Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-09-01
2002-08-27
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C528S332000, C528S335000, C528S338000, C525S420000, C525S422000, C525S425000, C525S432000, C524S174000, C524S177000, C524S411000, C524S430000, C524S442000
Reexamination Certificate
active
06441072
ABSTRACT:
The invention relates to a polyamide composition that is particularly suitable for the production of electrical and electronic components used in the so-called surface mounting technology, SMT, for printed circuits, which involves their being exposed to very high temperatures during the so-called reflow soldering process. The high temperatures used are obtained by, inter alia, infra-red radiation or hot-air circulation or a combination of both.
It goes without saying that the mechanical integrity of the component is to be retained at the high temperatures, which may increase to over 260° C. There are various reasons for the use of high-melting polyamide compositions in SMT components.
These compositions generally contain fibre-reinforcing materials, often glass fibre and materials promoting flame retardancy.
In view of the very high processing temperatures, 300° C. and higher, needed for the production of the SMT components from the high-melting polyamides, high demands are to be met in terms of the chemical stability of the flame retardants used and in particular flame retardants are suitable whose activity is based on incorporated halogen. Bromine-containing styrene polymers are very often applied.
In practice, in particular under tropical conditions, a disturbing phenomenon, blister formation on the surface of the soldered components, occurs when SMT components from the high-melting polyamides are reflow soldered. This phenomenon is ascribed to the fact that polyamides readily absorb water, which absorbed water will expand explosively in the component when the latter's temperature is raised in a very short time, for instance by means of infrared radiation, to for instance 260° C. Much research has therefore been aimed at reduction of the water absorption. JP-A-63-1959508, for instance, recommends drying the components to a water content of less than 0.8 wt. % before mounting them on the printed circuit board. This, however, necessitates an extra action and special logistics, while there must only be a short time between drying and mounting of the component. Factors interfering with the mounting process have a highly adverse effect.
Attempts have also been made, through incorporation of apolar polymers, or polymers having a low water absorption capacity, to reduce the water absorption capacity of the composition used, in particular of the aliphatic polyamide-4.6. To this end use has been made of, inter alia, aromatic polyesters, for instance polybutylene terephthalate, JP-A-03-190962, amorphous aliphatic polyamides with a long carbon chain, for instance polyamide-12, amorphous aromatic polyamides; for instance polyamide-6.6/6.T, JP-A-06-065502, and an aromatic polysulphone, JP-A-05-239344. In addition to sometimes negative effects on the mechanical properties of the components produced from these compositions, none of the compositions obtained proved to be an adequate solution to the blistering problem under all production conditions.
In particular under tropical conditions, 90% relative humidity and 35° C., in a number of cases blistering is still found to occur, even though the water absorption capacity of the compositions has been reduced strongly.
The aim of the invention is a polyamide composition for use in electrical and electronic components suitable for SMT that exhibit an even greater resistance to blistering during the infrared or hot-air circulation reflow soldering process. The invention is aimed in particular at a polyamide composition which, in spite of a water content higher than 0.8 wt. %, does not give rise to blistering if processed in electrical and electronic components in the said reflow soldering processes.
This aim is achieved with a composition comprising
(a) a polyamide having a melting point of at least about 280° C.
(b) a thermoplastic polymer having a melting point of lower than 230° C.
(c) a halogen-containing organic compound which, when heated at 300° C. for 10 minutes, generates at most 200 ppm, preferably at most 50 ppm, inorganic chlorine and at most 350 ppm, preferably at most 300 ppm, inorganic bromine.
(d) optionally a compound supporting the flame-retarding activity of (c)
(e) an inorganic reinforcement
(f) optionally other additives
The polyamide (a) may be either a homopolyamide or a copolyamide and be composed of repeating units derived from a diamine and a dicarboxylic acid or have been obtained by ring opening of a lactam.
Suitable diamines are aliphatic diamines, cycloaliphatic diamines and aromatic diamines. Examples are tetramethylene diamine, hexamethylene diamine, 2-methylpentamethylene diamine (2MP), 1,4-cyclohexane diamine (CHDA), 1,4-phenylene diamine (PDA) and p.xylylene diamine (PXDA). Suitable dicarboxylic acids are aliphatic, cycloaliphatic and aromatic dicarboxylic acids. Examples are adipic acid, pimelic acid, 1,4-cyclohexane dicarboxylic acid (CHDC), terephthalic acid (T) and isophthalic acid (I). Suitable homopolyamides and copolyamides include polyamide 4.6, polyamide-4.T, polyamide-4.6/4.T, polyamide-6.6/6.CHDC, polyamide-6.6/6.T, polyamide-6/6.6/6T and polyamide-6T/6I/2MPT. Several of these polyamides are commercially available under different trademarks. The polyamides can be obtained by means of polycondensation starting from the monomeric components or the corresponding nylon salts. These processes, which are known to one skilled in the art, are described in, inter alia, Encyclopaedia of polymer science and engineering, Vol. 11. p. 315 ff. (1988), ISBN 0-471-80943-8 (v.11) and the references quoted there.
Preferably, a polyamide with a melting point between 280° C. and 340° C. is chosen. At a higher temperature the stability of the constituent components is generally insufficient and processing by means of, inter alia, injection moulding presents technical problems.
Preferably, the melting point lies between 280 and 320° C. Both the blistering behaviour and the mechanical properties are favourably affected by a higher molecular weight of the polyamide. One skilled in the art will aim to use a polyamide with the highest possible degree of polymerization that can still readily be processed.
For the thermoplastic polymer (b) in principle any polymer having a melting point below 230° C. can be chosen. However, to retain the good physical properties of the composition, preference is given to a polymer that is at least to some degree compatible with polyamide (a). For this reason, polyolefins, for instance, are less suitable for use as polymer (b), but when the polyolefin is modified, for instance through grafting with maleic anhydride, so that polar groups are introduced that improve the compatibility with polyamide, polyolefins can be applied. The same holds for other apolar polymers that, through modification with inter alia carboxylic acid, amine and epoxy groups, are better compatible with polyamide. Preferably, the melting point is lower than 220° C., even more preferably lower than 200° C.
Particularly suitable are polyesters, copolyesters and polyamides. Examples of esters are polybutylene terephthalate, polyethylene terephthalate, polycarbonate and copolyether esters, for instance the copolyester derived from polybutylene terephthalate and polytetrahydrofuran. Suitable polyamides are polyamide 6, polyamide 8, polyamide 11 and polyamide 12. Preferably, the ratio of aliphatic C atoms: amide groups in the chain is at least 6, even more preferably at least 8.
Preferably, component (b) is very well dispersed in the matrix of (a). The best effects are obtained if the average cross-section of dispersed particles of component (b) in the composition in the solid state is less than 3 &mgr;m, preferably less than 1 &mgr;m for at least 90% of the particles .
The halogen-containing organic compound (c) is chosen from the group formed by halogen-containing organic compounds that are used as flame retardant for polyamide. The best known from this group are halogen-containing polymers. In view of the high processing temperatures and the complications that may present themselves at these temperatures due to the higher
Havenith Hubertus G. J.
Leeuwendal Robert M.
Sour Wilhelmus J. M.
Tijssen Johannes
Acquah Samuel A.
DSM N.V.
Pillsbury & Winthrop LLP
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