Electrical Test For PCBs

Demands for quality controls mean additional production costs. In the common interest of customers and
manufacturers, the question of optimal testing is repeatedly posed in the production process.

E-Test | PDF

The Dilemma is obvious:

The days of the first "printed circuits" which could be checked simply with the naked eye are over. Today, high-tech equipment is necessary for a thorough PCB test.

Optical checks with the aid of conventional micro-scopic equipment are sufficient for one sided and relatively simple double-sided PCBs. Residues, constricted conductors and subcorrosions can be discovered which would otherwise lead to PCB failure after a number of operational hours.

An electrical test is needed to check the actual functions of a PCB. It is an essential if inner layers need to be tested, as is the case with multi-layers. In two position measure-ments all connections in the PCB are checked for stoppages and short circuits. All SMD pads and plated through holes are checked. The electrical test can be conducted as a parallel or finger test.

Optische Kontrollen

unterstützt durch konventionelle Mikroskopiereinrichtungen. Hier werden Rückstände, Einschnürungen der Leiterzüge und Unterätzungen erkannt, die z. T. erst nach einer Reihe von Betriebsstunden zum Ausfall der Leiterplatte führen.

Elektrische Test (E-Test)

Die Überprüfung der tatsächlichen Funktionalität der Leiterplatte übernimmt der E-Test. Er ist unbedingt vorzunehmen, wenn auch Innenlagen geprüft werden müssen, wie es bei Multilayern der Fall ist. Im Rahmen von 2-Punkt-Messungen werden dabei sämtliche Verbindungen der Leiterplatte auf Unterbrechungen und Kurzschlüsse überprüft. Prüfpunkte sind sämtliche SMD-Pads und durchkontaktierte Bohrungen >0,60 mm. Die elektrische Prüfung kann per Parallel- oder Fingertest durchgeführt werden.

Parallel test

For the parallel test a control adaptor is constructed, consisting, e.g. of several synthetic sheets positioned above each other and fitted with control pins.

An adaptor programme must be written to reflect the actual holes in each individual synthetic layer of the adaptor. The pin positions are identical to the control points enabling an overall PCB test within seconds.

The control adaptors are all the more expensive the higher the density and the lower the grid distances between control points. The adaptors can also only be used for one specific type of PCB which explains the high set-up costs for the test.

Finger test

As PCB series are not always produced in mass, the finger test was developed as an alternative solution for small or medium-sized series. Manoeuverable probes move to the control points without necessitating an adaptor. As the points are checked individually, the time required for a complete check depends on the number of control points. In extremely complex layouts the test can take up to 30 minutes.

Electrical testing of PCBs is costly and time intensive. However, it is obvious that testing is necessary. Just consider the consequences of a defective PCB discovered only after assembling is completed!

As purchasers are generally unable to conduct tests themselves, there is no alternative to the electrical test.

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