On a flat note: the flat, leadless PQFN package traps flux from no-clean paste, leaving a gooey, conductive residue.Flux volatilization volatilization /vol·a·til·iza·tion/ (vol?ah-til-i-za´shun) conversion into vapor or gas without chemical change. vol·a·til·i·za·tion n. See evaporation. is a critical factor in maintaining ionic cleanliness levels that lend to reliable performance. In a no-clean process, when the flux is activated and fully volatilized vol·a·til·ize intr. & tr.v. vol·a·til·ized, vol·a·til·iz·ing, vol·a·til·iz·es 1. To become or make volatile. 2. To evaporate or cause to evaporate. , it leaves a benign residue carrier or solvent that will not have a negative impact on field performance. [TEXT NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] New and compact forms of IC packaging, particularly PQFN PQFN Power Quad Flat Pack No-Lead (power quad flatpack no-lead) packages, solve spacing issues for power electronics but have been shown to entrap flux residues by not permitting a volatilization path. This occurrence causes the entrapment entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g. of a high concentration of gooey See GUI. flux residues that can cause product failures. Though space is saved by a smaller footprint and replacement of axial leads, potential for flux entrapment becomes a huge concern to those using this type of package. In other words Adv. 1. in other words - otherwise stated; "in other words, we are broke" put differently , sometimes space conservation comes at a cost. A recent customer was using a PQFN package in an automotive application, and was experiencing stray voltage stray voltage accumulation of low voltages in the metalwork of a milking parlor due either to leakage from poor wiring or to poor earthing (grounding) with no outlet for static electricity. Very small voltages cause restlessness and a fall in milk yield. failures in the Z15 area where this package was located. This caused LEDs to stay on and, eventually, cause failures. Upon removing this package from the assembly, we found a visible flux residue underneath. This customer sent several assemblies for analysis to determine the source and type of ionic contamination. [ILLUSTRATION OMITTED] The process used was no-clean with some selective soldering Selective soldering is the process of soldering only through-hole electronic components onto a printed circuit board that has surface mount components on the under-side. This is usually done because the surface mounted component is not glued into place, instead solder paste is used sites. There were visible flux residues on the assembly, and the areas we looked at included the QFN QFN Quad Flat No-Lead QFN Queen Fan Newsletter (rock band) QFN Quad Flat No Leads package as well as several selective soldering sites. We also looked at the cleanliness of incoming bare boards and PQFN packages. The selective sites showed low residue levels, indicating that the no-clean flux had done its job and volatilized. The failure area around the PQFN package showed only one flux activator group, and gave readings of high weak organic acids (WOA WOA Wacken Open Air (music festival) WOA Work of Art WOA Western Orthopaedic Association WOA Web Offset Association (Nashville, TN) WOA World Airways, Inc (ICAO code) ). These residues were coming from marginal cleanliness levels of the bare board and components combined with the flux used in wave solder. These elements combined and trapped under the PQFN package create a very high risk for conductive pathways to form. To investigate this problem and process more fully, I visited the location assembling the boards with the PQFN. They used a nominal range profile for the reflow oven A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards. Types of Reflow Ovens Infrared and Convection Ovens with the no-clean paste. We tried modifying to a higher reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. profile, but there was still gooey, non-complexed flux under the PQFN package (Figure 1). [FIGURE 1 OMITTED] The reason for this phenomenon is that as the weight of the PQFN package collapses over the molten solder paste Solder paste (or solder cream) is a mix of small solder particles and flux. It is used extensively in the automated soldering processes wave soldering and reflow soldering. during reflow, flux from the outside is sinking over and becoming trapped underneath the package where it cannot be complexed and released. The PQFN, which sits atop solder paste at a 0.006" stencil stencil, cutout device of oiled or shellacked tough and resistant paper, thin metal, or other material used in applying paint, dye, or ink to reproduce its design or lettering upon a surface. thickness, is able to drop to 0.003-0.004", thus trapping the flux and blocking the solvent path of volatilization. To solve this, we suggested designing a 0.005" standoff to fit into the vent pipe vias that would physically hold up the part, and not permit the column of solder to collapse under the part weight. The part will hold up the solder until it cools. If this were an aluminum part, it would function as a heatsink fin. We are now working with this customer to enact this corrective design plan, and are also working to develop a remedial cleaning protocol to remove the flux residue from underneath PQFN packages. Smaller, more compact technology is always developing at a rapid pace, and sometimes challenging cleanliness issues are being discovered. Although this part is known for its heat dissipation Noun 1. heat dissipation - dissipation of heat chilling, cooling, temperature reduction - the process of becoming cooler; a falling temperature , great precaution is needed when incorporating PQFN packages into design to prevent flux entrapment with no path of volatilization.
Table 1. Cleanliness Results
Ion Chromatography
Sample C[l.sup.-] B[r.sup.-] WOA S[O.sup.-.sub.4] Na+
Description
Board #1 Z15 3.42 2.53 114.17 3.25 5.11
area
Board #1 3.09 2.77 16.33 3.69 5.33
Reference Area
Board #2 Z15 3.19 2.62 182.08 3.18 4.12
area
Board #2 3.49 2.44 14.63 3.27 4.32
Reference Area
Board #3 Z15 3.26 3.12 102.82 3.09 4.29
area
Board #3 3.41 3.08 18.37 3.39 4.01
Reference Area
Board #4 Z15 1.08 3.17 92.65 0.27 1.21
area
Board #4 0.89 2.98 11.52 0.23 1.47
Reference Area
Board #5 Z107 3.78 3.50 59.36 3.21 4.32
93ZOOE Z107
(GTC)
Board #5 Z107 3.62 3.14 18.78 3.09 4.74
93ZOOE Ref
area
Board #6 Z107 1.67 3.33 71.32 1.21 1.69
93ZOOI Z107
(KALEX)
Board #6 Z107 1.23 3.05 14.62 1.11 1.84
93ZOOI Ref
area
Board #7 Z107 3.60 3.27 68.36 3.34 4.69
93ZO15 Z107
(GTC)
Board #7 Z107 3.53 3.44 13.21 3.64 4.41
93ZO15 Ref
area
PQFN package 1.02 0.22 0.00 0.24 0.39
in tape and
reel #1
PQFN package 0.89 0.29 0.00 0.15 0.31
in tape and
reel #2
Ion Chromatography C3 Tester
Sample NH4+ Results Time (sec)
Description
Board #1 Z15 3.69 Dirty 33
area
Board #1 3.51 Clean 86
Reference Area
Board #2 Z15 4.02 Dirty 39
area
Board #2 3.87 Clean 74
Reference Area
Board #3 Z15 2.94 Dirty 41
area
Board #3 2.88 Clean 82
Reference Area
Board #4 Z15 0.27 Dirty 56
area
Board #4 0.33 Clean 180
Reference Area
Board #5 Z107 3.05 Dirty 59
93ZOOE Z107
(GTC)
Board #5 Z107 3.66 Clean 91
93ZOOE Ref
area
Board #6 Z107 1.21 Dirty 51
93ZOOI Z107
(KALEX)
Board #6 Z107 1.06 Clean 165
93ZOOI Ref
area
Board #7 Z107 3.55 Dirty 34
93ZO15 Z107
(GTC)
Board #7 Z107 3.16 Clean 82
93ZO15 Ref
area
PQFN package 0.15 Clean 180
in tape and
reel #1
PQFN package 0.11 Clean 180
in tape and
reel #2
Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. His column appears monthly. |
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