Repair and Rework Requirements for SMT Assemblies
In my last two columns, (August and September 2001), I discussed the broader issues of surface mount repair as they relate to rework process and equipment technologies. In this column, I will discuss generic rework requirements, regardless of the rework process or equipment used.
In through-hole components, removal is more critical than replacement. The main reason for this is that in the through-hole removal process, the solder first must be sucked from the hole before extracting the lead. This is where the problem lies. Even when the solder has been sucked out completely, the leads tend to stick to the barrel of the hole, separating traces connected to the barrel if the operator applies too much pressure during the lead extraction process. It is less of a problem when the operator removes two leaded devices such as axial components, but a major problem if the component is multi-leaded as is the case when removing dual in-line packages (DIP) and pin grid arrays (PGA).
Through hole assembly repair requires considerable operator skill, especially for removing multi-leaded through hole devices. Major causes of thermal damage in through hole assembly repair/rework are pad pressure, and uncontrolled temperature and time of desoldering and soldering. Conductive tool use, especially when using temperatures above 700°F, can char the board and cause delamination around the pad. Pressure on the pad is extremely harmful.
The repair equipment and processes for surface mounting fall into four main categories: laser, hot air, infrared and conductive tips. The conductive tips are not used as widely, but they are inexpensive, whereas the cost of other three systems can be significant. Hot air is the most widely used process but laser is the up-and-coming technology for surface mount repair because it allows rework of very tightly spaced components. Rework is the key driver for manufacturing design because adequate interpackage spacing is needed to allow rework.
The rework systems that allow tight interpackage spacing between surface mount components (especially fine-pitch, ball grid arrays and chip scale packages) while also offering real-time automated thermal profiling will have the edge.
No matter which rework equipment is used, the following requirements are key to reducing thermal damage to the board and should be followed closely when repairing through-hole or surface mount components.
1. When using hot air devices, care should be taken to prevent thermal damage to adjacent components or the substrate. The advantage of using a laser system is that damage to adjacent components is not a concern even if they are close to the part being removed.
2. When using soldering iron tip attachments or other resistance?heated conductive tips, pressure on the pads should be minimized.
3. Solder fillets resulting from repair/rework procedures should meet in-house or industry standard solder joint criteria requirements.
4. The number of times a part is removed and replaced should be kept to a minimum (maximum two times) to prevent internal thermal damage in the printed circuit board (PCB).1
5. Substrate preheating is recommended, especially during component removal. An oven or built?in preheating system in the repair machine or a stand?alone preheat system may be used for this purpose. Preheating generally is recommended for removing devices, especially if they are connected to large heat sinks or have thicker leads.
6. Tip attachment or conductive tip temperature should not exceed 370°C (700°F). The potential for thermal damage increases significantly above 700°F.1
7. Desoldering time (heat source on land) should not exceed 3 seconds when using conductive tips. When the tip temperature is kept between 550° and 850°F, it takes 1.5 seconds for the solder inside the plated through-hole to melt. Staying any longer than 3 seconds is not necessary for accomplishing solder joint reflow. Longer soldering or desoldering time with the iron tip simply increases the potential for thermal damage.
8. When using hot air removal/replacement equipment, an appropriate attachment should be used to direct the flow of hot air to the component to be removed or replaced. The time and temperature for desoldering/soldering permits rapid part removal without damaging the board or charring or burning flux. Longer times may discolor the board surface and warp the board.
9. Only low activity fluxes (e.g., rosin or no clean) should be used for repair/rework and components should be cleaned immediately after rework.
10. Wires of 30 gauge or thinner should be used to accommodate design changes. Whenever possible, via holes should be used to solder in jumper wires. If this is not possible, the J or gull wing wire leads of the surface mount packages should be used to solder the jumper wires. The wires should be hooked onto the leads but the solder fillet in the hook of the wire should be as small as possible to avoid stiffening the leads. Butt joints or lap joints on the solder pads also can be used instead of hooks on J or gull wing wire leads. However, in this case, the jumper wire must be glued to the board with adhesive to prevent detachment or butt/lap joint breaking.
11. Parts bonded to the substrate with adhesive should be removed with a tool that will cut through the adhesive bond. Another option is to hold the part rigid and twist its body, so that the bond between the part and the adhesive is sheared, keeping the part body intact. A gentle rocking or prying action should be used to pry the part loose from the board. Residual adhesive left after part removal may interfere with the new part replacement. Therefore, the residue should be
scraped carefully from the substrate surface. A uniform, thin layer of residue may be acceptable if it does not interfere with component placement. All adhesive should be removed from the lands to achieve an acceptable solder fillet.
12. To ensure that the components or boards are not damaged, it is important to develop a unique profile for each component to be removed or replaced by any given equipment.
13. Some systems heat all four sides of a component more uniformly than others. It is important to control the heating rate in rework. The quickest ramp rate in the hot air rework profile occurs when the nozzle is lowered onto the package after preheat.
14. Package peak temperature should be kept to a minimum. If the peak temperature is too high, the package or PCB may be damaged by excessive heat application. For eutectic tin/lead solder, this peak package temperature should not exceed 230°C during removal.
15. Regardless of the hot air system used, nozzle design is critical to obtaining an effective rework profile. Generally, the leads on the corners heat more quickly than the leads toward the center. Consequently, the corner leads reach the solder melting point before the center leads on each side of the package. The temperature lag between the corner and center leads is greater for larger packages — as much as 20°C for poorly designed nozzles. An excessive differential in temperature between the center and corner leads means that more heat than necessary is being supplied to the component body. It is better to have a uniform temperature throughout to prevent component body overheating.
16. When using laser systems for rework, there is no need for nozzles. However, it is important to develop profiles for both focused and defocused laser beams. Preheat is not always necessary when using laser because board or adjacent components generally do not get very hot; however, preheat is recommended when reworking high thermal mass assemblies.
Prasad, Ray P. "Contributing factors for thermal damage in PWB assemblies during hand soldering." IPC Technical Review, January 1982, pp. 9?17.
RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc., a supplier of automated mass rework and selective assembly systems, located in Portland, OR, and founder of the Ray Prasad Consultancy Group, which specializes in helping companies establish strong internal SMT infrastructure. Contact him through his web site: www.rayprasad.com.
In this column, I will discuss generic rework requirements, regardless of the rework process or equipment used.