Despite all effort put into design, process and material control, there always will be some assemblies that fail to meet requirements and must be repaired. This is true in every company. Even with conventional through-hole assemblies, which have been around since the 1950s, every company performs some level of repair. Surface mounting is no exception. There is no reason to think that the need for rework/repair will ever go away completely.
In this column, I compare and contrast some of the rework processes for surface mount repair. Before I delve into the details of different rework processes, let me first highlight some major issues, not necessarily in the order of importance, that we need to be aware of in surface mount repair.
Surface Mount Repair IssuesA key issue is to know, on a real-time basis if possible, what is happening to the temperatures of the component and board. This means that an automatic thermal management system is needed that not only monitors package temperature but also controls it within predefined thermal profile parameters.
Interpackage spacing is decreasing constantly. Even if companies have some design for manufacture (DFM) guidelines for interpackage spacing, those on the front line of manufacturing know very well that DFM guidelines are not always followed. So, using ministencils to print solder paste is becoming more difficult. Also, because a ministencil is needed for each size and type of part, it not only slows down the process but also quickly adds to the cost of repair.
Ministencil is not the only issue with ever-decreasing interpackage spacing. Using different hot air nozzles for each size and type of part being removed also adds to the cost and complexity of rework. Additionally, the potential for melting solder joints of neighboring components is a serious concern. In addition to increased intermetallic thickness because of unnecessary reflow, which weakens the solder joints, the boards must be baked before rework, increasing cycle time.
Throughput in rework is very important. Unfortunately, the typical time to remove and replace a simple component like a 32-pin plastic leaded chip carrier (PLCC) can take more than 12 minutes in the conventional process. Ball grid arrays (BGA) and some of the larger components can take at least 20 minutes per component. And some components can take more than an hour for each component. As discussed last month ("Mass Rework? Automated? You must be kidding!," SMT Magazine, August 2001, p. XX-XX), this can be quite a bottleneck if you have to do any major rework.
Another important issue in rework is board warpage. Warpage partly is due to intense local heating for a relatively long time necessary to remove the component. For surface mount repair, three rework processes are used today: conductive tools, hot air and laser. Conductive rework tools are the least expensive. Hot air is the most common. The up-and-coming rework process for surface mount repair is laser. Now, let us see how these processes address some of the major rework issues just highlighted.
Conductive Tools for Surface Mount RepairConductive tools such as soldering iron tip attachments are used in conjunction with the soldering iron for desoldering components from the substrate. The tip attachment is shaped such that all the solder joints on a particular device are heated simultaneously, and the part is lifted by the tool itself or by other aids such as tweezers.
Attachment size and shape will vary with the size and shape of the part to be removed. Thus, chips and small-outline integrated circuits have a heating attachment on two sides of the package, but PLCCs need an attachment on all four sides. Using the appropriate tip, heat is applied to the surface mount component to be desoldered until all solder melts. The component then is removed with a twisting motion. There is no preheating mechanism unless an external system is used to preheat the boards.
Conductive tools do increase the potential for lifting pads and board damage. The rework process is very slow and requires the largest interpackage spacing. This method does not address the ministencil issue but does not melt solder joints of neighboring components.
Hot Air Systems for Surface Mount RepairHot air systems are either totally manual or semiautomated. Essentially, they are sophisticated hair dryers that blow hot air on the part to be reworked. The part is pulled away from the board when the solder on all joints is molten. The hot air usually is directed on the leads by a nozzle designed specifically for that component. The package body is heated by the hot air impinging on the package and conduction within the package. Initially, the package is preheated with the nozzle some distance away (typically 1" or more) from the package body. Then the nozzle is lowered to a point just above the package body and lead temperature increases sharply until it reaches a peak. During this process of blowing hot air, the solder joints of neighboring components even half an inch away can reflow, an unwanted and undesirable result.
After the component is removed, paste application for reattachment is a most difficult and time-consuming process. Typically, a ministencil is used to apply the paste. Both hot air nozzles and ministencils are needed for each type and size of part being reworked. Both these items require sufficient interpackage spacing for rework.
Laser Systems for Surface Mount RepairUp-and-coming repair systems use from one to four lasers. Some of the laser systems are limited to reworking only peripheral components, in which the leads are in the laser's line of sight. However, the higher end laser systems that use multiple diode lasers can rework both peripheral and array type packages such as BGAs, chip scale packages (CSP) and flip chips by rapidly scanning top of package surfaces. This causes BGA/CSP/flip chip ball reflow underneath by conduction through the package, as is the case in hot air rework. Some of these high-end systems also have a built-in automated thermal management capability to monitor and control package temperatures within the specified limits to prevent overheating.
Unlike hot air systems, with their built-in temperature monitoring systems, the higher end laser rework systems can remove components, dispense paste, place and solder components automatically without nozzles or mini-stencils.
Even though there are various laser systems on the market for rework, to avoid conflict of interest, I want to disclose that I have financial interest in one of the higher end laser rework systems made by BeamWorks (www.beamworks.com). I am not only an investor in this company, but I also play a major role in its operation as my bio at the end of this column clearly indicates.
Hot Air or Laser?Interpackage spacing on surface mount boards is much smaller than that encountered on conventional boards. Because the laser beam is very narrow, components even 0.020" (20 mil) away do not experience any heat. Laser systems heat the package without melting the solder joints of neighboring components while keeping the board cool enough to touch. With hot air systems, melting solder joints of neighboring components is a serious concern. Using correct hot air nozzles and shielding adjacent components can minimize this problem. However, hot air nozzles are needed for each type and size of component. This adds to the cost and complexity of repair. And to shield adjacent components, adequate interpackage spacing is necessary, which is becoming difficult to come by because designers constantly reduce interpackage spacing as they try to pack more components into ever-smaller electronics devices. Even with shields, there is no way to completely prevent reflow of neighboring components because some air does leak out. Adjacent component reflow is not the only concern with hot air. The entire board must be baked for up to 24 hours before rework to prevent popcorning of both the part being reworked and neighboring components. This adds to the cycle time of rework. Because lasers only heat the lead, baking the board before rework is not necessary unless a part needs to be salvaged. This generally is not a major concern because most companies scrap reworked parts.