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A19E_EET_2_37x10_87_A19E.qxd 5/4/15 4:26 PM Page Three levels of process control CE MO UNT S U R FA in thermal profiling ( an d th r u - ho l e) s Tr ansf o r me r u cto r s WBy Patrick McWiggin & I nd ith today’s more sophisticated electronics market, which calls for more intricate Size does matter! from lowprofile .18"ht. • Audio Transformers • Pulse Transformers • DC-DC Converter Transformers • MultiPlex Data Bus Transformers • Power & EMI Inductors Se e P ico’s ful l Cata log imme diat ely w w w . p i c o e l e c t r o ni c s . c o m PICO Electronics,Inc. 143 Sparks Ave. Pelham, N.Y. 10803 E Mail: info@picoelectronics.com Pico Representatives Germany ELBV/Electronische Bauelemente Vertrieb E-mail: info@elbv.de Phone: 0049 89 4602852 Fax: 0049 89 46205442 England Ginsbury Electronics Ltd. E-mail: rbennett@ginsbury.co.uk Phone: 0044 1634 298900 Fax: 0044 1634 290904 and diverse requirements, together with decreased development cycles and improved capacity, the need to ensure products are produced correctly with the right process control and within tight constraints is ever more important. To guarantee products are soldered correctly and process parameters are met, thermal management is a necessity. To produce electronic assemblies without a known thermal profile is detrimental, as you ‘cannot manage what you do not measure’. Without correct profile management, increases in rework costs may occur and potentially give rise to early field failures. There are three levels of process control which are commonly employed today, all with differing benefits and failures starting with the traditional test board method, through to the advanced automatic profiling systems. Over the years thermal profiling has advanced significantly and now offers exact profiles aiding in the advancement of technology manufacture. We will first look at the traditional manual method. Fig. 1: APS - Box in situ. Traditional profiling devices The method of passing a profiling device through an oven to ensure the machine is set correctly is an important one, and having a test board to do the initial profile setup is still a fundamental step that needs to be performed. Thermocouples are placed on the crucial components of the board and when passed through the oven measurements are taken to ensure the thermal profile meets the target profile. With the introduction of more complex or smaller fragile assemblies, or when high production volumes of mobile phones and tablet PCs are involved, this is simply not enough. Verification of the oven profile must be performed much more frequently. Using the test board for on-going process checking is error prone, resulting in false alarms or measurement data with poor repeatability or accuracy. The main problem with this method is keeping the test board in good working order. The thermocouple sensors used to establish the profile during the initial set-up stage needs to respond quickly to temperature change and not influence the measurement. The result of this is that they are inherently fragile and need to be frequently replaced which is both time consuming and can result in measurement repeatability problems. Attachment of the sensors can also be difficult; they can be placed incorrectly on the board, fall off or get snagged in the machine. Together with the problem of repeated test runs damaging a PCB, resulting in an inaccurate measurement, the drawbacks to this method are great. Man hours is another drawback to the manual method. The process interrupts production and is very labour intensive. It is also very dependent on Fig. 2: Oven verification fixture to monitor accuracy of the APS thermal measurement. Patrick McWiggin is SolderStar’s Technical Director - www.solderstar.com www.electronics-eetimes.com Electronic Engineering Times Europe June 2015 43


EETE JUN 2015
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