SMT007 Magazine

SMT-July2014

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July 2014 • SMT Magazine 11 ELECTRoniC inTERConnECTionS ElECTRoNIC ASSEMBly WITH SolDER continues i) Blowholes: A term applied to a phenom- enon where a small hole is observed in a solder joint. Typically, the defect is found to be the result of discontinuities in the plated through- hole wall, which may absorb flux and then ex- plosively out gas during the soldering process. j) Cold Solder Joints: Solder joints that did not form completely a good metallurgical bond. They are often the result of the joint receiving sufficient heat to cause complete melting and joining of the solder. Cold solder joints are of- ten seen in cases where the component lead is connected to a large thermally conductive fea- ture or element and insufficient heat is retained near the lead to assure a good solder joint. With lead-free solders, the phenomenon pro- vides a greater challenge as the amount of heat which must be supplied is much greater than it might have been with a tin lead solder, thus potentially degrading device and assem- bly reliability. k) Brittle Solder Joints: Solder joints wherein the al- loy formed in the soldering process due to dissolution of elements within the finish or on the circuit board (e.g., gold), results in a solder joint that is less ductile than the solder used in the assembly process. l) Head-on-Pillow: A new type of defect which was identified only with the introduc- tion of lead-free soldering. It is an unsettling type of defect in that it is not easily detected but could result in an intermittent open in the operation of the assembly. The term was chosen because the phenomenon is reminiscent of an individual's head forming a depression on a pil- low. m) Graping: Another lead-free related de- fect wherein the small, often ball-like particles of solder in a solder paste do not reflow com- pletely, leaving a surface that looks like the sur- face of a bunch of grapes. Like head-in-pillow, it is a defect that may not be easily detected. n) Tombstoning: Tombstoning is a term that has been applied to the appearance of a de- fect related to discrete devices such as resistors and capacitors, wherein solder connections are not made simultaneously; the slight lag causes the first side to reflow to pull back and rotate up, resembling a grave marker (which is somewhat apropos given that the assembly will likely be dead if tombstones are present). o) Component Cracking: Component cracking can have multiple causes, one being a situation where there is a significant mismatch in terms of coefficient of thermal expansion be- tween the component and the printed circuit to which it is attached. It can also occur if the assembly is flexed in the area of the component, causing the device to crack. p) Popcorning: Popcorn- ing is a phenomenon mani- fest when moisture entrapped within a component outgases during assembly, causing a blister to form in the encapsu- lation material. With the ad- vent of lead-free soldering and its higher temperatures, the incidence of popcorning rises and in fact moisture sensitiv- ity levels of components are degraded to reflect the new reality. q) Solder Balling: Solder balling is a condition which hap- pens during the reflow of a solder paste on a surface mount assembly. It is a result of the high temperature of reflow causing rapid volatility station of the flux and spatter of the solder particles that are part of the flux. While a viable solder joint may be created even as solder balls are being formed, they represent a risk to the long term reliability of the assembly as po- tential shorting elements. r) Misregistration: Components with fine pitch leads, if jostled before or during the assem- bly, may be misregistered relative to the land pattern, resulting in a nonfunctional product. s) Insufficient Cleaning Under Devic- es: As mentioned previously, insufficient clean- ing under surface mount devices can result in latent failure through the formation of high re- sistance shorts or the growth of dendrites. Component cracking can have multiple causes, one being a situation where there is a significant mismatch in terms of coefficient of thermal expansion between the component and the printed circuit to which it is attached. " "

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