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July 2014 • The PCB Magazine 11 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 explosively 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 re- ceiving sufficient heat to cause complete melt- ing and joining of the solder. Cold solder joints are often seen in cases where the component lead is connected to a large thermally conduc- tive feature 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 alloy formed in the soldering pro- cess due to dissolution of ele- ments 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 in- troduction of lead-free soldering. It is an unset- tling type of defect in that it is not easily de- tected but could result in an intermittent open in the operation of the assembly. The term was chosen because the phenomenon is reminis- cent of an individual's head forming a depres- sion on a pillow. 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 some- what apropos given that the assembly will like- ly 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 as- sembly 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 happens 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 par- ticles 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 poten- tial shorting elements. r) Misregistration: Components with fine pitch leads, if jostled before or during the assembly, 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. ELECTRONIC ASSEMBLY WITH SOLDER continues 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. " "