May 19, 2022

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Tips for using EPIC / MEPIC detonators in fireworks applications

Negative components Tips for using EPIC / MEPIC detonators in fireworks applications

By Amin Zia*

Cheap BW igniters are often used to drive airbags or belt tensioners, but require complex designs. The EPIC and MEPIC technology ignition resistors are perfect here.

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EPIC (left) and MEPIC ignition resistors: can be used to drive airbags.

(Photo: © phonlamaiphoto – stock.adobe.com, Texas Instruments)

Pyrotechnic application markets such as igniters and starters require higher levels of performance, reliability, and safety. Today, bridge wire (BW) is the leading technology in a variety of industries, including automotive (airbags and seat belt pretensioners), mining (rock breaking), demolition (tunnels and buildings), and special effects (fireworks and movies). While BW technology offers good value for money, it requires special starter/starter designs where the head and wire must always be matched.

To provide design and process alternatives, Vishay has developed SMD solutions whose performance is not dependent on the head: EPIC (Electric Fireworks Initiator Chip) and MEPIC (Massive Photovoltaic Initiator Chip) series. In addition, these SMD solutions enable manufacturers to have an efficient assembly process based on the latest standard equipment (spool assembly, reflow soldering and subsequent cleaning).

Fireworks Application Requirements

The general concept is to deposit an active substance (fireworks) in close contact with the heating region of EPIC/MEPIC, which is called the active region. Upon electrical excitation (by a capacitor discharge or a DC pulse), the energy supplied to the EPIC/MEPIC is thermally transferred to the core pyrotechnic material either directly or through a thermally conductive substrate.

EPIC and MEPIC Component Manufacturing and Techniques

EPIC and MEPIC SMD components are manufactured using two different technologies:

  • EPIC – Case Size 0603: Material: alumina substrate – tantalum nitride resistance layer.
  • MEPC – Housing Size 0805: Material: FR4 epoxy based substrate – NiCr resistant layer on the adhesive.
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In both EPIC and MEPIC technologies, the active area is designed according to customer requirements and Vishay’s expertise to ensure an adequate active area ‘size’.

Welded using standard reflow process

Since EPIC and MEPIC are SMD components, they are often soldered on flat circuit boards using the standard reflow process. In addition to proper electrical contact procedure, great care must be taken during assembly to ensure that the active areas of the EPIC and MEPIC resistors remain free of solder paste flux residue to ensure intimate contact with the pyrotechnic material. Besides the standard zone patterns, the EPIC and MEPIC series have specific design and engineering features that greatly reduce the presence of flux in the active zone and improve heat transfer. However, thorough post cleaning is necessary to prevent dirt from forming which can be deposited on the active surface again.

Since the value of the resistance depends only on the dimensions of the active region generated by the size of the case (0603 for EPIC and 0805 for MEPIC), no circuit board modification is required, so that – regardless of the intended ignition performance of the EPIC or MEPIC – the standard contact layout is used.

Publication of primary pyrotechnics

The customer chooses the fireworks material. In this paper, we are concerned only with the pyrotechnic material (primer) that is in direct contact with the active face of the EPIC/MEPIC resistor and initiates the explosion. The heat generated by the EPIC/MEPIC components, when electrically stimulated, will cause a sufficiently high temperature to detonate the primer detonator. The heat from the first blast stage leads to a second blast stage, which then develops the desired effect (for example, gas development in airbags).

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The process for applying a pyrotechnic primer can be either a ‘wet process’ by applying varnish (brushing or dipping) or a ‘dry process’ by pressing a powder onto the active surface.

The most important characteristic of a pyrotechnic ignition substance is the self-ignition temperature. The table lists the main primers and their ignition temperatures.

Table: an overview of the main primer materials and their ignition temperatures.
Table: an overview of the main primer materials and their ignition temperatures.

(Photo: Texas Instruments)

Regardless of the chemistry of the primer, it is important to consider the size of the particles to ensure close contact with the active surface. For highly sensitive applications with small active areas, it is highly recommended that particle sizes are considered in the nanometer rather than the micrometer range. In order to ensure reproducibility of the response behaviour, the primer must be prepared in such a way that a highly homogeneous mixture without air bubbles is produced.

Advantages of EPIC and MEPIC over BW

Compared to BW resistors, EPIC and MEPIC resistors offer many advantages, mainly thanks to the use of standard SMD processes to design and manufacture igniters and starters.

The printed circuit board, called the header, has a standard SMD format (0603 or 0805), the components are delivered on tape, assembled using pick-and-place machines, and soldered in a standard reflow soldering process. Thanks to these standard processes, high productivity can be achieved. In addition, SMD technology makes it possible to do without expensive brushed conductors, which are indispensable using BW technology.

To meet customer’s special requirements, Vishay can customize the active area of ​​EPIC or MEPIC without changing the packet size and thus the required PCB or socket size; This allows developers to use a unified foundation for different applications. From a design and developer point of view, it is also important to consider that despite the interface with the pyrotechnic material, the use of EPIC and MEPIC devices avoids any reliability problems caused by electric welding during assembly and also allows a level of miniaturization where reflow soldering is possible.

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Finally, the tightly controlled manufacturing process of EPIC and MEPIC resistors combined with precise automated optical inspection (AOI) ensures high reproducibility and reliability, which remains a key safety factor in demanding applications.

* Amin Zia is Senior Marketing Manager for Thin Films and Flares at Vishay Sfernice in France.

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