Patent Application
20070290378
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
In describing the preferred embodiment of the present invention, reference will be made herein to
The subject invention provides reworkable underfill and dam compositions preferably silicone based soft reworkable underfill compositions (S-RUF) which have excellent chip protection properties and rework characteristics. The underfill compositions can be removed in xylene at room temperature; the modulus is as low as 100 cSt; is thermally stable above 180° C.; no chemical interaction with ATC mineral oil; and environmental protection to PbSn C4. More importantly, the type of reworkable underfill can be efficiently removed from micrometer level C4 gap, which could be as thin as 50-60 micro, simply by soaking the module into room temperature xylene, IPA or ketone, xylene is preferred or spin spray with aforementioned solvents.
Organic or polymeric materials used as the base resin of the RUFs are not reactive (non-crosslinkable) and include, but are not limited to, a polysiloxane having no or low cross-linkability such as the preferred linear polydimethyl siloxane, polyurethane, mineral oil, polyacrylate, polyacrylic ester, CTBN (carboxy-terminated butadiene-acrylonitrile) modified polyester, polystyrene, isoprene, epoxy modified siloxane, etc. The base resin is used to from a matrix for curable polymers which react to form a cross-linked polymer embedded in the base resin matrix. By non-reactive is meant that the base resin is not cross-linkable under normal cross-linking conditions, e.g., the base resin does not contain a reactive group such as vinyl, H, hydroxy-, carboxylic acid etc., but contains non-reactive groups such as methyl, ethyl, benzyl, isopropyl, etc.
The RUF compositions also comprise at least one and preferably two linear cross-linkable (curable or reactive) components such as a vinyl-terminated linear or branched siloxane and a hydrogen-terminated linear or branched siloxane. Other polymer or monomer components which are curable include ester terminated and carbocyclic anhydride terminated, or vinyl terminated linear or branched alkanes; acylic or acrylic ester; ester, cyano, anhydride, amide, vinyl, terminated CTBN, alphatic or aromatic anhydride, polyamide, polyimide, polyurethane, etc.
A curing catalyst, preferably Pt in a cyclic vinyl siloxane, is used to cure the cross-linkable polymers. H2PtCl6, Pt (C6H5)2, ZnO, etc inorganic catalysts are also preferred for the silicone system. Peroxide, amine, and metal chelate such as Co (AcAc)2 can also be used as catalysts in epoxy, acrylic, polyurethane and polystyrene base resin systems.
A filler such as a silane treated silica filler is preferably used in the composition to provide a dam composition which is used around the underfilled chip (perimeter), but may be used also in the underfill composition. Boronitride, carbon fiber, nano silica, nano carbon tube, alumina, etc and non-electroconductive fillers are also suitable for this application.
The MCM's or any other electronic component may be made using any process for chip join. Post chip join and test, the chips are underfilled and/or dammed with the composition of the invention. Following underfill and/or dam curing, the modules proceed through standard assembly and test. If rework is required, modules are disassembled, sealants and underfill removed, defect chip(s) replaced, the new chips re-underfilled and the module reassembled.
When lead containing or lead-free C4 interconnects (solders) are exposed to moisture, mobile ions under bias without protective insulation material in between, electrochemistry initiated chemical corrosion is very likely to happen. If there is alien corrosive chemical species, such as adipic acid, stearic acid, and other types organic acid formed during ATC paste thermal degradation and/or electrochemical corrosion may be greatly accelerated. Thus, to keep the moisture, mobile ion and alien carboxylic acids under a corrosion-initiated level, a protective material is needed to underfill the C4 joints.
More importantly, as process technologies continue to shrink, the critical charge required to cause a single-event-upset (SEU) is decreasing faster than the charge-collection area in the memory cell. Therefore, with smaller geometries, such as 90 nm, soft errors are more of concern.
At the process level, the use of purer packaging materials could lower alpha emission. However, tremendous efforts are needed to reduce the radioactive element level for current glass ceramic (GC) raw materials. GC fabrication processes, such as ball mill and in-house ceramic slurry formulation, and top surface metallurgy (TSM) C4 plating are very likely to introduce alpha particles. Therefore, a high purity with low alpha particle underfill is needed to protect C4 chip from alpha particle radiation damage.
To meet all the requirements mentioned above, a reworkable underfill is the easiest and lowest cost solution to high-end MCM's.
Referring to
Referring now to
The solder interconnections 13 of electronic component 10 as shown in
The reworkable composition of the invention comprises a linear thermoplastic base polymer, preferably poly(dimethylsiloxane) (PDMS), a linear curing agent preferably comprising a siloxane having reactive end groups such as a vinyl containing terminated PDMS and/or a hydrogen containing terminated PDMS, a curing catalyst and optionally a filler.
The preferred PDMS may be represented by the following formula and structure:
Me3SiO-(Me2SiO)n—SiMe3
wherein n is 10 to 2760 (MW 1250-204,000)
The vinyl terminated PDMS may be represented by the formula and structure:
wherein n is 2 to 5000.
The hydro terminated PDMS may be represented by the formula and structure:
H-Me2SiO-(Me2SiO)n—SiMe2-H
wherein n is 2 to 5000.
When the composition containing both vinyl terminated and hydro terminated siloxanes is cured a structure such as the following structure is obtained:
This structure is embedded in the base resin matrix forming an underfill having excellent reworkability properties.
It will be appreciated by those skilled in the art that the underfill and dam composition of the invention when cured forms a highly cross-linked domain in a base resin matrix, which cured composition has excellent reworkable characteristics. In particular, the cured composition is highly soluble in xylene and if the chip requires replacement, the underfill can easily be removed by immersion of the module or the chip in xylene or other suitable solvent. The reworkable underfill can be removed in xylene at room temperature and once removed, the chip can be removed from the substrate using heat to melt the solder interconnection. The substrate would then be dressed and cleaned providing a clean C4 pad site to which a new chip can be rejoined.
The base polymer forms a matrix for the underfill and is doped (mixed) with the reactive siloxanes. When cured, the reactive siloxanes react and cross-link to form the highly cross-linked domain comprising the reacted and cross-linked reactive siloxanes in the non-reactive base polymer matrix. The cross-linked domain has the properties described above and is easily removable in xylene at room temperature. The preferred siloxane base polymer preferably has a molecular weight of about 1250 to 204,000, or more. Only one of the curing agents need be used in the composition, but it is preferred to use both the vinyl terminated siloxane and hydrogen terminated siloxane curing agents in a weight ratio of 1:10 to about 10:1. The base resin is used in a weight ratio of base resin to curing agent of about 20:1 to 1:10 preferably from 10:1, to 1:5. The curing catalyst is present in the composition in an amount of about 0.001 wt.% to 5 wt.% or more. The filler if used in the SRUF composition, may be used in an amount of about 1 wt.% to 10 wt.%, or more. A silane surface treated silica is preferred.
Use of the underfill and/or dam composition of the invention has been found to provide excellent reworkable characteristics by dissolution in xylene, is not chemically interactive, non-corrosive and is commercially acceptable. Chips connected to a substrate forming an MCM and underfilled with the composition of the invention were easily reworked by dissolution in xylene at room temperature, melting of the solder interconnection, dressing and cleaning Solder residue by Cu block and reattachment of a chip and underfill with the composition of the invention.
The underfill composition is used by mixing the components together and underfilling the chips. The underfilled chip is then cured by reacting the composition by heating the module assembly to a temperature of 80 to 150° C., preferably 100 to 125° C. for 30 to 120 minutes, preferably 45 to 75 minutes.
While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.
