses. The following are common repair methods:

(1) Replace the damaged parts with new ones by heating and desoldering

Common methods for partial unsoldering of assembled boards include: simple soldering iron (Soldering Iron) type conduction method; and more complicated hot air (Hot Air) type convection method. The former is aimed at various extension and pin components, or passive components with caps at both ends. Iron Tip with suitable power should be used as a tool for heating and transporting solder during construction. Carefully remove the unsoldered old parts with special pliers, and then carefully weld the new parts on the original backing surface.

(2) The lack of thermal conductivity and hot air

(1) Heavy industry of simple parts

The heat conduction (Conduction) soldering iron method is simple, cheap, easy to learn, and fast to operate. It is often used for heavy-duty replacement of QFP, PLCC, or some discrete passive components. The disadvantage of this method is that it relies heavily on the skills of the technicians, and it is easier to cause scalds on the components or plates if heated too fast. Soldering irons with higher power may even cause the solder pads on the board to float away. Special equipment such as adjustable power (Power or watt) and self-adjustable soldering iron tips (Tips) should be used to facilitate the construction of different sizes of pads. For example, Sinait Heat from Metcal is a good commercial tool.

(2) Heavy industry of BGA/CSP

As for the BGA/CSP of Area Array ball feet, only Convection heating tools can be used to carry out more complicated repairs. These sophisticated special equipments are very expensive, and their hot air flow and temperature can also be adjusted randomly, but care must be taken not to damage other nearby components during construction. For high-priced BGAs, new solder must be used when re-soldering new parts, and all the old tin on the original pad surface must be removed to ensure the perfect metal detail of the new solder joints.

At this time, special hollow braided wire or braided wire made of thin copper wire can be used to absorb the molten tin on the cushion surface. If necessary, use precision dentist grinding tools or sandpaper, and use solvents to clean the copper pad in advance. It is necessary to completely remove the IMC that has grown on the copper surface to ensure the strength and reliability of the subsequent solder joints.

(3) Reprint solder paste and resolder

The solder paste is reprinted on the solder pad area with a special printed small steel plate, and then the new BGA/CSP is placed on the positioned solder paste with a precision alignment device, and then it is soldered firmly with hot air. Note that it must be done once to reduce the shear damage caused by the difference in CTE between the pad and the plate. Since the heat applied in the BGA/CSP heavy industry is much higher than that of the general small parts, it is necessary to be especially careful in the construction. After the completion of the work, a fluoroscopic inspection must be performed to ensure that the inner balls of the abdominal bottom have been welded properly.